[
    {
        "text": "[MUSIC PLAYING]",
        "start": 0.0,
        "duration": 3.479
    },
    {
        "text": "DAVID J. MALAN: All right, this\nis CS50, and this is week 4.",
        "start": 15.407,
        "duration": 4.873
    },
    {
        "text": "So let's consider first what we did\nlast time, which was focus on numbers",
        "start": 20.28,
        "duration": 5.15
    },
    {
        "text": "and focus on how we can search\nthem and how we can sort them.",
        "start": 25.43,
        "duration": 3.44
    },
    {
        "text": "And in particular, we took a\nlook at a number of algorithms",
        "start": 28.87,
        "duration": 3.2
    },
    {
        "text": "that we finally gave names to, a couple\nof which we actually saw back in week",
        "start": 32.07,
        "duration": 3.26
    },
    {
        "text": "0, but didn't treat as formally.",
        "start": 35.33,
        "duration": 2.06
    },
    {
        "text": "So we looked more formally\nlast time at linear search,",
        "start": 37.39,
        "duration": 2.84
    },
    {
        "text": "this process of searching elements\nfrom left to right or right to left",
        "start": 40.23,
        "duration": 3.74
    },
    {
        "text": "as though you're walking along\na line looking for some value.",
        "start": 43.97,
        "duration": 2.94
    },
    {
        "text": "We looked at binary search, a\ndivide and conquer algorithm",
        "start": 46.91,
        "duration": 2.94
    },
    {
        "text": "whereby we started, in\nthis case, in the middle,",
        "start": 49.85,
        "duration": 2.092
    },
    {
        "text": "and then we looked left to\nright and then went to the left,",
        "start": 51.942,
        "duration": 2.458
    },
    {
        "text": "went to the right, and then did\nthe same problem again and again.",
        "start": 54.4,
        "duration": 2.87
    },
    {
        "text": "We looked also at bubble sort.",
        "start": 57.27,
        "duration": 1.912
    },
    {
        "text": "How do you get to the\npoint where you can",
        "start": 59.182,
        "duration": 1.708
    },
    {
        "text": "use an algorithm like\nbinary search, which",
        "start": 60.89,
        "duration": 2.29
    },
    {
        "text": "assumes that your inputs are sorted?",
        "start": 63.18,
        "duration": 2.32
    },
    {
        "text": "So how do you get to that sorted state?",
        "start": 65.5,
        "duration": 1.76
    },
    {
        "text": "Well, bubble sort was one\nalgorithm that essentially",
        "start": 67.26,
        "duration": 2.34
    },
    {
        "text": "swapped elements pairwise from\nleft to right, from left to right,",
        "start": 69.6,
        "duration": 3.55
    },
    {
        "text": "until finally the biggest\nelements bubbled up to the top,",
        "start": 73.15,
        "duration": 2.48
    },
    {
        "text": "and the whole list\nwas ultimately sorted.",
        "start": 75.63,
        "duration": 2.3
    },
    {
        "text": "But we also looked at selection sort.",
        "start": 77.93,
        "duration": 1.74
    },
    {
        "text": "And selection sort is the\nname implied, selected,",
        "start": 79.67,
        "duration": 3.11
    },
    {
        "text": "then smallest element again and again\nand again, plucking it out of the list,",
        "start": 82.78,
        "duration": 4.22
    },
    {
        "text": "and putting it where it belongs.",
        "start": 87.0,
        "duration": 1.55
    },
    {
        "text": "Insertion sort, meanwhile, walked\nthrough the list really just once,",
        "start": 88.55,
        "duration": 3.55
    },
    {
        "text": "and as it encountered\neach element, it then",
        "start": 92.1,
        "duration": 2.34
    },
    {
        "text": "inserted it into the sorted half of\nthe list, the left half of the list",
        "start": 94.44,
        "duration": 4.7
    },
    {
        "text": "in its proper place.",
        "start": 99.14,
        "duration": 1.53
    },
    {
        "text": "And then lastly, and where we really\nconcluded and really got more bang",
        "start": 100.67,
        "duration": 4.6
    },
    {
        "text": "for our buck was this\nfinal algorithm merge",
        "start": 105.27,
        "duration": 2.36
    },
    {
        "text": "sort, which actually solved this same\nproblem no less correctly, but so much",
        "start": 107.63,
        "duration": 5.2
    },
    {
        "text": "more efficiently.",
        "start": 112.83,
        "duration": 1.12
    },
    {
        "text": "And indeed, lends itself perhaps\nto a fundamentally better design",
        "start": 113.95,
        "duration": 3.17
    },
    {
        "text": "in so far as it leverages a much\nbetter running time, so to speak.",
        "start": 117.12,
        "duration": 4.46
    },
    {
        "text": "In fact, we started to ascribe\nthese kinds of formulas",
        "start": 121.58,
        "duration": 2.9
    },
    {
        "text": "to the running times of or the\nperformance of our algorithms n",
        "start": 124.48,
        "duration": 3.49
    },
    {
        "text": "squared, otherwise known\nas quadratic, whereby",
        "start": 127.97,
        "duration": 2.66
    },
    {
        "text": "you take n times n number of\nsteps or minutes or seconds",
        "start": 130.63,
        "duration": 3.37
    },
    {
        "text": "or some unit of measure, n log in,\nwhich is actually what we achieved",
        "start": 134.0,
        "duration": 3.8
    },
    {
        "text": "with the merge source,\nwhich was much faster",
        "start": 137.8,
        "duration": 2.48
    },
    {
        "text": "recall than the end squared algorithms\nlike bubble sort and selection sort",
        "start": 140.28,
        "duration": 3.17
    },
    {
        "text": "and insertion sort in\ntheir upper bounds.",
        "start": 143.45,
        "duration": 2.32
    },
    {
        "text": "n would be something linear, like\na straight line relationship.",
        "start": 145.77,
        "duration": 2.85
    },
    {
        "text": "Logarithmic of n, log\nof n, log base 2 of n,",
        "start": 148.62,
        "duration": 2.944
    },
    {
        "text": "technically, would be\nsomething like week 0's divide",
        "start": 151.564,
        "duration": 2.166
    },
    {
        "text": "and conquer approach\nlooking for Mike Smith,",
        "start": 153.73,
        "duration": 2.18
    },
    {
        "text": "as well as last time's binary search.",
        "start": 155.91,
        "duration": 2.174
    },
    {
        "text": "And then one, which doesn't\nhave to be literally one step.",
        "start": 158.084,
        "duration": 2.416
    },
    {
        "text": "Maybe it's two, maybe it's 10, but\nit is a fixed finite constant number",
        "start": 160.5,
        "duration": 4.2
    },
    {
        "text": "of steps.",
        "start": 164.7,
        "duration": 0.63
    },
    {
        "text": "And that too might be the\nrunning time of some algorithm.",
        "start": 165.33,
        "duration": 2.56
    },
    {
        "text": "Now how do we describe the\nrunning times of algorithms?",
        "start": 167.89,
        "duration": 2.81
    },
    {
        "text": "Well, we use some special\nnotation, asymptotic notation,",
        "start": 170.7,
        "duration": 3.53
    },
    {
        "text": "so to speak, which while it might\nlook cryptic at first glance,",
        "start": 174.23,
        "duration": 2.71
    },
    {
        "text": "really is just a handy way of succinctly\nexpressing the fact that you know what",
        "start": 176.94,
        "duration": 3.81
    },
    {
        "text": "the upper bound on some\nalgorithms running time is,",
        "start": 180.75,
        "duration": 2.7
    },
    {
        "text": "what the lower bounds on some\nalgorithms running time is.",
        "start": 183.45,
        "duration": 2.62
    },
    {
        "text": "And if those are one in\nthe same formulas in theta,",
        "start": 186.07,
        "duration": 3.09
    },
    {
        "text": "do you have a coincidence of the two\nof big O, so speak, and capital theta.",
        "start": 189.16,
        "duration": 5.49
    },
    {
        "text": "So that, while Greek,\nliterally is just a way",
        "start": 194.65,
        "duration": 2.17
    },
    {
        "text": "of expressing a bit more succinctly\nwhat these running times are.",
        "start": 196.82,
        "duration": 2.96
    },
    {
        "text": "And we'll continue to\nrevisit this issue as we",
        "start": 199.78,
        "duration": 1.916
    },
    {
        "text": "look at more algorithms and\nsoon data structures still.",
        "start": 201.696,
        "duration": 2.894
    },
    {
        "text": "But this time, we apologize.",
        "start": 204.59,
        "duration": 3.58
    },
    {
        "text": "We pull back a layer here and admit\nthat there is no such thing as a string.",
        "start": 208.17,
        "duration": 6.73
    },
    {
        "text": "Indeed, all this time we've been saying\nthat there's ins, and there's floats,",
        "start": 214.9,
        "duration": 3.35
    },
    {
        "text": "and there's chars, and\nthere's doubles and more.",
        "start": 218.25,
        "duration": 2.075
    },
    {
        "text": "And we've also been\nsaying there are strings,",
        "start": 220.325,
        "duration": 1.875
    },
    {
        "text": "but there really aren't strings.",
        "start": 222.2,
        "duration": 1.76
    },
    {
        "text": "This is sort of a figment of the\nimagination of our so-called CS50",
        "start": 223.96,
        "duration": 3.25
    },
    {
        "text": "library.",
        "start": 227.21,
        "duration": 0.85
    },
    {
        "text": "But it's a pedagogical\nsimplification that we've",
        "start": 228.06,
        "duration": 2.28
    },
    {
        "text": "been using for the past\nseveral weeks, so as to not get",
        "start": 230.34,
        "duration": 3.32
    },
    {
        "text": "lost in the weeds, the\nlower level implementation",
        "start": 233.66,
        "duration": 2.234
    },
    {
        "text": "details of what a string\nis, so that we can just get",
        "start": 235.894,
        "duration": 2.166
    },
    {
        "text": "real work done in these first weeks.",
        "start": 238.06,
        "duration": 2.01
    },
    {
        "text": "But now we'll begin to\nlook underneath that hood",
        "start": 240.07,
        "duration": 2.58
    },
    {
        "text": "and see what a string actually\nis and what the implications are.",
        "start": 242.65,
        "duration": 3.87
    },
    {
        "text": "And it turns out, while more\ncomplicated in some sense,",
        "start": 246.52,
        "duration": 3.18
    },
    {
        "text": "it really just boils down\nto some first principles,",
        "start": 249.7,
        "duration": 2.95
    },
    {
        "text": "what it is the computer is\ndoing underneath that hood.",
        "start": 252.65,
        "duration": 3.01
    },
    {
        "text": "So let's take a look at string first\nby way of a couple of examples.",
        "start": 255.66,
        "duration": 4.9
    },
    {
        "text": "Let's go in to CS50 IDE, create a new\nfile, save it as Compare zero dot C,",
        "start": 260.56,
        "duration": 5.27
    },
    {
        "text": "and look at the little program\nthat actually doesn't necessarily",
        "start": 265.83,
        "duration": 3.45
    },
    {
        "text": "do what we think it's going to do.",
        "start": 269.28,
        "duration": 2.72
    },
    {
        "text": "In particular, let me go\nahead and include our typical,",
        "start": 272.0,
        "duration": 2.91
    },
    {
        "text": "include CS 50 dot H. And let me go ahead\nand include standard IO dot H as well.",
        "start": 274.91,
        "duration": 5.29
    },
    {
        "text": "Let me go ahead and use\nmain void, so I'm not",
        "start": 280.2,
        "duration": 2.01
    },
    {
        "text": "going to worry about any\ncommand line arguments for now.",
        "start": 282.21,
        "duration": 2.47
    },
    {
        "text": "And then I'm going to go ahead\nand just prompt the user.",
        "start": 284.68,
        "duration": 2.333
    },
    {
        "text": "Hey user, give me a string\ncalled S for instance.",
        "start": 287.013,
        "duration": 2.144
    },
    {
        "text": "And then I'm going to have no newline.",
        "start": 289.157,
        "duration": 1.583
    },
    {
        "text": "I just want that all in one same line.",
        "start": 290.74,
        "duration": 1.99
    },
    {
        "text": "Now I'm going to go\nahead and do string S,",
        "start": 292.73,
        "duration": 3.22
    },
    {
        "text": "gets gets string, open\nparen and close parens,",
        "start": 295.95,
        "duration": 2.51
    },
    {
        "text": "so as to get a string from the user.",
        "start": 298.46,
        "duration": 2.12
    },
    {
        "text": "And then, let me do this.",
        "start": 300.58,
        "duration": 1.09
    },
    {
        "text": "Let me also print T colon and ask\nthe user, essentially, for a string",
        "start": 301.67,
        "duration": 4.33
    },
    {
        "text": "that I'll call T, since T\ncomes after S quite simply.",
        "start": 306.0,
        "duration": 4.32
    },
    {
        "text": "And now let me just compare these\nstrings as the filename suggests.",
        "start": 310.32,
        "duration": 3.37
    },
    {
        "text": "So I know how to compare,\nnot with if s equals t,",
        "start": 313.69,
        "duration": 4.706
    },
    {
        "text": "because that's the assignment operator.",
        "start": 318.396,
        "duration": 1.624
    },
    {
        "text": "But we know that s equals equals t\nshould compare the values on the left",
        "start": 320.02,
        "duration": 5.062
    },
    {
        "text": "and on the right.",
        "start": 325.082,
        "duration": 0.708
    },
    {
        "text": "So let's try this.",
        "start": 325.79,
        "duration": 0.88
    },
    {
        "text": "So if s equals equals t, then I'm\ngoing to go ahead and print out same.",
        "start": 326.67,
        "duration": 5.46
    },
    {
        "text": "Elts, they are presumably\ndifference, so I'm",
        "start": 332.13,
        "duration": 2.43
    },
    {
        "text": "going to go ahead and print out\ndifferent with a newline character",
        "start": 334.56,
        "duration": 3.98
    },
    {
        "text": "and then save it.",
        "start": 338.54,
        "duration": 1.08
    },
    {
        "text": "So a pretty simple program, so let me\ngo down into my terminal window and run",
        "start": 339.62,
        "duration": 3.69
    },
    {
        "text": "make compare 0.",
        "start": 343.31,
        "duration": 2.12
    },
    {
        "text": "Now let me go ahead and\nrun dot slash compare 0. s,",
        "start": 345.43,
        "duration": 4.49
    },
    {
        "text": "I'm going to go ahead\nand type in Zamyla.",
        "start": 349.92,
        "duration": 3.84
    },
    {
        "text": "And I'm going to go ahead\nnow and type in Maria.",
        "start": 353.76,
        "duration": 3.127
    },
    {
        "text": "OK, and they're different.",
        "start": 356.887,
        "duration": 1.083
    },
    {
        "text": "I expected as much.",
        "start": 357.97,
        "duration": 1.22
    },
    {
        "text": "Now let's go ahead and run this again.",
        "start": 359.19,
        "duration": 1.83
    },
    {
        "text": "Again with Zamyla.",
        "start": 361.02,
        "duration": 1.696
    },
    {
        "text": "And let's just say\nZamyla again, different.",
        "start": 362.716,
        "duration": 4.744
    },
    {
        "text": "What did I do wrong?",
        "start": 367.46,
        "duration": 1.004
    },
    {
        "text": "Let me try this again.",
        "start": 368.464,
        "duration": 0.916
    },
    {
        "text": "Maybe it's the capitalization.",
        "start": 369.38,
        "duration": 1.63
    },
    {
        "text": "So Zamyla in all lower case, different.",
        "start": 371.01,
        "duration": 5.11
    },
    {
        "text": "Well, maybe it's just Zamyla's name.",
        "start": 376.12,
        "duration": 1.69
    },
    {
        "text": "Let me try Rob or RLB?",
        "start": 377.81,
        "duration": 3.76
    },
    {
        "text": "How about RLB?",
        "start": 381.57,
        "duration": 1.2
    },
    {
        "text": "Those are different, as is Rob and Rob.",
        "start": 382.77,
        "duration": 2.86
    },
    {
        "text": "So what is going on?",
        "start": 385.63,
        "duration": 1.82
    },
    {
        "text": "Those strings pretty\nmuch look the same to me.",
        "start": 387.45,
        "duration": 2.12
    },
    {
        "text": "I'm typing the same\nincantation of strings,",
        "start": 389.57,
        "duration": 2.273
    },
    {
        "text": "so what is it that's going on here?",
        "start": 391.843,
        "duration": 2.777
    },
    {
        "text": "You know what, let me do a\ntest with something else.",
        "start": 394.62,
        "duration": 2.46
    },
    {
        "text": "Let me go ahead and\ncreate a new file here.",
        "start": 397.08,
        "duration": 2.61
    },
    {
        "text": "I'm going to call this copyzero.c.",
        "start": 399.69,
        "duration": 1.66
    },
    {
        "text": "Maybe I'm just misunderstanding\nhow comparison works.",
        "start": 401.35,
        "duration": 2.78
    },
    {
        "text": "But surely I should be able to copy\na string and make an identical copy,",
        "start": 404.13,
        "duration": 3.11
    },
    {
        "text": "so let's do that now.",
        "start": 407.24,
        "duration": 1.14
    },
    {
        "text": "Let me go ahead and create\na file called copyzero.c.",
        "start": 408.38,
        "duration": 4.58
    },
    {
        "text": "And let me do an include cs50.h.",
        "start": 412.96,
        "duration": 2.75
    },
    {
        "text": "Include standard io.h into\nmain void, so just as before.",
        "start": 415.71,
        "duration": 6.027
    },
    {
        "text": "And now, let me go ahead and\njust prompt the user like before.",
        "start": 421.737,
        "duration": 2.583
    },
    {
        "text": "Give me a string s.",
        "start": 424.32,
        "duration": 1.3
    },
    {
        "text": "And I'll put that in a variable,\ns, calling get string as before.",
        "start": 425.62,
        "duration": 4.23
    },
    {
        "text": "And now, I remember from prior\nclasses that I'm supposed to do,",
        "start": 429.85,
        "duration": 5.09
    },
    {
        "text": "if s equals equals null, maybe I\nneed to do some error checking here.",
        "start": 434.94,
        "duration": 4.71
    },
    {
        "text": "And if that's the case, I'm\ngoing to arbitrarily return 1.",
        "start": 439.65,
        "duration": 2.48
    },
    {
        "text": "So recall that main can return values.",
        "start": 442.13,
        "duration": 2.12
    },
    {
        "text": "0 means all is well.",
        "start": 444.25,
        "duration": 1.2
    },
    {
        "text": "1 or any other non-zero value\nmeans something is wrong.",
        "start": 445.45,
        "duration": 2.6
    },
    {
        "text": "So maybe that's what I was doing wrong\nearlier, just not error checking.",
        "start": 448.05,
        "duration": 3.0
    },
    {
        "text": "So let's try that.",
        "start": 451.05,
        "duration": 1.04
    },
    {
        "text": "Now, let me declare a variable of type\nstring called t and assign it to s.",
        "start": 452.09,
        "duration": 5.94
    },
    {
        "text": "So in other words, I\nwant to copy s into t.",
        "start": 458.03,
        "duration": 3.09
    },
    {
        "text": "And I know that this happens\nfrom the right to the left.",
        "start": 461.12,
        "duration": 2.35
    },
    {
        "text": "So that's as I think it should be.",
        "start": 463.47,
        "duration": 2.41
    },
    {
        "text": "And then, if strleng of\nt is greater than 0--",
        "start": 465.88,
        "duration": 7.302
    },
    {
        "text": "you know what, I'm going\nto do one other thing.",
        "start": 473.182,
        "duration": 1.958
    },
    {
        "text": "Not just copy it.",
        "start": 475.14,
        "duration": 0.924
    },
    {
        "text": "I'm going to go ahead and do this.",
        "start": 476.064,
        "duration": 1.416
    },
    {
        "text": "Recall, that you can\ntreat strings as arrays.",
        "start": 477.48,
        "duration": 2.02
    },
    {
        "text": "So I'm going to say the zeroth\ncharacter, the first character of t--",
        "start": 479.5,
        "duration": 3.477
    },
    {
        "text": "you know what?",
        "start": 482.977,
        "duration": 0.583
    },
    {
        "text": "I want to uppercase it.",
        "start": 483.56,
        "duration": 1.61
    },
    {
        "text": "I want to really make sure that\nthese two strings are indeed",
        "start": 485.17,
        "duration": 2.79
    },
    {
        "text": "different as I intend.",
        "start": 487.96,
        "duration": 1.58
    },
    {
        "text": "And let's go ahead now and do this.",
        "start": 489.54,
        "duration": 3.3
    },
    {
        "text": "Let me go ahead now and do printf of s.",
        "start": 492.84,
        "duration": 4.5
    },
    {
        "text": "And let me plug-in s's\nvalue in a new line.",
        "start": 497.34,
        "duration": 2.18
    },
    {
        "text": "Plug-in s.",
        "start": 499.52,
        "duration": 0.7
    },
    {
        "text": "And then printf t:%s\nfor a placeholder again.",
        "start": 500.22,
        "duration": 4.87
    },
    {
        "text": "Comma t semi-colon.",
        "start": 505.09,
        "duration": 1.74
    },
    {
        "text": "So in other words, if the length\nof t is greater than zero,",
        "start": 506.83,
        "duration": 3.69
    },
    {
        "text": "let's capitalize it by\nchanging that first character.",
        "start": 510.52,
        "duration": 2.764
    },
    {
        "text": "And then, just print out s and t.",
        "start": 513.284,
        "duration": 1.375
    },
    {
        "text": "And surely, by capitalizing only t, I\nshould see only one capitalized word.",
        "start": 514.659,
        "duration": 5.091
    },
    {
        "text": "Now, I'm using both\nstrleng and two upper.",
        "start": 519.75,
        "duration": 1.812
    },
    {
        "text": "So rather than let Clang have a chance\nto yell at me, I'm going to go in here",
        "start": 521.562,
        "duration": 3.208
    },
    {
        "text": "and preemptively add ctype.h.",
        "start": 524.77,
        "duration": 3.056
    },
    {
        "text": "Which recall is a library\nyou might have seen",
        "start": 527.826,
        "duration": 1.874
    },
    {
        "text": "or certainly will soon see that\nhas a number of functions in it.",
        "start": 529.7,
        "duration": 3.34
    },
    {
        "text": "Among them, two upper.",
        "start": 533.04,
        "duration": 1.18
    },
    {
        "text": "And then, I also need to include\nstring.h, so that I can use strlen--",
        "start": 534.22,
        "duration": 4.094
    },
    {
        "text": "the function that gives me\nthe length of the string.",
        "start": 538.314,
        "duration": 2.166
    },
    {
        "text": "So Clang would have yelled at me if I\nforgot that, but let me preemptively",
        "start": 540.48,
        "duration": 2.89
    },
    {
        "text": "solve that problem.",
        "start": 543.37,
        "duration": 0.82
    },
    {
        "text": "And now, do make copies zero.",
        "start": 544.19,
        "duration": 2.35
    },
    {
        "text": "All seems to be well.",
        "start": 546.54,
        "duration": 1.03
    },
    {
        "text": "Let's run dot slash copy 0.",
        "start": 547.57,
        "duration": 1.861
    },
    {
        "text": "All right.",
        "start": 549.431,
        "duration": 0.499
    },
    {
        "text": "Let's go ahead and type in-- how about\njust my own name in all lowercase.",
        "start": 549.93,
        "duration": 6.05
    },
    {
        "text": "Huh?",
        "start": 555.98,
        "duration": 1.31
    },
    {
        "text": "Now, why is this confusing?",
        "start": 557.29,
        "duration": 2.04
    },
    {
        "text": "So I wrote code that got a string\nfrom the user and stored it in s.",
        "start": 559.33,
        "duration": 5.53
    },
    {
        "text": "I then wrote code that\ndeclared a second variable, t.",
        "start": 564.86,
        "duration": 3.48
    },
    {
        "text": "And I set t equal to s,\nthereby making I would",
        "start": 568.34,
        "duration": 2.88
    },
    {
        "text": "think a copy-- as in past weeks\nof using the assignment operator.",
        "start": 571.22,
        "duration": 3.7
    },
    {
        "text": "Then down here, I made\nsure t was long enough.",
        "start": 574.92,
        "duration": 2.21
    },
    {
        "text": "That's just a quick sanity check.",
        "start": 577.13,
        "duration": 1.71
    },
    {
        "text": "And then on this line\nhere, I'm just saying,",
        "start": 578.84,
        "duration": 3.18
    },
    {
        "text": "set the first character of t, t bracket\n0, equal to the result of upper casing",
        "start": 582.02,
        "duration": 5.85
    },
    {
        "text": "the first character of t.",
        "start": 587.87,
        "duration": 2.13
    },
    {
        "text": "So the only code that's\ntouching t is this one here.",
        "start": 590.0,
        "duration": 3.37
    },
    {
        "text": "And yet, somehow my name gets\ncapitalized both in s and in t.",
        "start": 593.37,
        "duration": 5.38
    },
    {
        "text": "So what is it that's\nactually going on here?",
        "start": 598.75,
        "duration": 3.67
    },
    {
        "text": "It seems to be broken still.",
        "start": 602.42,
        "duration": 2.559
    },
    {
        "text": "In fact, let me go ahead and open\nanother example, rather than type",
        "start": 604.979,
        "duration": 2.791
    },
    {
        "text": "this one out ourselves.",
        "start": 607.77,
        "duration": 1.73
    },
    {
        "text": "Let me go ahead and open up\nan example called no swap.",
        "start": 609.5,
        "duration": 4.42
    },
    {
        "text": "As the name suggests,\nit's a bit of a spoiler.",
        "start": 613.92,
        "duration": 2.59
    },
    {
        "text": "I wrote this program in\nadvance to do the following.",
        "start": 616.51,
        "duration": 2.82
    },
    {
        "text": "First, I've included standard\nio.h, so I can use printf.",
        "start": 619.33,
        "duration": 3.61
    },
    {
        "text": "I have a prototype of my function\ncalled swap up here, because indeed,",
        "start": 622.94,
        "duration": 3.55
    },
    {
        "text": "the goal at hand I decided was I just\nwant to write a simple function that",
        "start": 626.49,
        "duration": 4.11
    },
    {
        "text": "swaps two values.",
        "start": 630.6,
        "duration": 1.69
    },
    {
        "text": "Given a and b, make b a and a, b.",
        "start": 632.29,
        "duration": 2.86
    },
    {
        "text": "And then, return.",
        "start": 635.15,
        "duration": 0.99
    },
    {
        "text": "So I'm just going to arbitrarily test\nthis out by declaring a variable x.",
        "start": 636.14,
        "duration": 3.06
    },
    {
        "text": "Setting it equal to 1.",
        "start": 639.2,
        "duration": 1.17
    },
    {
        "text": "A variable y setting it equal to 2.",
        "start": 640.37,
        "duration": 2.35
    },
    {
        "text": "And then, just as a sanity check, I'm\ngoing to print out x is such and such,",
        "start": 642.72,
        "duration": 3.3
    },
    {
        "text": "y is such and such.",
        "start": 646.02,
        "duration": 1.18
    },
    {
        "text": "And then, I'm going to\nclaim swapping dot dot dot.",
        "start": 647.2,
        "duration": 2.92
    },
    {
        "text": "And then, the key line\nis apparently this one.",
        "start": 650.12,
        "duration": 2.68
    },
    {
        "text": "Call a function called\nswap, passing in x and y.",
        "start": 652.8,
        "duration": 3.51
    },
    {
        "text": "And if I implemented swap correctly,\nthis should swap the two variables.",
        "start": 656.31,
        "duration": 4.09
    },
    {
        "text": "Thereafter, I'm going to claim swapped\nx as such and such, y as such and such.",
        "start": 660.4,
        "duration": 4.69
    },
    {
        "text": "So let's run this program and see\nwhat else is apparently a lie.",
        "start": 665.09,
        "duration": 5.19
    },
    {
        "text": "Make no swap in my source directory.",
        "start": 670.28,
        "duration": 6.62
    },
    {
        "text": "./noswap enter.",
        "start": 676.9,
        "duration": 3.71
    },
    {
        "text": "And if we scroll back up in my\nhistory, you'll see x is 1, y is 2.",
        "start": 680.61,
        "duration": 4.95
    },
    {
        "text": "Swapping swapped.",
        "start": 685.56,
        "duration": 1.66
    },
    {
        "text": "x is 1, y is 2.",
        "start": 687.22,
        "duration": 1.431
    },
    {
        "text": "All right.",
        "start": 688.651,
        "duration": 0.499
    },
    {
        "text": "So maybe just the swap\nfunction is buggy.",
        "start": 689.15,
        "duration": 1.96
    },
    {
        "text": "This isn't necessarily\nindicative of a misunderstanding.",
        "start": 691.11,
        "duration": 3.17
    },
    {
        "text": "So let's look at the\nimplementation of swap.",
        "start": 694.28,
        "duration": 2.01
    },
    {
        "text": "Swap returns no value and I think\nthat's OK, so long as it takes inputs.",
        "start": 696.29,
        "duration": 4.5
    },
    {
        "text": "Swap takes two inputs, an ints and\nan ints, called a and b respectively.",
        "start": 700.79,
        "duration": 4.64
    },
    {
        "text": "And then, let's consider how this works.",
        "start": 705.43,
        "duration": 1.72
    },
    {
        "text": "So I've declared a temporary\nvariable, called temp--",
        "start": 707.15,
        "duration": 3.225
    },
    {
        "text": "though I could call it anything I want.",
        "start": 710.375,
        "duration": 1.625
    },
    {
        "text": "And I'm storing in it the value a.",
        "start": 712.0,
        "duration": 2.28
    },
    {
        "text": "So I'm taking a-- and it's the number 1.",
        "start": 714.28,
        "duration": 1.975
    },
    {
        "text": "And I'm just temporarily\nstoring the number 1",
        "start": 716.255,
        "duration": 1.875
    },
    {
        "text": "in this additional temporary\nvariable, so that I now",
        "start": 718.13,
        "duration": 2.22
    },
    {
        "text": "have two copies of the number\none-- in a and in temp.",
        "start": 720.35,
        "duration": 3.34
    },
    {
        "text": "Then, I change the\nvalue of a to equal b.",
        "start": 723.69,
        "duration": 3.09
    },
    {
        "text": "So at this point in the story,\na should have a value of 2.",
        "start": 726.78,
        "duration": 3.92
    },
    {
        "text": "b should have a value of 2.",
        "start": 730.7,
        "duration": 1.62
    },
    {
        "text": "Which could be problematic, except\nI've saved the original value 1",
        "start": 732.32,
        "duration": 3.88
    },
    {
        "text": "in my temporary variable.",
        "start": 736.2,
        "duration": 1.59
    },
    {
        "text": "And so, indeed I change b to\nequal that temporary value.",
        "start": 737.79,
        "duration": 3.97
    },
    {
        "text": "So indeed, I can't do\nthis magical switcheroo.",
        "start": 741.76,
        "duration": 2.63
    },
    {
        "text": "Because if I simultaneously\ntook one value and one value",
        "start": 744.39,
        "duration": 3.0
    },
    {
        "text": "and put them in each other's\nplaces, I would essentially",
        "start": 747.39,
        "duration": 2.8
    },
    {
        "text": "risk clobbering one or the other.",
        "start": 750.19,
        "duration": 1.554
    },
    {
        "text": "Because the computer typically\ncan only do one thing",
        "start": 751.744,
        "duration": 2.166
    },
    {
        "text": "at a time underneath the hood,\neven though it's super fast.",
        "start": 753.91,
        "duration": 2.97
    },
    {
        "text": "So I run the risk of somehow getting\ntwo copies of that same value,",
        "start": 756.88,
        "duration": 5.43
    },
    {
        "text": "unless I'm careful.",
        "start": 762.31,
        "duration": 0.96
    },
    {
        "text": "I really want to take one value,\nput it in a temporary space.",
        "start": 763.27,
        "duration": 3.71
    },
    {
        "text": "Then, copy one value over.",
        "start": 766.98,
        "duration": 1.72
    },
    {
        "text": "And then, do that switcheroo.",
        "start": 768.7,
        "duration": 1.84
    },
    {
        "text": "But it doesn't seem to work.",
        "start": 770.54,
        "duration": 1.54
    },
    {
        "text": "And yet, this is a good\nopportunity for one more tool.",
        "start": 772.08,
        "duration": 2.98
    },
    {
        "text": "Let me do a further check.",
        "start": 775.06,
        "duration": 1.48
    },
    {
        "text": "Let me go ahead and run\ndebug 50 on no swap.",
        "start": 776.54,
        "duration": 4.5
    },
    {
        "text": "And you know what?",
        "start": 781.04,
        "duration": 0.75
    },
    {
        "text": "I'm going to set a break point right at\nthe beginning of this program on line",
        "start": 781.79,
        "duration": 3.33
    },
    {
        "text": "10 by simply clicking in the so-called\ngutter of the line number here.",
        "start": 785.12,
        "duration": 3.99
    },
    {
        "text": "A red dot appears, so that\nnow when I run debug 50,",
        "start": 789.11,
        "duration": 3.04
    },
    {
        "text": "notice my right hand bar pops out.",
        "start": 792.15,
        "duration": 2.17
    },
    {
        "text": "Notice that the line 10 in question\nis highlighted, because execution is",
        "start": 794.32,
        "duration": 3.96
    },
    {
        "text": "paused at this line.",
        "start": 798.28,
        "duration": 1.2
    },
    {
        "text": "Let's take a look at what's going on\nthe right hand side of my debugger.",
        "start": 799.48,
        "duration": 3.0
    },
    {
        "text": "Notice that x has a value\nof 1 and y has a value of 2.",
        "start": 802.48,
        "duration": 4.1
    },
    {
        "text": "And that's what I intend.",
        "start": 806.58,
        "duration": 1.15
    },
    {
        "text": "Now, let's begin to\nstep over these lines.",
        "start": 807.73,
        "duration": 2.34
    },
    {
        "text": "So let's step over the printf.",
        "start": 810.07,
        "duration": 1.77
    },
    {
        "text": "And we see on my terminal\nwindow that x is 1.",
        "start": 811.84,
        "duration": 2.41
    },
    {
        "text": "Step over the next printf\nand we see that y is 2.",
        "start": 814.25,
        "duration": 2.99
    },
    {
        "text": "Now, let's step over that and we\nsee indeed swapping dot dot dot.",
        "start": 817.24,
        "duration": 3.62
    },
    {
        "text": "But rather than step over my call to\nthe swap function, let's step into it.",
        "start": 820.86,
        "duration": 4.31
    },
    {
        "text": "So I can actually see\nmaybe my logic is buggy.",
        "start": 825.17,
        "duration": 2.67
    },
    {
        "text": "So let's go ahead and step into swap.",
        "start": 827.84,
        "duration": 2.37
    },
    {
        "text": "And notice that debug 50 immediately\njumps down to the first line and swap.",
        "start": 830.21,
        "duration": 4.42
    },
    {
        "text": "And notice now on the right\nhand side, what's going on?",
        "start": 834.63,
        "duration": 3.17
    },
    {
        "text": "a is 1 and b is 2.",
        "start": 837.8,
        "duration": 2.21
    },
    {
        "text": "And that's as expected,\nbecause I passed an x and y.",
        "start": 840.01,
        "duration": 3.22
    },
    {
        "text": "Swap calls it's two inputs a and\nb respectively, so a is 1, b is 2.",
        "start": 843.23,
        "duration": 4.8
    },
    {
        "text": "Temp though, very curiously is 32,767.",
        "start": 848.03,
        "duration": 6.629
    },
    {
        "text": "And where is that value coming?",
        "start": 854.659,
        "duration": 1.291
    },
    {
        "text": "Well, it turns out when\nyou declare a variable,",
        "start": 855.95,
        "duration": 3.38
    },
    {
        "text": "but haven't yet said it's of some\nvalue it's going to by definition have",
        "start": 859.33,
        "duration": 3.58
    },
    {
        "text": "an undefined value.",
        "start": 862.91,
        "duration": 1.18
    },
    {
        "text": "A so-called garbage value.",
        "start": 864.09,
        "duration": 1.51
    },
    {
        "text": "Remnants of other\naspects of your program",
        "start": 865.6,
        "duration": 2.77
    },
    {
        "text": "having used that chunk of memory before.",
        "start": 868.37,
        "duration": 2.17
    },
    {
        "text": "And we'll see why this is in just a bit.",
        "start": 870.54,
        "duration": 1.77
    },
    {
        "text": "So this is anomalous.",
        "start": 872.31,
        "duration": 1.25
    },
    {
        "text": "It's not a bug in the program.",
        "start": 873.56,
        "duration": 1.25
    },
    {
        "text": "It's to be expected, because\nI haven't yet assigned",
        "start": 874.81,
        "duration": 2.22
    },
    {
        "text": "temp a value until-- let me zoom out.",
        "start": 877.03,
        "duration": 3.11
    },
    {
        "text": "Until I step over line 21 and\nactually let temp equal a.",
        "start": 880.14,
        "duration": 6.58
    },
    {
        "text": "And notice, if you watch the\nright hand side of my screen now,",
        "start": 886.72,
        "duration": 2.75
    },
    {
        "text": "temp is currently 32,767.",
        "start": 889.47,
        "duration": 4.03
    },
    {
        "text": "And when I step over it, OK.",
        "start": 893.5,
        "duration": 2.51
    },
    {
        "text": "It becomes 1.",
        "start": 896.01,
        "duration": 1.04
    },
    {
        "text": "So that's as expected.",
        "start": 897.05,
        "duration": 1.29
    },
    {
        "text": "Now, a is about to become b.",
        "start": 898.34,
        "duration": 2.61
    },
    {
        "text": "And indeed, we now see that a\nis 2 b is 2 and temp is still 1.",
        "start": 900.95,
        "duration": 4.2
    },
    {
        "text": "So now we have to put temp into\nb, so let's step over that line.",
        "start": 905.15,
        "duration": 3.4
    },
    {
        "text": "And indeed, we've done\na switcheroo of a and b.",
        "start": 908.55,
        "duration": 3.57
    },
    {
        "text": "a is 2, b is 1.",
        "start": 912.12,
        "duration": 2.18
    },
    {
        "text": "OK.",
        "start": 914.3,
        "duration": 0.5
    },
    {
        "text": "Let's go ahead and play this out.",
        "start": 914.8,
        "duration": 2.55
    },
    {
        "text": "Let's continue here.",
        "start": 917.35,
        "duration": 1.06
    },
    {
        "text": "I claim that I've swapped.",
        "start": 918.41,
        "duration": 1.52
    },
    {
        "text": "I'm printing x-- and somehow\nx and y are still 1 and 2.",
        "start": 919.93,
        "duration": 6.68
    },
    {
        "text": "Even though the debunker\ntold me it would",
        "start": 926.61,
        "duration": 1.97
    },
    {
        "text": "seem that my swap function was correct.",
        "start": 928.58,
        "duration": 2.33
    },
    {
        "text": "So what is happening after all of these\nweeks of CS50 and all of these problems",
        "start": 930.91,
        "duration": 4.24
    },
    {
        "text": "solved thus far?",
        "start": 935.15,
        "duration": 0.792
    },
    {
        "text": "It would seem that all of\nour understanding of things",
        "start": 935.942,
        "duration": 2.208
    },
    {
        "text": "is kind of unraveling.",
        "start": 938.15,
        "duration": 1.059
    },
    {
        "text": "And that's because we've been very\ncareful over the past few weeks,",
        "start": 939.209,
        "duration": 2.791
    },
    {
        "text": "at least in class.",
        "start": 942.0,
        "duration": 0.93
    },
    {
        "text": "And in sections to try to avoid\ntripping over some of these lower-level",
        "start": 942.93,
        "duration": 3.41
    },
    {
        "text": "implementation details.",
        "start": 946.34,
        "duration": 1.26
    },
    {
        "text": "And there's not that many of them.",
        "start": 947.6,
        "duration": 1.56
    },
    {
        "text": "But today, is now a time\nto peel back this layer",
        "start": 949.16,
        "duration": 4.12
    },
    {
        "text": "and understand exactly\nwhat it is that's going on.",
        "start": 953.28,
        "duration": 2.83
    },
    {
        "text": "Indeed, all of this time when you\nrun a program on your computer,",
        "start": 956.11,
        "duration": 4.06
    },
    {
        "text": "double-click an icon or run the\nprogram's name with dot slash",
        "start": 960.17,
        "duration": 2.89
    },
    {
        "text": "something or other at your\nterminal window-- what happens",
        "start": 963.06,
        "duration": 2.56
    },
    {
        "text": "is that your computer gives that program\nthe illusion of a really big chunk",
        "start": 965.62,
        "duration": 4.93
    },
    {
        "text": "of memory all of its own.",
        "start": 970.55,
        "duration": 1.34
    },
    {
        "text": "Maybe two gigabytes of memory,\neven though it doesn't necessarily",
        "start": 971.89,
        "duration": 3.05
    },
    {
        "text": "use all of that memory.",
        "start": 974.94,
        "duration": 1.337
    },
    {
        "text": "And that memory-- if you just\nthink of your computer's memory,",
        "start": 976.277,
        "duration": 2.583
    },
    {
        "text": "as we've done before, is\nlike a rectangular region.",
        "start": 978.86,
        "duration": 2.125
    },
    {
        "text": "And we could number of\nthe bytes in my memory.",
        "start": 980.985,
        "duration": 1.922
    },
    {
        "text": "But it doesn't really matter what the\naddresses are, what the numbers are,",
        "start": 982.907,
        "duration": 3.083
    },
    {
        "text": "just that it exists.",
        "start": 985.99,
        "duration": 1.78
    },
    {
        "text": "It turns out that when\nyou run a program,",
        "start": 987.77,
        "duration": 3.19
    },
    {
        "text": "your operating system typically lays\nout your program's memory in a fairly",
        "start": 990.96,
        "duration": 3.79
    },
    {
        "text": "standard way.",
        "start": 994.75,
        "duration": 1.65
    },
    {
        "text": "There's a chunk of memory down here for\nsomething called environment variables.",
        "start": 996.4,
        "duration": 3.52
    },
    {
        "text": "There's a bigger growable chunk\nof memory down here called",
        "start": 999.92,
        "duration": 3.66
    },
    {
        "text": "the so-called stack.",
        "start": 1003.58,
        "duration": 1.58
    },
    {
        "text": "On the opposite side of the\npicture is a so-called heap.",
        "start": 1005.16,
        "duration": 2.379
    },
    {
        "text": "Another chunk of memory that actually\ngrows in the other direction.",
        "start": 1007.539,
        "duration": 2.791
    },
    {
        "text": "So long story short, bad things\ncan happen if both of those",
        "start": 1010.33,
        "duration": 3.04
    },
    {
        "text": "grow bigger than you intend.",
        "start": 1013.37,
        "duration": 1.78
    },
    {
        "text": "Then, there's some kind of uninitialized\nand initialized data up top.",
        "start": 1015.15,
        "duration": 3.53
    },
    {
        "text": "And then, text.",
        "start": 1018.68,
        "duration": 1.39
    },
    {
        "text": "Now, it turns out text is\nthe segment of memory where",
        "start": 1020.07,
        "duration": 3.2
    },
    {
        "text": "your program's zeros and ones live.",
        "start": 1023.27,
        "duration": 2.299
    },
    {
        "text": "So when you double-click\nan icon on your Mac or PC",
        "start": 1025.569,
        "duration": 2.111
    },
    {
        "text": "or when you run the command\ndot slash something,",
        "start": 1027.68,
        "duration": 2.07
    },
    {
        "text": "those zeros and ones are loaded from\nyour hard disk or solid state disk",
        "start": 1029.75,
        "duration": 3.54
    },
    {
        "text": "into RAM or memory.",
        "start": 1033.29,
        "duration": 1.64
    },
    {
        "text": "And it's put conceptually\nat the top of the memory",
        "start": 1034.93,
        "duration": 3.38
    },
    {
        "text": "that your computer program is using.",
        "start": 1038.31,
        "duration": 1.64
    },
    {
        "text": "And below that is the actual\ndata that your program is using.",
        "start": 1039.95,
        "duration": 4.149
    },
    {
        "text": "The variables and the\nvalues inside of it.",
        "start": 1044.099,
        "duration": 2.451
    },
    {
        "text": "Now, each of these types of\nmemory have different purposes.",
        "start": 1046.55,
        "duration": 3.06
    },
    {
        "text": "And we'll see in just a moment\nwhat it is that's going on.",
        "start": 1049.61,
        "duration": 3.49
    },
    {
        "text": "And we'll ultimately\npeel back these layers.",
        "start": 1053.1,
        "duration": 2.31
    },
    {
        "text": "So what is it that's actually\ngoing on underneath the hood here?",
        "start": 1055.41,
        "duration": 3.53
    },
    {
        "text": "Well, let's consider this\nto be my computer's memory.",
        "start": 1058.94,
        "duration": 2.77
    },
    {
        "text": "So focusing on just that\nbottom most portion,",
        "start": 1061.71,
        "duration": 2.99
    },
    {
        "text": "which I called the stack a moment ago.",
        "start": 1064.7,
        "duration": 2.08
    },
    {
        "text": "So if we draw just the bottom of my\ncomputer's memory kind of like this,",
        "start": 1066.78,
        "duration": 3.645
    },
    {
        "text": "the bottom of it has technically\nenvironment variables.",
        "start": 1070.425,
        "duration": 2.574
    },
    {
        "text": "But let's focus on the\nregion known as the stack.",
        "start": 1072.999,
        "duration": 2.041
    },
    {
        "text": "And the stack, as the name implies,\nis kind of like the stack of trays",
        "start": 1075.04,
        "duration": 3.68
    },
    {
        "text": "that you might see in a\ncafeteria or a dining hall,",
        "start": 1078.72,
        "duration": 2.18
    },
    {
        "text": "where you put trays on top of another\nuntil it can get potentially pretty",
        "start": 1080.9,
        "duration": 3.89
    },
    {
        "text": "tall.",
        "start": 1084.79,
        "duration": 0.62
    },
    {
        "text": "And it turns out when\nyou run a program, not",
        "start": 1085.41,
        "duration": 2.74
    },
    {
        "text": "only is your program given the illusion\nof this really big memory space laid",
        "start": 1088.15,
        "duration": 3.51
    },
    {
        "text": "out as proposed, but\nit also by convention",
        "start": 1091.66,
        "duration": 3.23
    },
    {
        "text": "uses this memory in a\nfairly standard way.",
        "start": 1094.89,
        "duration": 2.94
    },
    {
        "text": "Specifically, when main is called,\nmain is given a chunk of memory",
        "start": 1097.83,
        "duration": 5.47
    },
    {
        "text": "at the bottom, so to\nspeak, of this stack space.",
        "start": 1103.3,
        "duration": 2.48
    },
    {
        "text": "And so, let me go ahead\nhere and write main.",
        "start": 1105.78,
        "duration": 2.67
    },
    {
        "text": "And any local variables that main\nhas and any arguments to main,",
        "start": 1108.45,
        "duration": 5.0
    },
    {
        "text": "namely argc an argv, end up inside here.",
        "start": 1113.45,
        "duration": 3.5
    },
    {
        "text": "So if indeed you are using\nsomething like argv and argc,",
        "start": 1116.95,
        "duration": 4.68
    },
    {
        "text": "you might have a value\nlike this down here.",
        "start": 1121.63,
        "duration": 2.83
    },
    {
        "text": "And you might have another chunk\nof memory carved out here for argv.",
        "start": 1124.46,
        "duration": 4.46
    },
    {
        "text": "And if you have a couple of\nlocal variables, for instance x",
        "start": 1128.92,
        "duration": 4.03
    },
    {
        "text": "and another one, y, those two would\nbe allocated space in this slice",
        "start": 1132.95,
        "duration": 5.48
    },
    {
        "text": "if you will.",
        "start": 1138.43,
        "duration": 0.53
    },
    {
        "text": "This frame of memory.",
        "start": 1138.96,
        "duration": 2.21
    },
    {
        "text": "Meanwhile, if main calls a\nfunction, like swap-- swap",
        "start": 1141.17,
        "duration": 4.72
    },
    {
        "text": "is allocated a swath of memory, a\nframe of memory, above main by design.",
        "start": 1145.89,
        "duration": 5.45
    },
    {
        "text": "So if I've called swap,\nits memory ends up here.",
        "start": 1151.34,
        "duration": 3.56
    },
    {
        "text": "And if the swap function\nitself has arguments,",
        "start": 1154.9,
        "duration": 3.26
    },
    {
        "text": "like a and b or any\nother local variables,",
        "start": 1158.16,
        "duration": 2.94
    },
    {
        "text": "those values too are put inside\nof the so-called stack frame.",
        "start": 1161.1,
        "duration": 4.1
    },
    {
        "text": "So this might be a and this might be b.",
        "start": 1165.2,
        "duration": 3.659
    },
    {
        "text": "In other words, the concepts\nthat we've been taking for",
        "start": 1168.859,
        "duration": 2.291
    },
    {
        "text": "granted, both in Scratch and in C, at\nthe end of the day, boil down to values",
        "start": 1171.15,
        "duration": 4.48
    },
    {
        "text": "needing to go somewhere physically.",
        "start": 1175.63,
        "duration": 2.1
    },
    {
        "text": "And so, if you assume that\nthe big rectangular region",
        "start": 1177.73,
        "duration": 2.31
    },
    {
        "text": "here is your computer's memory.",
        "start": 1180.04,
        "duration": 1.44
    },
    {
        "text": "And then, you consider that\nthe operating system really",
        "start": 1181.48,
        "duration": 2.49
    },
    {
        "text": "just slices and dices this memory,\nsuch that mains memory is down here.",
        "start": 1183.97,
        "duration": 5.16
    },
    {
        "text": "Any function that main calls\nis immediately above it.",
        "start": 1189.13,
        "duration": 2.6
    },
    {
        "text": "And frankly, if swap called its own\nfunction, it would end up above it.",
        "start": 1191.73,
        "duration": 4.54
    },
    {
        "text": "But now, given this basic\ndefinition of memory management",
        "start": 1196.27,
        "duration": 4.16
    },
    {
        "text": "and the layout of computer\nprogram's memory space,",
        "start": 1200.43,
        "duration": 2.87
    },
    {
        "text": "you can perhaps start to infer\nwhy all of these failures",
        "start": 1203.3,
        "duration": 4.23
    },
    {
        "text": "have started to happen in my program.",
        "start": 1207.53,
        "duration": 2.08
    },
    {
        "text": "A moment ago, I didn't\nhave argv and argc.",
        "start": 1209.61,
        "duration": 2.88
    },
    {
        "text": "I just had for instance x and y.",
        "start": 1212.49,
        "duration": 2.04
    },
    {
        "text": "And I had the value 1\nand I had the value 2.",
        "start": 1214.53,
        "duration": 4.21
    },
    {
        "text": "Main then called swap and put a copy\nof 1 there and a copy of 2 there.",
        "start": 1218.74,
        "duration": 6.53
    },
    {
        "text": "And indeed, that's the key insight.",
        "start": 1225.27,
        "duration": 2.34
    },
    {
        "text": "When a function calls another function,\npassing in arguments as inputs,",
        "start": 1227.61,
        "duration": 4.55
    },
    {
        "text": "that the function is being\npassed to copies of those inputs.",
        "start": 1232.16,
        "duration": 3.72
    },
    {
        "text": "So at this point in time, if you\nopened up the lid of your computer",
        "start": 1235.88,
        "duration": 3.01
    },
    {
        "text": "and looked inside digitally,\nyou would see 1 and 2 down here.",
        "start": 1238.89,
        "duration": 3.302
    },
    {
        "text": "And you would see\nanother pattern of bits",
        "start": 1242.192,
        "duration": 1.708
    },
    {
        "text": "representing 1 and 2\nup here in duplicate.",
        "start": 1243.9,
        "duration": 2.87
    },
    {
        "text": "So now when my swap\nfunction operates, it",
        "start": 1246.77,
        "duration": 2.98
    },
    {
        "text": "declares a temporary\nvariable recall, called temp.",
        "start": 1249.75,
        "duration": 2.69
    },
    {
        "text": "So let me draw that here.",
        "start": 1252.44,
        "duration": 2.62
    },
    {
        "text": "And as I recall, it stores\nin temp, which value?",
        "start": 1255.06,
        "duration": 3.97
    },
    {
        "text": "The value of a.",
        "start": 1259.03,
        "duration": 1.52
    },
    {
        "text": "The value of a is 1.",
        "start": 1260.55,
        "duration": 2.11
    },
    {
        "text": "It then took the value of b, put it\nin a-- which puts that value here.",
        "start": 1262.66,
        "duration": 7.47
    },
    {
        "text": "And now at this point in the\nstory, a and b are incorrect.",
        "start": 1270.13,
        "duration": 3.15
    },
    {
        "text": "We still need to put the value 1 in b.",
        "start": 1273.28,
        "duration": 2.45
    },
    {
        "text": "And that's why we then took\ntemps value, put it in b.",
        "start": 1275.73,
        "duration": 3.93
    },
    {
        "text": "Thereby giving the me ultimately\nthe number 1 in b's slot as well.",
        "start": 1279.66,
        "duration": 7.08
    },
    {
        "text": "And so, at this point in the\nstory, temp still exists.",
        "start": 1286.74,
        "duration": 3.1
    },
    {
        "text": "And a and b have the correct\nanswers, 2 and 1 respectively.",
        "start": 1289.84,
        "duration": 3.79
    },
    {
        "text": "But the catch is the moment\nthat swap returns, this happens.",
        "start": 1293.63,
        "duration": 6.87
    },
    {
        "text": "Essentially, everything that was\nbeing used above main disappears.",
        "start": 1300.5,
        "duration": 4.36
    },
    {
        "text": "It's not actually deleted.",
        "start": 1304.86,
        "duration": 1.099
    },
    {
        "text": "All of those bits are still\nthere, so technically the numbers",
        "start": 1305.959,
        "duration": 2.541
    },
    {
        "text": "are still there.",
        "start": 1308.5,
        "duration": 0.67
    },
    {
        "text": "But the computer just\nforgets about those values.",
        "start": 1309.17,
        "duration": 2.2
    },
    {
        "text": "And indeed, the key\ntakeaway here is that when",
        "start": 1311.37,
        "duration": 3.34
    },
    {
        "text": "execution returns to main-- that\nis when swap says, return I'm done.",
        "start": 1314.71,
        "duration": 3.96
    },
    {
        "text": "And main takes over operation again,\n1 and 2 are completely unchanged.",
        "start": 1318.67,
        "duration": 5.017
    },
    {
        "text": "So all of the word that\nwe just did up there",
        "start": 1323.687,
        "duration": 1.833
    },
    {
        "text": "was a complete waste\nof time in some sense.",
        "start": 1325.52,
        "duration": 2.0
    },
    {
        "text": "Because even though it was correct\nin so far as it swapped a and b,",
        "start": 1327.52,
        "duration": 3.23
    },
    {
        "text": "it did not actually have a\npermanent impact on the variables",
        "start": 1330.75,
        "duration": 3.78
    },
    {
        "text": "that I actually cared about.",
        "start": 1334.53,
        "duration": 1.67
    },
    {
        "text": "Let's consider another example now.",
        "start": 1336.2,
        "duration": 2.06
    },
    {
        "text": "Recall my previous example where when I\nwhipped up some code to compare values,",
        "start": 1338.26,
        "duration": 4.79
    },
    {
        "text": "I called get string once and\nstored its return value in s.",
        "start": 1343.05,
        "duration": 3.09
    },
    {
        "text": "I called get string twice and\nstored its return value in t.",
        "start": 1346.14,
        "duration": 3.66
    },
    {
        "text": "And then, I just tried\nto compare s and t.",
        "start": 1349.8,
        "duration": 2.92
    },
    {
        "text": "Well, what does that actually mean?",
        "start": 1352.72,
        "duration": 2.11
    },
    {
        "text": "It turns out that when\nyou call get string,",
        "start": 1354.83,
        "duration": 3.11
    },
    {
        "text": "it's not technically\nreturning a string to per say.",
        "start": 1357.94,
        "duration": 3.3
    },
    {
        "text": "It's not technically returning\nto you a sequence of characters.",
        "start": 1361.24,
        "duration": 3.81
    },
    {
        "text": "It's actually returning\nsomething much simpler.",
        "start": 1365.05,
        "duration": 3.36
    },
    {
        "text": "When I call get string and do this--\nstring s gets get underscore string,",
        "start": 1368.41,
        "duration": 11.76
    },
    {
        "text": "all that's happening is this.",
        "start": 1380.17,
        "duration": 2.69
    },
    {
        "text": "The left hand side of this expression\nis telling the computer, hey, computer.",
        "start": 1382.86,
        "duration": 3.42
    },
    {
        "text": "I need a variable called s that's big\nenough to fit a string so to speak.",
        "start": 1386.28,
        "duration": 5.51
    },
    {
        "text": "On the right hand side, get string\nindeed gets a string from the user,",
        "start": 1391.79,
        "duration": 3.09
    },
    {
        "text": "like D-A-V-I-D or Z-A-M-Y-L-A.",
        "start": 1394.88,
        "duration": 3.5
    },
    {
        "text": "But where does that string come from?",
        "start": 1398.38,
        "duration": 1.8
    },
    {
        "text": "It turns out it comes from\nsomewhere else in memory.",
        "start": 1400.18,
        "duration": 2.79
    },
    {
        "text": "And suppose that the user has indeed\ntyped in Z-A-M-Y-L-A quote unquote",
        "start": 1402.97,
        "duration": 6.12
    },
    {
        "text": "so to speak.",
        "start": 1409.09,
        "duration": 0.67
    },
    {
        "text": "That's just kind of floating\nsomewhere in my computer's memory.",
        "start": 1409.76,
        "duration": 2.88
    },
    {
        "text": "And if this string happens to\nexist at address number 123,",
        "start": 1412.64,
        "duration": 5.66
    },
    {
        "text": "byte number 123 in my computer's memory.",
        "start": 1418.3,
        "duration": 3.3
    },
    {
        "text": "What's actually going to get\nreturned is the value 123.",
        "start": 1421.6,
        "duration": 6.38
    },
    {
        "text": "In other words, a string is\ntechnically just the address",
        "start": 1427.98,
        "duration": 4.46
    },
    {
        "text": "of a character in memory.",
        "start": 1432.44,
        "duration": 1.92
    },
    {
        "text": "In fact, if we zoom\nin further on Zamyla,",
        "start": 1434.36,
        "duration": 2.32
    },
    {
        "text": "recall that Zamyla's name really\nlooks like this underneath the hood.",
        "start": 1436.68,
        "duration": 4.88
    },
    {
        "text": "It's an array, a block of\ncontiguous memory with a backslash 0",
        "start": 1441.56,
        "duration": 4.68
    },
    {
        "text": "at the very end.",
        "start": 1446.24,
        "duration": 1.11
    },
    {
        "text": "And if this first byte just\nhappens to be the number 123,",
        "start": 1447.35,
        "duration": 4.05
    },
    {
        "text": "the second byte is going to be 124.",
        "start": 1451.4,
        "duration": 2.28
    },
    {
        "text": "The third byte it's going to be 125.",
        "start": 1453.68,
        "duration": 2.39
    },
    {
        "text": "And so forth.",
        "start": 1456.07,
        "duration": 0.647
    },
    {
        "text": "In other words, if my\ncomputer has a billion bytes",
        "start": 1456.717,
        "duration": 2.083
    },
    {
        "text": "of memory, 2 billion bytes of memory--\nlike a gigabyte or two gigabytes,",
        "start": 1458.8,
        "duration": 3.949
    },
    {
        "text": "you can certainly number each\nof those bytes and the computer",
        "start": 1462.749,
        "duration": 2.541
    },
    {
        "text": "does underneath the hood.",
        "start": 1465.29,
        "duration": 1.38
    },
    {
        "text": "And so Z-A-M-Y-L-A backslash 0 is simply\na sequence of 7 characters that live",
        "start": 1466.67,
        "duration": 8.67
    },
    {
        "text": "somewhere physically in memory.",
        "start": 1475.34,
        "duration": 2.08
    },
    {
        "text": "And what a string technically is,\nis just kind of a breadcrumb--",
        "start": 1477.42,
        "duration": 5.97
    },
    {
        "text": "specifically the address of, the number\nof the first byte of Zamyla's name.",
        "start": 1483.39,
        "duration": 5.57
    },
    {
        "text": "So when you return a string, you're not\nhanding back Z-A-M-Y-L-A backslash 0,",
        "start": 1488.96,
        "duration": 5.349
    },
    {
        "text": "you're handing back\nsomething much smaller.",
        "start": 1494.309,
        "duration": 1.791
    },
    {
        "text": "A little scrap of paper, if you\nwill, that's a map to Zamyla's name",
        "start": 1496.1,
        "duration": 4.31
    },
    {
        "text": "somewhere in your computer's memory.",
        "start": 1500.41,
        "duration": 3.01
    },
    {
        "text": "And so over here, this number 123 is\ngenerally called, not just a number,",
        "start": 1503.42,
        "duration": 5.09
    },
    {
        "text": "but an address.",
        "start": 1508.51,
        "duration": 1.4
    },
    {
        "text": "Much like post boxes and houses have\naddresses physically that uniquely",
        "start": 1509.91,
        "duration": 4.92
    },
    {
        "text": "identify them, 123 uniquely\nidentifies that byte in memory.",
        "start": 1514.83,
        "duration": 3.66
    },
    {
        "text": "And so via this address,\nthis map if you will,",
        "start": 1518.49,
        "duration": 2.905
    },
    {
        "text": "can my program later on find Zamyla's\nname and do something with it.",
        "start": 1521.395,
        "duration": 5.095
    },
    {
        "text": "Like printed out, or capitalize it,\nor compare it, or anything else.",
        "start": 1526.49,
        "duration": 4.23
    },
    {
        "text": "And so, how though\ndoes the computer know",
        "start": 1530.72,
        "duration": 3.14
    },
    {
        "text": "where Zamyla's name begins and ends?",
        "start": 1533.86,
        "duration": 2.43
    },
    {
        "text": "Well, it begins at 123.",
        "start": 1536.29,
        "duration": 2.79
    },
    {
        "text": "And again, recall from two weeks\nago, it ends with backslash 0.",
        "start": 1539.08,
        "duration": 5.07
    },
    {
        "text": "So with those two markers,\nwhere does it begin?",
        "start": 1544.15,
        "duration": 2.51
    },
    {
        "text": "Where does it end?",
        "start": 1546.66,
        "duration": 0.82
    },
    {
        "text": "Can you infer the\nentirety of someone's name",
        "start": 1547.48,
        "duration": 2.67
    },
    {
        "text": "or the entirety of a string in between?",
        "start": 1550.15,
        "duration": 1.83
    },
    {
        "text": "Now, consider my second line of code.",
        "start": 1551.98,
        "duration": 2.11
    },
    {
        "text": "String t gets get string.",
        "start": 1554.09,
        "duration": 3.98
    },
    {
        "text": "So what does this mean?",
        "start": 1558.07,
        "duration": 1.8
    },
    {
        "text": "This is another function call.",
        "start": 1559.87,
        "duration": 1.85
    },
    {
        "text": "And suppose that I just happen to type\nin Z-A-M-Y-L-A enter, just like before.",
        "start": 1561.72,
        "duration": 5.54
    },
    {
        "text": "What's happening there is that the\ncomputer, blind to the fact that,",
        "start": 1567.26,
        "duration": 4.13
    },
    {
        "text": "that might already exist\nsomewhere in memory,",
        "start": 1571.39,
        "duration": 2.26
    },
    {
        "text": "is going to give me another chunk of\nmemory somewhere with Z-A-M-Y-L-A.",
        "start": 1573.65,
        "duration": 6.65
    },
    {
        "text": "And then, backslash 0.",
        "start": 1580.3,
        "duration": 2.02
    },
    {
        "text": "Whereby, this is effectively\nan array of its own.",
        "start": 1582.32,
        "duration": 2.972
    },
    {
        "text": "And I don't know where it is.",
        "start": 1585.292,
        "duration": 1.208
    },
    {
        "text": "Maybe it's at address number\n234, followed by 235 dot dot dot.",
        "start": 1586.5,
        "duration": 6.28
    },
    {
        "text": "It's somewhere else in memory.",
        "start": 1592.78,
        "duration": 1.25
    },
    {
        "text": "I don't know or need to\neven care where it is.",
        "start": 1594.03,
        "duration": 2.26
    },
    {
        "text": "But the key detail here is that t in\nso far is it is a variable itself.",
        "start": 1596.29,
        "duration": 7.51
    },
    {
        "text": "It gets what value?",
        "start": 1603.8,
        "duration": 2.64
    },
    {
        "text": "Well, if the string's Zamyla,\nthe second time I type it in it",
        "start": 1606.44,
        "duration": 2.67
    },
    {
        "text": "just happens to end up at location 234.",
        "start": 1609.11,
        "duration": 2.88
    },
    {
        "text": "What ends up in t is\nsimply the number 234.",
        "start": 1611.99,
        "duration": 5.44
    },
    {
        "text": "So if we change back to my code where\nI was comparing s and t, as I am here.",
        "start": 1617.43,
        "duration": 6.901
    },
    {
        "text": "I've called get string\nonce, stored the value in s.",
        "start": 1624.331,
        "duration": 2.124
    },
    {
        "text": "Called get string twice\nstored the value in t.",
        "start": 1626.455,
        "duration": 1.916
    },
    {
        "text": "And then, I'm quite simply saying,\nif s equals equals t, then say same.",
        "start": 1628.371,
        "duration": 4.524
    },
    {
        "text": "Else, say different.",
        "start": 1632.895,
        "duration": 2.315
    },
    {
        "text": "Now, when you consider what is really\ngoing on underneath the hood all",
        "start": 1635.21,
        "duration": 4.91
    },
    {
        "text": "these weeks, certainly 123\nis not the same as 234.",
        "start": 1640.12,
        "duration": 5.39
    },
    {
        "text": "And so, s does not equal t.",
        "start": 1645.51,
        "duration": 4.9
    },
    {
        "text": "Capitalization meanwhile.",
        "start": 1650.41,
        "duration": 1.419
    },
    {
        "text": "What happens with that other program?",
        "start": 1651.829,
        "duration": 1.541
    },
    {
        "text": "Well, we'll do this one more quickly.",
        "start": 1653.37,
        "duration": 2.07
    },
    {
        "text": "But if I for instance,\ncall string s gets get",
        "start": 1655.44,
        "duration": 6.88
    },
    {
        "text": "underscore string open\nparen close paren.",
        "start": 1662.32,
        "duration": 4.07
    },
    {
        "text": "That again, is going to\ngive me something like this.",
        "start": 1666.39,
        "duration": 2.26
    },
    {
        "text": "And it's going to give me something\nlike Z-A-M-Y-L-A backslash 0,",
        "start": 1668.65,
        "duration": 7.31
    },
    {
        "text": "all of which I can think of, as before,\nlike an array-- if sloppily drawn.",
        "start": 1675.96,
        "duration": 5.65
    },
    {
        "text": "Maybe starting again at address 123.",
        "start": 1681.61,
        "duration": 2.36
    },
    {
        "text": "And so that's what ends up in s.",
        "start": 1683.97,
        "duration": 2.085
    },
    {
        "text": "But in my copy 0 program,\nrecall that I did this.",
        "start": 1686.055,
        "duration": 3.315
    },
    {
        "text": "String t gets s.",
        "start": 1689.37,
        "duration": 4.61
    },
    {
        "text": "I didn't call get string again.",
        "start": 1693.98,
        "duration": 1.65
    },
    {
        "text": "I just said, store s inside of t.",
        "start": 1695.63,
        "duration": 3.08
    },
    {
        "text": "Effectively, I thought,\nmaking a copy of it.",
        "start": 1698.71,
        "duration": 2.99
    },
    {
        "text": "What this left-hand side gives\nme is another box called t.",
        "start": 1701.7,
        "duration": 3.11
    },
    {
        "text": "If this is s, this is t.",
        "start": 1704.81,
        "duration": 2.74
    },
    {
        "text": "But what goes inside of this new box t?",
        "start": 1707.55,
        "duration": 3.32
    },
    {
        "text": "Well, what goes inside\nis literally a copy of s.",
        "start": 1710.87,
        "duration": 2.34
    },
    {
        "text": "What is s? s is 123.",
        "start": 1713.21,
        "duration": 1.94
    },
    {
        "text": "OK. t then, is 123, which\nmeans later in my copy program,",
        "start": 1715.15,
        "duration": 6.55
    },
    {
        "text": "when I simply decided to\ncapitalize t bracket 0,",
        "start": 1721.7,
        "duration": 6.08
    },
    {
        "text": "the first character in\nmy string t-- that's",
        "start": 1727.78,
        "duration": 3.65
    },
    {
        "text": "kind of misleading, because my\nstring t is really just my string s.",
        "start": 1731.43,
        "duration": 4.65
    },
    {
        "text": "They're sort of synonyms for\none another at this point,",
        "start": 1736.08,
        "duration": 2.36
    },
    {
        "text": "because one indeed equals the other.",
        "start": 1738.44,
        "duration": 2.02
    },
    {
        "text": "And so what has happened is that I\nhave gone to the first character in t,",
        "start": 1740.46,
        "duration": 5.9
    },
    {
        "text": "which of course is z.",
        "start": 1746.36,
        "duration": 1.37
    },
    {
        "text": "And recall from my example earlier when\nI typed it in all lowercase, David,",
        "start": 1747.73,
        "duration": 3.52
    },
    {
        "text": "for instance, with a lowercase d, it\ncapitalized it not only in t, but also",
        "start": 1751.25,
        "duration": 5.31
    },
    {
        "text": "in s.",
        "start": 1756.56,
        "duration": 0.786
    },
    {
        "text": "In other words-- and frankly, it\ndoesn't really matter, typically,",
        "start": 1757.346,
        "duration": 4.184
    },
    {
        "text": "what these addresses are.",
        "start": 1761.53,
        "duration": 1.24
    },
    {
        "text": "I'm just using 123 and 234 because\nthey're sort of easy to say.",
        "start": 1762.77,
        "duration": 3.98
    },
    {
        "text": "But you can really think of\ns as again, being a map that",
        "start": 1766.75,
        "duration": 3.29
    },
    {
        "text": "leads you to the string you care about.",
        "start": 1770.04,
        "duration": 2.62
    },
    {
        "text": "A pointer, if you will.",
        "start": 1772.66,
        "duration": 1.15
    },
    {
        "text": "Literally an arrow.",
        "start": 1773.81,
        "duration": 0.96
    },
    {
        "text": "And t, similarly, can be\nthought of as a pointer.",
        "start": 1774.77,
        "duration": 3.17
    },
    {
        "text": "And the key detail here is that\nbecause I've set s equal to t-- or t",
        "start": 1777.94,
        "duration": 6.5
    },
    {
        "text": "equal to s-- they are effectively\npointing at the same thing.",
        "start": 1784.44,
        "duration": 5.88
    },
    {
        "text": "So strings are a lie.",
        "start": 1790.32,
        "duration": 1.33
    },
    {
        "text": "There is no such thing\nas a string data type.",
        "start": 1791.65,
        "duration": 2.09
    },
    {
        "text": "There are things called\nchars, characters",
        "start": 1793.74,
        "duration": 2.4
    },
    {
        "text": "that can live somewhere in memory.",
        "start": 1796.14,
        "duration": 1.45
    },
    {
        "text": "And we humans can\narbitrarily decide that hey,",
        "start": 1797.59,
        "duration": 2.66
    },
    {
        "text": "if we put a backslash 0 character at the\nend of a sequence of other characters,",
        "start": 1800.25,
        "duration": 4.74
    },
    {
        "text": "we can all just agree universally\nto treat that as the end of a quote",
        "start": 1804.99,
        "duration": 4.03
    },
    {
        "text": "unquote \"string,\" that\nis a word or a phrase",
        "start": 1809.02,
        "duration": 2.27
    },
    {
        "text": "or paragraph or anything even bigger.",
        "start": 1811.29,
        "duration": 2.3
    },
    {
        "text": "But we need a convention for\nremembering where strings begin.",
        "start": 1813.59,
        "duration": 3.696
    },
    {
        "text": "We've already solved the\nwhere do they end problem.",
        "start": 1817.286,
        "duration": 2.124
    },
    {
        "text": "So where does the string begin?",
        "start": 1819.41,
        "duration": 1.32
    },
    {
        "text": "It begins at an address.",
        "start": 1820.73,
        "duration": 1.83
    },
    {
        "text": "It begins with a pointer.",
        "start": 1822.56,
        "duration": 2.21
    },
    {
        "text": "And so this special data type\nthat we declare in CS50's library",
        "start": 1824.77,
        "duration": 2.94
    },
    {
        "text": "called string really is just\nin CS50 IDE an 8-byte value,",
        "start": 1827.71,
        "duration": 5.23
    },
    {
        "text": "a 64-bit value that is just a\nreally big number that represents",
        "start": 1832.94,
        "duration": 5.51
    },
    {
        "text": "the address in memory of a string.",
        "start": 1838.45,
        "duration": 1.74
    },
    {
        "text": "And I say really big just\nbecause the IDE gives you access",
        "start": 1840.19,
        "duration": 2.69
    },
    {
        "text": "to lots of memory, certainly\nnumbers bigger than 123.",
        "start": 1842.88,
        "duration": 3.14
    },
    {
        "text": "But a string is just a number, is\njust an address, AKA a pointer.",
        "start": 1846.02,
        "duration": 6.43
    },
    {
        "text": "And that explains, then, why\nall three of these examples",
        "start": 1852.45,
        "duration": 3.07
    },
    {
        "text": "did not behave as I might\nhave hoped, because rather",
        "start": 1855.52,
        "duration": 3.38
    },
    {
        "text": "they were taking things\na little too literally.",
        "start": 1858.9,
        "duration": 3.89
    },
    {
        "text": "Or I was failing to appreciate\nwhat's actually going on.",
        "start": 1862.79,
        "duration": 3.62
    },
    {
        "text": "Let's pause for a moment,\ntake things down a notch.",
        "start": 1866.41,
        "duration": 2.49
    },
    {
        "text": "Make things a little more\nreal with a bit of claymation",
        "start": 1868.9,
        "duration": 2.49
    },
    {
        "text": "that will motivate, eventually,\npeeling back this layer further",
        "start": 1871.39,
        "duration": 3.07
    },
    {
        "text": "and seeing what's really going on.",
        "start": 1874.46,
        "duration": 3.21
    },
    {
        "text": "[STRUMMED CHORD]",
        "start": 1877.67,
        "duration": 1.4
    },
    {
        "text": "NICK PARLANTE: Hey, Binky.",
        "start": 1879.437,
        "duration": 1.083
    },
    {
        "text": "Wake up.",
        "start": 1880.52,
        "duration": 0.74
    },
    {
        "text": "It's time for pointer fun.",
        "start": 1881.26,
        "duration": 2.51
    },
    {
        "text": "BINKY: What's that?",
        "start": 1883.77,
        "duration": 1.21
    },
    {
        "text": "Learn about pointers?",
        "start": 1884.98,
        "duration": 1.748
    },
    {
        "text": "Oh, goodie!",
        "start": 1886.728,
        "duration": 2.789
    },
    {
        "text": "DAVID J. MALAN: Binky, who exist\nhere only in claymation form,",
        "start": 1889.517,
        "duration": 2.583
    },
    {
        "text": "is the product of a good friend of\nours, Nick Parlante at Stanford,",
        "start": 1892.1,
        "duration": 3.372
    },
    {
        "text": "who teaches computer science there.",
        "start": 1895.472,
        "duration": 1.458
    },
    {
        "text": "You'll see more of Binky and hear\nmore of Nick in just a moment.",
        "start": 1896.93,
        "duration": 3.62
    },
    {
        "text": "But these here are\nsort of metaphorically",
        "start": 1900.55,
        "duration": 2.39
    },
    {
        "text": "the training wheels that we've\nhad on for the past few weeks.",
        "start": 1902.94,
        "duration": 2.56
    },
    {
        "text": "And the goal now at hand\nis to take these off,",
        "start": 1905.5,
        "duration": 2.37
    },
    {
        "text": "and to finally start looking at what's\nreally going on underneath the hood.",
        "start": 1907.87,
        "duration": 4.8
    },
    {
        "text": "And starting to remove,\nif you will, let's",
        "start": 1912.67,
        "duration": 2.426
    },
    {
        "text": "see if-- [BANG] --probably\nnot the best idea.",
        "start": 1915.096,
        "duration": 4.564
    },
    {
        "text": "Remove, if you will,\nthese training wheels,",
        "start": 1919.66,
        "duration": 2.02
    },
    {
        "text": "and actually see what's\ngoing on, and understand",
        "start": 1921.68,
        "duration": 2.87
    },
    {
        "text": "and take advantage of the same.",
        "start": 1924.55,
        "duration": 1.57
    },
    {
        "text": "As follows.",
        "start": 1926.12,
        "duration": 0.78
    },
    {
        "text": "Let's go ahead now into CS50\nIDE, and go ahead and open up,",
        "start": 1926.9,
        "duration": 4.18
    },
    {
        "text": "let's say, compare1.c, which I\nwrote in advance to look as follows.",
        "start": 1931.08,
        "duration": 6.41
    },
    {
        "text": "And you'll notice that it works a\nlittle differently from version zero.",
        "start": 1937.49,
        "duration": 3.04
    },
    {
        "text": "Here we have a prompt for string s.",
        "start": 1940.53,
        "duration": 2.26
    },
    {
        "text": "And we store in it the\nreturn value of get string.",
        "start": 1942.79,
        "duration": 3.8
    },
    {
        "text": "But notice what's on the left-hand side.",
        "start": 1946.59,
        "duration": 3.64
    },
    {
        "text": "Char star s, all of a sudden.",
        "start": 1950.23,
        "duration": 2.58
    },
    {
        "text": "Indeed, all of this time,\nI've been treating things",
        "start": 1952.81,
        "duration": 2.18
    },
    {
        "text": "as though they are strings, literally.",
        "start": 1954.99,
        "duration": 3.54
    },
    {
        "text": "But it turns out a\nstring is just a synonym",
        "start": 1958.53,
        "duration": 5.11
    },
    {
        "text": "for a data type known as a char star.",
        "start": 1963.64,
        "duration": 2.54
    },
    {
        "text": "And the new syntax today,\nthen, is this star operator.",
        "start": 1966.18,
        "duration": 2.51
    },
    {
        "text": "The asterisk that actually has special\nmeaning in certain contexts, not just",
        "start": 1968.69,
        "duration": 3.78
    },
    {
        "text": "multiplication.",
        "start": 1972.47,
        "duration": 0.94
    },
    {
        "text": "But in this case, it specifies\nthat the type of a variable",
        "start": 1973.41,
        "duration": 3.74
    },
    {
        "text": "is not a char literally,\nbut a char star, the address",
        "start": 1977.15,
        "duration": 3.79
    },
    {
        "text": "of a char, a pointer to a char.",
        "start": 1980.94,
        "duration": 3.3
    },
    {
        "text": "Now, why char?",
        "start": 1984.24,
        "duration": 0.7
    },
    {
        "text": "I thought we were talking about strings.",
        "start": 1984.94,
        "duration": 1.666
    },
    {
        "text": "But again, recall that a string\nis just a sequence of characters",
        "start": 1986.606,
        "duration": 3.174
    },
    {
        "text": "back to back to back, and\ntherefore, you can define a string",
        "start": 1989.78,
        "duration": 4.51
    },
    {
        "text": "by the address of its first character.",
        "start": 1994.29,
        "duration": 2.8
    },
    {
        "text": "Ergo, what we really\nneed underneath the hood",
        "start": 1997.09,
        "duration": 2.31
    },
    {
        "text": "is a data type that lets us\nstore the address of a character.",
        "start": 1999.4,
        "duration": 5.29
    },
    {
        "text": "There is no string.",
        "start": 2004.69,
        "duration": 1.68
    },
    {
        "text": "And so what does this allow us to do?",
        "start": 2006.37,
        "duration": 1.92
    },
    {
        "text": "Let me go into CS50 IDE, and let\nme declare then, on this line",
        "start": 2008.29,
        "duration": 4.01
    },
    {
        "text": "here, that s, this time, will\nnot be a \"string\" quote unquote.",
        "start": 2012.3,
        "duration": 4.98
    },
    {
        "text": "That was from the CS50 library.",
        "start": 2017.28,
        "duration": 1.66
    },
    {
        "text": "But rather it's going to be a char star.",
        "start": 2018.94,
        "duration": 3.022
    },
    {
        "text": "It turns out that all this time\nget string, again, does not",
        "start": 2021.962,
        "duration": 2.458
    },
    {
        "text": "return a string, it returns\nthe address of a string, AKA",
        "start": 2024.42,
        "duration": 4.33
    },
    {
        "text": "the address of the first\ncharacter in a string.",
        "start": 2028.75,
        "duration": 3.65
    },
    {
        "text": "And so the type of value it's\nreturning is not just a number.",
        "start": 2032.4,
        "duration": 3.83
    },
    {
        "text": "It's not just an int.",
        "start": 2036.23,
        "duration": 0.96
    },
    {
        "text": "It's a special type of an int.",
        "start": 2037.19,
        "duration": 2.09
    },
    {
        "text": "It's used for a different purpose.",
        "start": 2039.28,
        "duration": 1.51
    },
    {
        "text": "It's simply an integer that\nrepresents the address of a char.",
        "start": 2040.79,
        "duration": 3.87
    },
    {
        "text": "And the way you type out address\nof a char is literally char star.",
        "start": 2044.66,
        "duration": 4.74
    },
    {
        "text": "So this, then, is identical to my\npreviously in weeks past having typed",
        "start": 2049.4,
        "duration": 5.27
    },
    {
        "text": "string s.",
        "start": 2054.67,
        "duration": 1.08
    },
    {
        "text": "Now I'm going to start\ntyping it as char star.",
        "start": 2055.75,
        "duration": 2.76
    },
    {
        "text": "Meanwhile, t is going to be the same.",
        "start": 2058.51,
        "duration": 1.709
    },
    {
        "text": "So when I prompt the user\nfor another string there,",
        "start": 2060.219,
        "duration": 2.125
    },
    {
        "text": "I'm going to store that\nreturn value inside of t.",
        "start": 2062.344,
        "duration": 3.006
    },
    {
        "text": "And now, notice, just for\ngood measure, I'm making sure",
        "start": 2065.35,
        "duration": 2.77
    },
    {
        "text": "that both s and t are not null.",
        "start": 2068.12,
        "duration": 2.71
    },
    {
        "text": "I'm using a bit of conditional\nlogic there, saying if s is not null",
        "start": 2070.83,
        "duration": 3.04
    },
    {
        "text": "and t is not null, it's safe to proceed.",
        "start": 2073.87,
        "duration": 2.099
    },
    {
        "text": "Because recall that get string can\naccidentally return null sometimes",
        "start": 2075.969,
        "duration": 3.595
    },
    {
        "text": "if your computer is out of memory,\nor something else goes wrong.",
        "start": 2079.564,
        "duration": 2.666
    },
    {
        "text": "Or not so much\naccidentally, but by design.",
        "start": 2082.23,
        "duration": 3.17
    },
    {
        "text": "But notice this new chunk of code.",
        "start": 2085.4,
        "duration": 2.85
    },
    {
        "text": "It turns out-- and we\nknow now from a moment",
        "start": 2088.25,
        "duration": 2.8
    },
    {
        "text": "ago-- you can't just\ncompare s against t.",
        "start": 2091.05,
        "duration": 1.822
    },
    {
        "text": "They're not going to\nequal the same thing",
        "start": 2092.872,
        "duration": 1.708
    },
    {
        "text": "if you type in two independent strings.",
        "start": 2094.58,
        "duration": 2.34
    },
    {
        "text": "We need a special function that actually\ncompares strings in a conceptual way.",
        "start": 2096.92,
        "duration": 5.33
    },
    {
        "text": "I mean that a string is equal to another\nstring if every one of its characters",
        "start": 2102.25,
        "duration": 5.83
    },
    {
        "text": "equals every one of the\nother string's characters.",
        "start": 2108.08,
        "duration": 2.83
    },
    {
        "text": "Thankfully, there exists in C a function\nthat does exactly that called strcomp,",
        "start": 2110.91,
        "duration": 4.67
    },
    {
        "text": "string compare, and it\ntakes two arguments.",
        "start": 2115.58,
        "duration": 3.09
    },
    {
        "text": "The first is a string.",
        "start": 2118.67,
        "duration": 1.14
    },
    {
        "text": "The second is a string.",
        "start": 2119.81,
        "duration": 1.01
    },
    {
        "text": "Or more properly, the first\nis the address of a string.",
        "start": 2120.82,
        "duration": 2.58
    },
    {
        "text": "Or even more properly, the first\nis the address of a character.",
        "start": 2123.4,
        "duration": 4.3
    },
    {
        "text": "The second is the\naddress of a character.",
        "start": 2127.7,
        "duration": 2.25
    },
    {
        "text": "And str compare is just going to\nhope that both of those strings",
        "start": 2129.95,
        "duration": 4.92
    },
    {
        "text": "eventually end in a\nbackslash zero, so that they",
        "start": 2134.87,
        "duration": 4.01
    },
    {
        "text": "don't loop forever through memory.",
        "start": 2138.88,
        "duration": 1.6
    },
    {
        "text": "They eventually hit that\nspecial null terminating byte.",
        "start": 2140.48,
        "duration": 2.85
    },
    {
        "text": "And if so, and those characters are\nall entirely equal, you print same.",
        "start": 2143.33,
        "duration": 4.23
    },
    {
        "text": "Else, as before, we print out different.",
        "start": 2147.56,
        "duration": 4.24
    },
    {
        "text": "So now, when I compile this\nprogram, make compare 1,",
        "start": 2151.8,
        "duration": 3.64
    },
    {
        "text": "and then I do ./compare1.",
        "start": 2155.44,
        "duration": 4.73
    },
    {
        "text": "Now I'll type in david in all\nlowercase, david in all lowercase.",
        "start": 2160.17,
        "duration": 3.951
    },
    {
        "text": "They're indeed the same.",
        "start": 2164.121,
        "duration": 0.999
    },
    {
        "text": "Let's do it again.",
        "start": 2165.12,
        "duration": 0.94
    },
    {
        "text": "Zamyla with a capital Z. Zamyla with a\ncapital Z. There are indeed the same.",
        "start": 2166.06,
        "duration": 4.93
    },
    {
        "text": "Let's do Zamyla with a capital\nZ. zamyla with a lowercase z.",
        "start": 2170.99,
        "duration": 3.97
    },
    {
        "text": "Different.",
        "start": 2174.96,
        "duration": 1.09
    },
    {
        "text": "And then FUBAR, clearly different.",
        "start": 2176.05,
        "duration": 2.95
    },
    {
        "text": "Now we're actually comparing\nthese things properly.",
        "start": 2179.0,
        "duration": 2.7
    },
    {
        "text": "Because now we're appreciating what\nit actually means to be a string,",
        "start": 2181.7,
        "duration": 3.76
    },
    {
        "text": "and we are underneath the hood\ncomparing what we should be doing.",
        "start": 2185.46,
        "duration": 3.69
    },
    {
        "text": "Now, underneath the hood,\nwhat is str compare doing?",
        "start": 2189.15,
        "duration": 2.21
    },
    {
        "text": "Honestly, it's probably just\na while loop or a for loop",
        "start": 2191.36,
        "duration": 2.71
    },
    {
        "text": "that is iterating over the\nstring and their lengths",
        "start": 2194.07,
        "duration": 2.67
    },
    {
        "text": "and looking at the i-th\ncharacter in each string,",
        "start": 2196.74,
        "duration": 3.05
    },
    {
        "text": "and making sure they're\nall in fact equal.",
        "start": 2199.79,
        "duration": 2.49
    },
    {
        "text": "Let's go ahead and fix\ncopy with version 1 here.",
        "start": 2202.28,
        "duration": 2.58
    },
    {
        "text": "Copy 1.c that I've written in\nadvance now looks like this.",
        "start": 2204.86,
        "duration": 4.06
    },
    {
        "text": "I still prompt the user for a\nstring s with these lines here.",
        "start": 2208.92,
        "duration": 4.73
    },
    {
        "text": "I then, just to be safe, say\nhey, wait a minute, if s equals",
        "start": 2213.65,
        "duration": 3.0
    },
    {
        "text": "equals null, return 1.",
        "start": 2216.65,
        "duration": 1.179
    },
    {
        "text": "And again, 1 is just arbitrary.",
        "start": 2217.829,
        "duration": 1.291
    },
    {
        "text": "I just want to get out, lest\nI break something later.",
        "start": 2219.12,
        "duration": 3.59
    },
    {
        "text": "Now, down here, this\nis a new line of code.",
        "start": 2222.71,
        "duration": 2.227
    },
    {
        "text": "And this is perhaps one of\nthe most powerful ingredients",
        "start": 2224.937,
        "duration": 2.333
    },
    {
        "text": "we'll see this week, is this\nnew function called malloc,",
        "start": 2227.27,
        "duration": 3.55
    },
    {
        "text": "memory allocate.",
        "start": 2230.82,
        "duration": 1.64
    },
    {
        "text": "This is a special function via which\nyou can ask the operating system, Linux",
        "start": 2232.46,
        "duration": 5.05
    },
    {
        "text": "in the case here, or Mac OS, or Windows,\nif you're running the code locally,",
        "start": 2237.51,
        "duration": 3.4
    },
    {
        "text": "hey operating system, please give\nme a bunch of bytes of memory.",
        "start": 2240.91,
        "duration": 5.22
    },
    {
        "text": "Now, why do I want this?",
        "start": 2246.13,
        "duration": 1.17
    },
    {
        "text": "This program is copy 1.c.",
        "start": 2247.3,
        "duration": 1.64
    },
    {
        "text": "The goal at hand is to create a\nprogram that copies a string s",
        "start": 2248.94,
        "duration": 4.24
    },
    {
        "text": "and stores the copy in t, so to speak.",
        "start": 2253.18,
        "duration": 2.47
    },
    {
        "text": "Last time, it was not sufficient\njust to say t equals s,",
        "start": 2255.65,
        "duration": 3.06
    },
    {
        "text": "because that copied the addresses.",
        "start": 2258.71,
        "duration": 1.87
    },
    {
        "text": "That didn't give me a copy of\nZ-A-M-Y-- Z-A-M-Y-L-A backlash 0.",
        "start": 2260.58,
        "duration": 7.31
    },
    {
        "text": "It instead just gave me\na copy of the address.",
        "start": 2267.89,
        "duration": 2.27
    },
    {
        "text": "So how do I get a complete\ncopy of Zamyla's name?",
        "start": 2270.16,
        "duration": 4.18
    },
    {
        "text": "I need to preemptively do\na little bit of arithmetic",
        "start": 2274.34,
        "duration": 2.63
    },
    {
        "text": "and say, all right, how\nlong is Zamyla's name?",
        "start": 2276.97,
        "duration": 2.05
    },
    {
        "text": "Well, it's the length\nof s, str len of s.",
        "start": 2279.02,
        "duration": 4.06
    },
    {
        "text": "But plus one.",
        "start": 2283.08,
        "duration": 1.223
    },
    {
        "text": "Why plus one?",
        "start": 2284.303,
        "duration": 2.878
    },
    {
        "text": "Why plus one?",
        "start": 2287.181,
        "duration": 1.409
    },
    {
        "text": "Yeah?",
        "start": 2288.59,
        "duration": 2.05
    },
    {
        "text": "Exactly.",
        "start": 2290.64,
        "duration": 0.59
    },
    {
        "text": "We now were hit the lowest\nlevel of the computer.",
        "start": 2291.23,
        "duration": 2.91
    },
    {
        "text": "If we don't ask the operating system for\nmemory for that extra backslash 0 byte,",
        "start": 2294.14,
        "duration": 5.047
    },
    {
        "text": "we're not going to get it.",
        "start": 2299.187,
        "duration": 1.083
    },
    {
        "text": "So we have to explicitly\nsay, give me one more byte,",
        "start": 2300.27,
        "duration": 2.266
    },
    {
        "text": "because I know how strings are\nimplemented underneath the hood.",
        "start": 2302.536,
        "duration": 2.624
    },
    {
        "text": "I need to put that backslash\nzero there, ultimately,",
        "start": 2305.16,
        "duration": 2.3
    },
    {
        "text": "and then whatever that expression is,\nthe length of Zamyla, so Z-A-M-Y-L-A,",
        "start": 2307.46,
        "duration": 4.45
    },
    {
        "text": "six, plus one, seven bytes.",
        "start": 2311.91,
        "duration": 2.95
    },
    {
        "text": "Times the size of a character.",
        "start": 2314.86,
        "duration": 1.9
    },
    {
        "text": "Turns out it's always going\nto be one, by definition.",
        "start": 2316.76,
        "duration": 2.26
    },
    {
        "text": "But just for good measure, I'm\nclearly saying, give me seven times",
        "start": 2319.02,
        "duration": 3.49
    },
    {
        "text": "the size of a char,\nwhich is going to be one.",
        "start": 2322.51,
        "duration": 2.37
    },
    {
        "text": "That gives me seven total bytes.",
        "start": 2324.88,
        "duration": 1.96
    },
    {
        "text": "So just to simplify.",
        "start": 2326.84,
        "duration": 1.41
    },
    {
        "text": "If you multiply all this out,\nbecause the line looks unnecessarily",
        "start": 2328.25,
        "duration": 2.81
    },
    {
        "text": "cryptic at the moment, this really\nis equivalent, at the moment,",
        "start": 2331.06,
        "duration": 6.3
    },
    {
        "text": "to just this.",
        "start": 2337.36,
        "duration": 1.18
    },
    {
        "text": "Call the function malloc.",
        "start": 2338.54,
        "duration": 1.45
    },
    {
        "text": "Give it the number seven, so that\nmalloc, and in turn, the operating",
        "start": 2339.99,
        "duration": 3.66
    },
    {
        "text": "system, looks inside of its memory\nbank, so to speak, and says,",
        "start": 2343.65,
        "duration": 2.7
    },
    {
        "text": "hmm, where are there are seven available\nbytes that aren't currently in use?",
        "start": 2346.35,
        "duration": 3.58
    },
    {
        "text": "Ah, here is a chunk of them.",
        "start": 2349.93,
        "duration": 1.73
    },
    {
        "text": "And it's a contiguous chunk.",
        "start": 2351.66,
        "duration": 1.37
    },
    {
        "text": "It's going to find a block of memory,\na rectangular region, if you will,",
        "start": 2353.03,
        "duration": 3.44
    },
    {
        "text": "and grab seven bytes, and\nreturn them to my main function.",
        "start": 2356.47,
        "duration": 6.64
    },
    {
        "text": "But what do I mean to\nreturn a chunk of memory?",
        "start": 2363.11,
        "duration": 2.34
    },
    {
        "text": "Well, just as get\nstring returns a string",
        "start": 2365.45,
        "duration": 3.92
    },
    {
        "text": "by returning the address of the\nfirst character in that string,",
        "start": 2369.37,
        "duration": 3.95
    },
    {
        "text": "so does malloc equivalently\nsimply return the address",
        "start": 2373.32,
        "duration": 3.9
    },
    {
        "text": "of the first byte of memory.",
        "start": 2377.22,
        "duration": 3.7
    },
    {
        "text": "But the danger now is\nthat unlike a string,",
        "start": 2380.92,
        "duration": 3.07
    },
    {
        "text": "malloc is not giving you characters.",
        "start": 2383.99,
        "duration": 1.88
    },
    {
        "text": "It's just giving you seven bytes in\na row that you are now free to use.",
        "start": 2385.87,
        "duration": 3.93
    },
    {
        "text": "It does not give you a backslash\nzero at the end of them.",
        "start": 2389.8,
        "duration": 3.19
    },
    {
        "text": "If you want to remember the\nlength of the chunk of memory",
        "start": 2392.99,
        "duration": 3.43
    },
    {
        "text": "you just allocated, the burden is\nentirely on you, the programmer.",
        "start": 2396.42,
        "duration": 3.682
    },
    {
        "text": "And indeed, one of the most common\nsources of bugs in writing code in C",
        "start": 2400.102,
        "duration": 2.958
    },
    {
        "text": "is to forget about how long\nwas this chunk of memory,",
        "start": 2403.06,
        "duration": 2.55
    },
    {
        "text": "and to accidentally, with a loop,\ngo too far past the end of it.",
        "start": 2405.61,
        "duration": 3.376
    },
    {
        "text": "And we'll see what can\nhappen in those cases.",
        "start": 2408.986,
        "duration": 1.874
    },
    {
        "text": "So now, assuming I do have in t the\naddress of that chunk of memory,",
        "start": 2410.86,
        "duration": 3.92
    },
    {
        "text": "let me just say, if t equals\nequals null, return 1.",
        "start": 2414.78,
        "duration": 3.73
    },
    {
        "text": "Something happened that's bad,\nprobably the operating system just",
        "start": 2418.51,
        "duration": 3.21
    },
    {
        "text": "didn't have seven extra\nbytes of memory to give me.",
        "start": 2421.72,
        "duration": 2.27
    },
    {
        "text": "So fine, I'll quit.",
        "start": 2423.99,
        "duration": 1.51
    },
    {
        "text": "Then down here, what do I want to do?",
        "start": 2425.5,
        "duration": 1.88
    },
    {
        "text": "Well, I now need to implement, at\nleast in this example, my own copying",
        "start": 2427.38,
        "duration": 4.98
    },
    {
        "text": "process.",
        "start": 2432.36,
        "duration": 1.46
    },
    {
        "text": "Here, at this point in the story,\nI have two variables, s and t.",
        "start": 2433.82,
        "duration": 4.12
    },
    {
        "text": "s contains the address\nof Zamyla's name. t",
        "start": 2437.94,
        "duration": 2.975
    },
    {
        "text": "contains the address of a new\nchunk of memory of length seven.",
        "start": 2440.915,
        "duration": 3.495
    },
    {
        "text": "So here's what I want to do.",
        "start": 2444.41,
        "duration": 1.22
    },
    {
        "text": "Just like a couple of\nweeks ago, I'm going",
        "start": 2445.63,
        "duration": 1.92
    },
    {
        "text": "to iterate from zero on up\nto the length of the string.",
        "start": 2447.55,
        "duration": 3.65
    },
    {
        "text": "But not up to, but up through\nthe length of the string.",
        "start": 2451.2,
        "duration": 2.684
    },
    {
        "text": "Because in this case, I actually\nwant to iterate with a for loop",
        "start": 2453.884,
        "duration": 2.666
    },
    {
        "text": "up through that backslash 0 byte.",
        "start": 2456.55,
        "duration": 3.33
    },
    {
        "text": "And then just this syntax\nfrom a couple of weeks ago,",
        "start": 2459.88,
        "duration": 2.41
    },
    {
        "text": "when we simply manipulated strings as\nfor our cryptography ciphers, character",
        "start": 2462.29,
        "duration": 6.06
    },
    {
        "text": "by character.",
        "start": 2468.35,
        "duration": 0.98
    },
    {
        "text": "Make the i-th character of t\nequal to the i-th character of s.",
        "start": 2469.33,
        "duration": 3.95
    },
    {
        "text": "And this is perfectly valid,\nbecause so long as this loop doesn't",
        "start": 2473.28,
        "duration": 3.32
    },
    {
        "text": "go past n, the number of characters\nthat I allocated, seven, in this case,",
        "start": 2476.6,
        "duration": 5.58
    },
    {
        "text": "I can go to t bracket 0, bracket 1,\nbracket 2, all the way up through n,",
        "start": 2482.18,
        "duration": 5.64
    },
    {
        "text": "effectively copying the string.",
        "start": 2487.82,
        "duration": 2.66
    },
    {
        "text": "And so now when I actually print out s\nand t, I should see truly a copy of t.",
        "start": 2490.48,
        "duration": 8.12
    },
    {
        "text": "Because even when I\nforce its first character",
        "start": 2498.6,
        "duration": 2.37
    },
    {
        "text": "to lower case with this same\nline of code here as before,",
        "start": 2500.97,
        "duration": 3.24
    },
    {
        "text": "I'm actually changing different memory.",
        "start": 2504.21,
        "duration": 2.884
    },
    {
        "text": "So let's compile this.",
        "start": 2507.094,
        "duration": 0.916
    },
    {
        "text": "Make copy 1, ./copy1.",
        "start": 2508.01,
        "duration": 3.62
    },
    {
        "text": "And let me go ahead and type\nin zamyla in all lowercase,",
        "start": 2511.63,
        "duration": 3.89
    },
    {
        "text": "and now notice the\nprogram does seem to work.",
        "start": 2515.52,
        "duration": 2.86
    },
    {
        "text": "Zamyla is reprinted in lower\ncase for s, but it's then",
        "start": 2518.38,
        "duration": 3.67
    },
    {
        "text": "print in uppercase for\nits first letter for t.",
        "start": 2522.05,
        "duration": 2.62
    },
    {
        "text": "And because the z's look pretty\nsimilar, let's do my name again,",
        "start": 2524.67,
        "duration": 2.93
    },
    {
        "text": "whereby I type david in all lowercase.",
        "start": 2527.6,
        "duration": 2.47
    },
    {
        "text": "Type Enter.",
        "start": 2530.07,
        "duration": 0.64
    },
    {
        "text": "And now, you see s is\nstill david all lowercase,",
        "start": 2530.71,
        "duration": 3.14
    },
    {
        "text": "but t has only now been\ncapitalized itself.",
        "start": 2533.85,
        "duration": 3.36
    },
    {
        "text": "It hasn't had a side\neffect of some sort on s,",
        "start": 2537.21,
        "duration": 1.98
    },
    {
        "text": "because they're different\nchunks of memory.",
        "start": 2539.19,
        "duration": 2.571
    },
    {
        "text": "Why?",
        "start": 2541.761,
        "duration": 0.499
    },
    {
        "text": "Well, what has just happened\nin this program is this.",
        "start": 2542.26,
        "duration": 3.72
    },
    {
        "text": "We have, again, done\nstring s gets get string.",
        "start": 2545.98,
        "duration": 4.73
    },
    {
        "text": "And when we typed get\nstring, this gives me",
        "start": 2553.34,
        "duration": 5.52
    },
    {
        "text": "a chunk of memory for the address of s.",
        "start": 2558.86,
        "duration": 2.32
    },
    {
        "text": "Get string gives me a name,\nlike D-A-V-I-D in all lowercase,",
        "start": 2561.18,
        "duration": 4.84
    },
    {
        "text": "plus that backslash 0.",
        "start": 2566.02,
        "duration": 1.64
    },
    {
        "text": "Which again, is really just an\narray underneath the hood like this,",
        "start": 2567.66,
        "duration": 3.6
    },
    {
        "text": "that starts at some byte, and maybe\nit's again, by coincidence, 123.",
        "start": 2571.26,
        "duration": 3.446
    },
    {
        "text": "Just because it's easy\nto say, but that's not",
        "start": 2574.706,
        "duration": 1.874
    },
    {
        "text": "where it's necessarily going to end up.",
        "start": 2576.58,
        "duration": 1.624
    },
    {
        "text": "And so what ends up here is 123.",
        "start": 2578.204,
        "duration": 3.176
    },
    {
        "text": "And then later, when\nI allocate t, I again",
        "start": 2581.38,
        "duration": 3.95
    },
    {
        "text": "get this little chunk of\nmemory that's supposed",
        "start": 2585.33,
        "duration": 3.08
    },
    {
        "text": "to store the address of a character.",
        "start": 2588.41,
        "duration": 2.41
    },
    {
        "text": "And actually, recall\nthat we're now doing this",
        "start": 2590.82,
        "duration": 2.22
    },
    {
        "text": "as char star, not even string.",
        "start": 2593.04,
        "duration": 3.31
    },
    {
        "text": "So t is similarly a char star.",
        "start": 2596.35,
        "duration": 2.36
    },
    {
        "text": "And what happens, malloc,\nwhen I ask it for seven bytes,",
        "start": 2598.71,
        "duration": 3.57
    },
    {
        "text": "gives me 1, 2, 3, 4, 5,\n6, 7 bytes of memory.",
        "start": 2602.28,
        "duration": 5.47
    },
    {
        "text": "There's no null terminating\ncharacter just yet.",
        "start": 2607.75,
        "duration": 2.02
    },
    {
        "text": "It's just a block of memory.",
        "start": 2609.77,
        "duration": 1.19
    },
    {
        "text": "And frankly, there could be\nsome random values here, as",
        "start": 2610.96,
        "duration": 3.42
    },
    {
        "text": "denoted with question marks here.",
        "start": 2614.38,
        "duration": 1.79
    },
    {
        "text": "It's just a chunk of memory that\nmight have been used previously",
        "start": 2616.17,
        "duration": 2.68
    },
    {
        "text": "in my program for some other purpose.",
        "start": 2618.85,
        "duration": 1.66
    },
    {
        "text": "But what gets stored here, if\nthis happens to be at address 234,",
        "start": 2620.51,
        "duration": 5.12
    },
    {
        "text": "is this value here, 234.",
        "start": 2625.63,
        "duration": 2.85
    },
    {
        "text": "And if you're not liking\nthe numbers, again, you",
        "start": 2628.48,
        "duration": 2.18
    },
    {
        "text": "can think of these as just\nbeing pointers, arrows,",
        "start": 2630.66,
        "duration": 3.85
    },
    {
        "text": "to these chunks of memory.",
        "start": 2634.51,
        "duration": 1.74
    },
    {
        "text": "But now, in my C code, when I have\na few lines above this loop whereby",
        "start": 2636.25,
        "duration": 9.24
    },
    {
        "text": "I am copying from s\nbracket i into t bracket",
        "start": 2645.49,
        "duration": 3.79
    },
    {
        "text": "i, each of the characters in my\nloop, what's actually happening?",
        "start": 2649.28,
        "duration": 5.2
    },
    {
        "text": "Well, fairly intuitively, this lower\ncase d ends up getting copied here.",
        "start": 2654.48,
        "duration": 4.96
    },
    {
        "text": "This lower case a ends\nup getting copied here.",
        "start": 2659.44,
        "duration": 2.692
    },
    {
        "text": "V-I-D, on through.",
        "start": 2662.132,
        "duration": 3.218
    },
    {
        "text": "And David can't count,\nso-- backslash-- oh, right.",
        "start": 2665.35,
        "duration": 3.467
    },
    {
        "text": "David's name is shorter than Zamyla's\nname, which means we didn't actually",
        "start": 2668.817,
        "duration": 3.083
    },
    {
        "text": "ask for this many characters over here.",
        "start": 2671.9,
        "duration": 2.79
    },
    {
        "text": "But we have taken the\ncomputer more literally now.",
        "start": 2674.69,
        "duration": 2.49
    },
    {
        "text": "Give me six bytes, not\nseven bytes, in this case.",
        "start": 2677.18,
        "duration": 2.53
    },
    {
        "text": "And then literally copy each of the\ncharacters from the original string",
        "start": 2679.71,
        "duration": 3.94
    },
    {
        "text": "into this new string all the way up\nthrough that backslash 0 character.",
        "start": 2683.65,
        "duration": 5.0
    },
    {
        "text": "And then when you capitalize\nthe first character in t,",
        "start": 2688.65,
        "duration": 3.73
    },
    {
        "text": "you are literally only changing\nthis-- we can do better than this.",
        "start": 2692.38,
        "duration": 5.27
    },
    {
        "text": "We are only changing this\nfirst character here,",
        "start": 2697.65,
        "duration": 4.22
    },
    {
        "text": "which looks like that now.",
        "start": 2701.87,
        "duration": 3.46
    },
    {
        "text": "And that's what's going\non underneath the hood.",
        "start": 2705.33,
        "duration": 2.54
    },
    {
        "text": "So this is why, then, in\nthe beginning of the class,",
        "start": 2707.87,
        "duration": 2.18
    },
    {
        "text": "we don't introduce\nstrings as char stars,",
        "start": 2710.05,
        "duration": 2.19
    },
    {
        "text": "because you very quickly get\ncaught up in a lot of this minutia.",
        "start": 2712.24,
        "duration": 2.797
    },
    {
        "text": "But at the end of the day,\nit's not all that complicated",
        "start": 2715.037,
        "duration": 2.333
    },
    {
        "text": "once you realize that a\nstring is just an address,",
        "start": 2717.37,
        "duration": 2.95
    },
    {
        "text": "and malloc, this new function,\nalso just returns an address.",
        "start": 2720.32,
        "duration": 3.25
    },
    {
        "text": "This is very powerful,\nbecause now you have",
        "start": 2723.57,
        "duration": 2.17
    },
    {
        "text": "these sort of breadcrumbs that can\nlead you to different places in memory.",
        "start": 2725.74,
        "duration": 4.15
    },
    {
        "text": "A little map, so to speak, that can\nlead you to actual strings in memory,",
        "start": 2729.89,
        "duration": 3.95
    },
    {
        "text": "and can actually now solve\nproblems more effectively.",
        "start": 2733.84,
        "duration": 2.78
    },
    {
        "text": "For instance, we can go back\nand solve one other problem",
        "start": 2736.62,
        "duration": 2.47
    },
    {
        "text": "we saw a moment ago, which was swap.",
        "start": 2739.09,
        "duration": 4.17
    },
    {
        "text": "So this version of swap was broken why?",
        "start": 2743.26,
        "duration": 4.18
    },
    {
        "text": "What was the source of\nthis fundamental problem?",
        "start": 2747.44,
        "duration": 3.99
    },
    {
        "text": "Yeah?",
        "start": 2751.43,
        "duration": 1.319
    },
    {
        "text": "STUDENT: [INAUDIBLE]",
        "start": 2752.749,
        "duration": 0.833
    },
    {
        "text": "DAVID J. MALAN: Yeah.",
        "start": 2755.765,
        "duration": 0.875
    },
    {
        "text": "When you went back to main, you\nerased the memory on top of it,",
        "start": 2756.64,
        "duration": 3.0
    },
    {
        "text": "and the fundamental problem there\nwas when I passed in x and y,",
        "start": 2759.64,
        "duration": 5.55
    },
    {
        "text": "they became copies called a and\nb, in different chunks of memory.",
        "start": 2765.19,
        "duration": 4.48
    },
    {
        "text": "And so the fundamental\nproblem seems to be",
        "start": 2769.67,
        "duration": 2.93
    },
    {
        "text": "that swap is incorrectly implemented.",
        "start": 2772.6,
        "duration": 2.61
    },
    {
        "text": "It's logically correct.",
        "start": 2775.21,
        "duration": 1.58
    },
    {
        "text": "It does swap two values.",
        "start": 2776.79,
        "duration": 1.2
    },
    {
        "text": "And we saw that with debug 50, but it's\nkind of fundamentally flawed in so far",
        "start": 2777.99,
        "duration": 4.77
    },
    {
        "text": "as it requires, it\nseems, by design of C,",
        "start": 2782.76,
        "duration": 3.81
    },
    {
        "text": "that a and b be passed in by\nvalue as copies, so to speak.",
        "start": 2786.57,
        "duration": 4.8
    },
    {
        "text": "We need some way to change this function\nto say main, hey, uh-uh, don't give me",
        "start": 2791.37,
        "duration": 4.62
    },
    {
        "text": "copies of your variables.",
        "start": 2795.99,
        "duration": 1.58
    },
    {
        "text": "Give me a treasure map that\nwill lead me to your variables.",
        "start": 2797.57,
        "duration": 3.49
    },
    {
        "text": "Give me the address of x.",
        "start": 2801.06,
        "duration": 2.12
    },
    {
        "text": "Give me the address of y.",
        "start": 2803.18,
        "duration": 1.91
    },
    {
        "text": "And I'll still call them a\nand b, or whatever I want,",
        "start": 2805.09,
        "duration": 2.37
    },
    {
        "text": "but lead me to the original values.",
        "start": 2807.46,
        "duration": 2.5
    },
    {
        "text": "Don't just pass me\ncopies of those values.",
        "start": 2809.96,
        "duration": 2.93
    },
    {
        "text": "And so we can change swap as follows\nfrom a program or a function that's",
        "start": 2812.89,
        "duration": 4.32
    },
    {
        "text": "incorrect entirely, but\ninto one that is correct.",
        "start": 2817.21,
        "duration": 5.23
    },
    {
        "text": "And we need to change\nthe syntax a little bit.",
        "start": 2822.44,
        "duration": 2.12
    },
    {
        "text": "So before is what we had here.",
        "start": 2824.56,
        "duration": 3.2
    },
    {
        "text": "After is what we now have.",
        "start": 2827.76,
        "duration": 2.23
    },
    {
        "text": "Before, after.",
        "start": 2829.99,
        "duration": 2.12
    },
    {
        "text": "Before, after.",
        "start": 2832.11,
        "duration": 1.619
    },
    {
        "text": "So if you see it in\nrapid succession there,",
        "start": 2833.729,
        "duration": 1.791
    },
    {
        "text": "all you see is that a whole bunch\nof stars are appearing in the code.",
        "start": 2835.52,
        "duration": 3.64
    },
    {
        "text": "And unfortunately, C was not\ndesigned in the best of ways",
        "start": 2839.16,
        "duration": 3.85
    },
    {
        "text": "to make clear what star\nmeans in different contexts,",
        "start": 2843.01,
        "duration": 2.57
    },
    {
        "text": "but it's all related as follows.",
        "start": 2845.58,
        "duration": 2.14
    },
    {
        "text": "The fact that I have now\nchanged a and b to be not ints,",
        "start": 2847.72,
        "duration": 4.61
    },
    {
        "text": "but ints stars, int pointers, if you\nwill, means that when main calls swap,",
        "start": 2852.33,
        "duration": 7.42
    },
    {
        "text": "it is by design of the C language,\ngoing to pass in the address of x",
        "start": 2859.75,
        "duration": 4.07
    },
    {
        "text": "and the address of y.",
        "start": 2863.82,
        "duration": 0.95
    },
    {
        "text": "So that's what the star means.",
        "start": 2864.77,
        "duration": 1.25
    },
    {
        "text": "Give me the address of an int and the\naddress of an int, not actual ints.",
        "start": 2866.02,
        "duration": 4.08
    },
    {
        "text": "Now, down here, the star unfortunately\nmeans something slightly different,",
        "start": 2870.1,
        "duration": 3.23
    },
    {
        "text": "but related in spirit.",
        "start": 2873.33,
        "duration": 1.79
    },
    {
        "text": "Int temp just gives me an integer,\nan int variable called temp.",
        "start": 2875.12,
        "duration": 4.78
    },
    {
        "text": "Star a, in this context, without\nthe word int in front of it again,",
        "start": 2879.9,
        "duration": 5.01
    },
    {
        "text": "means go to that location.",
        "start": 2884.91,
        "duration": 2.66
    },
    {
        "text": "Follow the treasure map, so to speak.",
        "start": 2887.57,
        "duration": 1.8
    },
    {
        "text": "Go to the address that is in a.",
        "start": 2889.37,
        "duration": 2.71
    },
    {
        "text": "So for instance, if the\naddress of a is say,",
        "start": 2892.08,
        "duration": 3.78
    },
    {
        "text": "33 Oxford Street,\nCambridge, Massachusetts.",
        "start": 2895.86,
        "duration": 2.739
    },
    {
        "text": "That happens to be the address\nof the CS building at Harvard.",
        "start": 2898.599,
        "duration": 2.541
    },
    {
        "text": "Star a means go to 33 Oxford\nStreet in Cambridge, Massachusetts.",
        "start": 2901.14,
        "duration": 5.06
    },
    {
        "text": "The star just means go to\nthat particular address.",
        "start": 2906.2,
        "duration": 4.12
    },
    {
        "text": "So what does that mean, then, down\nhere, when I say star a gets star b?",
        "start": 2910.32,
        "duration": 4.17
    },
    {
        "text": "That means go to the address\nin b and get its value,",
        "start": 2914.49,
        "duration": 3.56
    },
    {
        "text": "and store it at whenever\na is pointing at to.",
        "start": 2918.05,
        "duration": 3.45
    },
    {
        "text": "So go to a and wait for me for a value.",
        "start": 2921.5,
        "duration": 2.97
    },
    {
        "text": "Go to b, get a value, and put\nthat value at the location in a.",
        "start": 2924.47,
        "duration": 4.61
    },
    {
        "text": "And then lastly, this just\nmeans go to the location in b,",
        "start": 2929.08,
        "duration": 3.645
    },
    {
        "text": "go to whatever building\nthat is, so to speak,",
        "start": 2932.725,
        "duration": 2.295
    },
    {
        "text": "and put the value that is in\ntemp inside of that building.",
        "start": 2935.02,
        "duration": 4.88
    },
    {
        "text": "So a pointer is just an address.",
        "start": 2939.9,
        "duration": 2.96
    },
    {
        "text": "These stars just mean\npointers are involved.",
        "start": 2942.86,
        "duration": 3.75
    },
    {
        "text": "Give me the address of an int,\ngive me the address of an int.",
        "start": 2946.61,
        "duration": 3.04
    },
    {
        "text": "And again, confusing,\nadmittedly, the star",
        "start": 2949.65,
        "duration": 2.58
    },
    {
        "text": "in this context where we don't have\nthe word int in front of it again,",
        "start": 2952.23,
        "duration": 3.73
    },
    {
        "text": "on the side of the equal sign,\njust means go to that address.",
        "start": 2955.96,
        "duration": 2.83
    },
    {
        "text": "Go to that building.",
        "start": 2958.79,
        "duration": 1.29
    },
    {
        "text": "Go to that other building\nand put something there.",
        "start": 2960.08,
        "duration": 3.55
    },
    {
        "text": "So we can now fix our\nswap program correctly.",
        "start": 2963.63,
        "duration": 3.98
    },
    {
        "text": "We can now open up, as I will here,\nswap dot c, which I wrote in advance.",
        "start": 2967.61,
        "duration": 5.1
    },
    {
        "text": "That looks almost the same, except\nthat I've changed the swap function",
        "start": 2972.71,
        "duration": 3.94
    },
    {
        "text": "as follows. a is now\nint*, b is now int*,",
        "start": 2976.65,
        "duration": 3.5
    },
    {
        "text": "and I also borrowed the stars\ninside of the function, as well.",
        "start": 2980.15,
        "duration": 3.62
    },
    {
        "text": "But something's gotta change.",
        "start": 2983.77,
        "duration": 1.88
    },
    {
        "text": "There's one more line of code I need\nto change for all of this to work.",
        "start": 2985.65,
        "duration": 4.46
    },
    {
        "text": "What is that?",
        "start": 2990.11,
        "duration": 0.67
    },
    {
        "text": "What line needs to change?",
        "start": 2994.161,
        "duration": 1.649
    },
    {
        "text": "Well who cares about swap?",
        "start": 2995.81,
        "duration": 1.73
    },
    {
        "text": "It's main that was calling\nthis thing in the first place,",
        "start": 2997.54,
        "duration": 3.07
    },
    {
        "text": "so let's go back to the original story.",
        "start": 3000.61,
        "duration": 2.29
    },
    {
        "text": "Main, here, declares x and y as 1 and\n2, does some printfs here, as before.",
        "start": 3002.9,
        "duration": 5.73
    },
    {
        "text": "But notice this line has to change.",
        "start": 3008.63,
        "duration": 1.64
    },
    {
        "text": "So one more piece of syntax today.",
        "start": 3010.27,
        "duration": 2.739
    },
    {
        "text": "And we're running out of new symbols.",
        "start": 3013.009,
        "duration": 1.541
    },
    {
        "text": "We've seen most of C already.",
        "start": 3014.55,
        "duration": 2.11
    },
    {
        "text": "&x and &y means get me the address\nof x, and get me the address of y,",
        "start": 3016.66,
        "duration": 7.0
    },
    {
        "text": "and pass those in instead.",
        "start": 3023.66,
        "duration": 1.89
    },
    {
        "text": "So x,y would just mean pass in\na copy of x and a copy of y,",
        "start": 3025.55,
        "duration": 4.11
    },
    {
        "text": "or the values thereof.",
        "start": 3029.66,
        "duration": 2.23
    },
    {
        "text": "&x &y means give me a little, you know,\nmap with the address of x and a little",
        "start": 3031.89,
        "duration": 5.36
    },
    {
        "text": "map with the address of y, so that\nswap-- who's receiving those maps--",
        "start": 3037.25,
        "duration": 4.71
    },
    {
        "text": "can go there.",
        "start": 3041.96,
        "duration": 1.98
    },
    {
        "text": "So what does this mean\nin pictorial form?",
        "start": 3043.94,
        "duration": 2.81
    },
    {
        "text": "If we now go back to the\nbeginning of this story,",
        "start": 3046.75,
        "duration": 2.26
    },
    {
        "text": "where we were looking at my computer's\nmemory as this big rectangular region",
        "start": 3049.01,
        "duration": 4.53
    },
    {
        "text": "like this.",
        "start": 3053.54,
        "duration": 1.26
    },
    {
        "text": "With main's chunk of\nmemory at the bottom here.",
        "start": 3054.8,
        "duration": 4.61
    },
    {
        "text": "And inside of main was two variables,\nlike x, and another variable y.",
        "start": 3059.41,
        "duration": 6.16
    },
    {
        "text": "And inside of those were\nthe numbers 1 and 2.",
        "start": 3065.57,
        "duration": 3.4
    },
    {
        "text": "And then I called swap.",
        "start": 3068.97,
        "duration": 1.92
    },
    {
        "text": "And so swap gets its own frame\non the stack, so to speak.",
        "start": 3070.89,
        "duration": 3.57
    },
    {
        "text": "This is swap's frame.",
        "start": 3074.46,
        "duration": 1.77
    },
    {
        "text": "It, too, had a variable called\na and a variable called b.",
        "start": 3076.23,
        "duration": 6.472
    },
    {
        "text": "But what goes in there now?",
        "start": 3082.702,
        "duration": 1.638
    },
    {
        "text": "It's not 1 and 2.",
        "start": 3084.34,
        "duration": 1.49
    },
    {
        "text": "We need to know a little something\nmore about my computer's memory.",
        "start": 3085.83,
        "duration": 2.8
    },
    {
        "text": "And I don't know where\neverything's laid out,",
        "start": 3088.63,
        "duration": 2.22
    },
    {
        "text": "but let me just arbitrarily\nassume that, you know,",
        "start": 3090.85,
        "duration": 3.42
    },
    {
        "text": "it's inside of my computer's memory.",
        "start": 3094.27,
        "duration": 1.92
    },
    {
        "text": "Maybe this is byte number 90.",
        "start": 3096.19,
        "duration": 2.48
    },
    {
        "text": "This is going to be 91.",
        "start": 3098.67,
        "duration": 1.97
    },
    {
        "text": "This here is going to be 92,\n93, 94, 95, and so forth.",
        "start": 3100.64,
        "duration": 3.91
    },
    {
        "text": "I just need to know that there's\nsome kind of numbering scheme there.",
        "start": 3104.55,
        "duration": 3.33
    },
    {
        "text": "So what goes inside of a is 91.",
        "start": 3107.88,
        "duration": 3.51
    },
    {
        "text": "What goes inside of b is 92.",
        "start": 3111.39,
        "duration": 3.94
    },
    {
        "text": "And not the values 1 and 2, but rather\nthe addresses of those values 1 and 2.",
        "start": 3115.33,
        "duration": 7.22
    },
    {
        "text": "Because now my code for the swap\nfunction, consider what it does.",
        "start": 3122.55,
        "duration": 4.23
    },
    {
        "text": "It says, upon receiving\nthe address of an integer,",
        "start": 3126.78,
        "duration": 3.44
    },
    {
        "text": "called a, upon receiving the address\nof another integer, called b,",
        "start": 3130.22,
        "duration": 4.531
    },
    {
        "text": "go there and store that value in temp.",
        "start": 3134.751,
        "duration": 3.469
    },
    {
        "text": "Go to the address in b and store\nthat value at the address in a.",
        "start": 3138.22,
        "duration": 4.46
    },
    {
        "text": "Store the value in temp\nat the address in b.",
        "start": 3142.68,
        "duration": 3.6
    },
    {
        "text": "So let's see what happens then.",
        "start": 3146.28,
        "duration": 2.26
    },
    {
        "text": "So first of all, I need another\nvariable here, called temp.",
        "start": 3148.54,
        "duration": 4.51
    },
    {
        "text": "Temp, meanwhile, is not a pointer.",
        "start": 3153.05,
        "duration": 3.56
    },
    {
        "text": "It's just an integer,\nbut what does it store?",
        "start": 3156.61,
        "duration": 3.54
    },
    {
        "text": "Well, according to my\ncode, temp gets the value",
        "start": 3160.15,
        "duration": 4.55
    },
    {
        "text": "of going to a, going\nto the address in a.",
        "start": 3164.7,
        "duration": 3.25
    },
    {
        "text": "So what is a?",
        "start": 3167.95,
        "duration": 1.583
    },
    {
        "text": "a is 99.",
        "start": 3169.533,
        "duration": 1.777
    },
    {
        "text": "That's like a treasure map leading\nto, OK, this chunk of memory",
        "start": 3171.31,
        "duration": 3.4
    },
    {
        "text": "down here in my computer.",
        "start": 3174.71,
        "duration": 1.61
    },
    {
        "text": "And what value is there\nonce I've gone there?",
        "start": 3176.32,
        "duration": 2.11
    },
    {
        "text": "Once I've gone to the CS building\ninside of it, I see the number 1,",
        "start": 3178.43,
        "duration": 3.8
    },
    {
        "text": "and so I put the number 1 in temp.",
        "start": 3182.23,
        "duration": 3.12
    },
    {
        "text": "Meanwhile, my second line of\ncode says go to the address in b",
        "start": 3185.35,
        "duration": 4.795
    },
    {
        "text": "and grab its value, and put\nit at the address that's in a.",
        "start": 3190.145,
        "duration": 5.015
    },
    {
        "text": "So what does that mean?",
        "start": 3195.16,
        "duration": 1.64
    },
    {
        "text": "Well, star b means\nstart here and go to 92.",
        "start": 3196.8,
        "duration": 4.767
    },
    {
        "text": "So it's like an arrow-- kind\nof like chutes and ladders,",
        "start": 3201.567,
        "duration": 2.333
    },
    {
        "text": "if you know the game--\nlike go to address 92.",
        "start": 3203.9,
        "duration": 2.64
    },
    {
        "text": "What value is there?",
        "start": 3206.54,
        "duration": 0.99
    },
    {
        "text": "The number 2.",
        "start": 3207.53,
        "duration": 0.98
    },
    {
        "text": "And the other half of the\nequation, on the left,",
        "start": 3208.51,
        "duration": 2.0
    },
    {
        "text": "said, go to the address\nin a, which is here,",
        "start": 3210.51,
        "duration": 2.79
    },
    {
        "text": "and copy the 2 into that location.",
        "start": 3213.3,
        "duration": 4.89
    },
    {
        "text": "And then the last line--\nonly one more line--",
        "start": 3218.19,
        "duration": 2.61
    },
    {
        "text": "is this, get the value in temp-- that's\neasy-- and put it at the address in b.",
        "start": 3220.8,
        "duration": 5.53
    },
    {
        "text": "So we have to go to b\nand put temp in there.",
        "start": 3226.33,
        "duration": 3.29
    },
    {
        "text": "So to do that, here's\ntemp, it's the number 1.",
        "start": 3229.62,
        "duration": 4.35
    },
    {
        "text": "I have to go to the address in b.",
        "start": 3233.97,
        "duration": 1.69
    },
    {
        "text": "The address and b is 92.",
        "start": 3235.66,
        "duration": 1.18
    },
    {
        "text": "So let's go there, and\naha, let me go ahead then,",
        "start": 3236.84,
        "duration": 3.44
    },
    {
        "text": "and overwrite the value that's\nthere with the number 1.",
        "start": 3240.28,
        "duration": 7.39
    },
    {
        "text": "So now this frame of\nmemory on the stack--",
        "start": 3247.67,
        "duration": 3.5
    },
    {
        "text": "the 91, the 92, and the\ntemporary variable--",
        "start": 3251.17,
        "duration": 2.45
    },
    {
        "text": "they are, by design of my\nnew function, disposable.",
        "start": 3253.62,
        "duration": 3.43
    },
    {
        "text": "I really don't care, after swap\nreturns, if those things continue--",
        "start": 3257.05,
        "duration": 5.07
    },
    {
        "text": "I did care a little bit about that.",
        "start": 3262.12,
        "duration": 1.46
    },
    {
        "text": "I don't care if those\nthings continue to exist.",
        "start": 3267.43,
        "duration": 2.52
    },
    {
        "text": "All I care about is that\nx and y continue to exist.",
        "start": 3269.95,
        "duration": 4.21
    },
    {
        "text": "So in this way is the new and\nimproved version of the swap function",
        "start": 3274.16,
        "duration": 3.91
    },
    {
        "text": "actually having a permanent\nimpact on my data?",
        "start": 3278.07,
        "duration": 2.89
    },
    {
        "text": "And with the frame, the\nmemory still looks like that,",
        "start": 3280.96,
        "duration": 3.07
    },
    {
        "text": "because it's gone to the address in a.",
        "start": 3284.03,
        "duration": 2.01
    },
    {
        "text": "Gone to the address in b, which\nleads it to the original x and y.",
        "start": 3286.04,
        "duration": 4.15
    },
    {
        "text": "And so by way of pointers,\nby way of these addresses,",
        "start": 3290.19,
        "duration": 2.87
    },
    {
        "text": "do we have the ability to actually\ngo much, much deeper into a program",
        "start": 3293.06,
        "duration": 4.75
    },
    {
        "text": "and actually get at\nvalues that previously we",
        "start": 3297.81,
        "duration": 3.43
    },
    {
        "text": "had no way of even expressing.",
        "start": 3301.24,
        "duration": 3.04
    },
    {
        "text": "So it's at this point in the story where\nI usually admit that, at least for me,",
        "start": 3304.28,
        "duration": 5.13
    },
    {
        "text": "this has been among the most challenging\ntopics when I, myself, was a student.",
        "start": 3309.41,
        "duration": 3.91
    },
    {
        "text": "And in fact, all these years later--\nit's like, 20, 20 year-- yeah,",
        "start": 3313.32,
        "duration": 2.905
    },
    {
        "text": "I think we're up to 20 years ago.",
        "start": 3316.225,
        "duration": 1.375
    },
    {
        "text": "20 years ago-- I didn't\ntake this photo then--",
        "start": 3317.6,
        "duration": 2.96
    },
    {
        "text": "but I sat in what was, at the time,\nthe back right hand corner of Elliot",
        "start": 3320.56,
        "duration": 3.59
    },
    {
        "text": "House's dining hall, here at Harvard.",
        "start": 3324.15,
        "duration": 2.07
    },
    {
        "text": "And I sat down with my teaching\nfellow, who of all the TFs I had",
        "start": 3326.22,
        "duration": 2.93
    },
    {
        "text": "as an undergrad, still remember to\nthis day, [? Nishat ?] [? Meda ?].",
        "start": 3329.15,
        "duration": 2.42
    },
    {
        "text": "And we just reconnected on\nFacebook, all these years later.",
        "start": 3331.57,
        "duration": 2.52
    },
    {
        "text": "Very exciting.",
        "start": 3334.09,
        "duration": 0.96
    },
    {
        "text": "And it was he who\nwonderfully sat down with me",
        "start": 3335.05,
        "duration": 2.419
    },
    {
        "text": "at office hours one\nday in the dining hall,",
        "start": 3337.469,
        "duration": 1.791
    },
    {
        "text": "trying to help me understand pointers,\nbecause it was just so much more",
        "start": 3339.26,
        "duration": 3.03
    },
    {
        "text": "technical than all the other stuff.",
        "start": 3342.29,
        "duration": 1.458
    },
    {
        "text": "Like, there is no puzzle piece in\nScratch for the address of something",
        "start": 3343.748,
        "duration": 3.142
    },
    {
        "text": "that leads you somewhere so\npowerfully as these stars seem",
        "start": 3346.89,
        "duration": 3.53
    },
    {
        "text": "to be able to, here.",
        "start": 3350.42,
        "duration": 1.011
    },
    {
        "text": "And this is only to say that\nthis is among those topics that",
        "start": 3351.431,
        "duration": 2.499
    },
    {
        "text": "might take a little bit of\ntime to sink in, but it does.",
        "start": 3353.93,
        "duration": 3.33
    },
    {
        "text": "And when it does, it really is that\nproverbial light bulb that goes off.",
        "start": 3357.26,
        "duration": 3.39
    },
    {
        "text": "And for me, that light bulb\nwent off right then and there.",
        "start": 3360.65,
        "duration": 4.9
    },
    {
        "text": "Now, what more can we do with these\nthings, after that motivational speech?",
        "start": 3365.55,
        "duration": 3.37
    },
    {
        "text": "Pointer arithmetic.",
        "start": 3368.92,
        "duration": 1.22
    },
    {
        "text": "So, sort of complicated sounding\ntopic, but really, it just",
        "start": 3370.14,
        "duration": 3.58
    },
    {
        "text": "goes back to first principles,\nas to what a pointer actually is.",
        "start": 3373.72,
        "duration": 3.12
    },
    {
        "text": "And it allows us now\nto do things like this.",
        "start": 3376.84,
        "duration": 1.99
    },
    {
        "text": "Let me go ahead and open up one other\nprogram that I wrote in advance here,",
        "start": 3378.83,
        "duration": 3.38
    },
    {
        "text": "called pointers dot c.",
        "start": 3382.21,
        "duration": 2.11
    },
    {
        "text": "And take a look at what\nthis thing does here.",
        "start": 3384.32,
        "duration": 2.42
    },
    {
        "text": "It works a little differently\nfrom the syntax we're used to,",
        "start": 3386.74,
        "duration": 2.72
    },
    {
        "text": "and from any of our\ncrypto problems thus far.",
        "start": 3389.46,
        "duration": 3.09
    },
    {
        "text": "So notice on this first line here,\nI get string and I store in s.",
        "start": 3392.55,
        "duration": 4.08
    },
    {
        "text": "No more string right\nnow, just char star.",
        "start": 3396.63,
        "duration": 1.85
    },
    {
        "text": "We can be real and talk about\nit as the address of a char.",
        "start": 3398.48,
        "duration": 3.55
    },
    {
        "text": "A little sanity check,\nis s equal equal to null?",
        "start": 3402.03,
        "duration": 2.44
    },
    {
        "text": "If so, just return.",
        "start": 3404.47,
        "duration": 0.94
    },
    {
        "text": "Something went wrong, so\nlet's not deal with it now.",
        "start": 3405.41,
        "duration": 2.32
    },
    {
        "text": "Down here, a for loop.",
        "start": 3407.73,
        "duration": 1.75
    },
    {
        "text": "For i gets 0 all the way up to n.",
        "start": 3409.48,
        "duration": 3.71
    },
    {
        "text": "So this is just a standard syntax\nwe've used a few times now, even back",
        "start": 3413.19,
        "duration": 3.05
    },
    {
        "text": "in week 1 when we just\nwanted iterate over.",
        "start": 3416.24,
        "duration": 2.36
    },
    {
        "text": "Or in week 2, when we wanted iterate\nover the characters in a string.",
        "start": 3418.6,
        "duration": 3.67
    },
    {
        "text": "But we've never seen this\nkind of craziness before.",
        "start": 3422.27,
        "duration": 2.85
    },
    {
        "text": "A star, and then some\narithmetic in parentheses.",
        "start": 3425.12,
        "duration": 3.47
    },
    {
        "text": "In the past, when we wanted to print\nout a character, as implied by %c here,",
        "start": 3428.59,
        "duration": 5.0
    },
    {
        "text": "we quite simply, as I recall, did this.",
        "start": 3433.59,
        "duration": 3.7
    },
    {
        "text": "Which was nice and intuitive, right?",
        "start": 3437.29,
        "duration": 1.5
    },
    {
        "text": "The square brackets\ndenote to treat the string",
        "start": 3438.79,
        "duration": 2.327
    },
    {
        "text": "as though it's an array, which it\nreally is, an array of characters.",
        "start": 3441.117,
        "duration": 2.833
    },
    {
        "text": "And that means get the\ni-th character of s.",
        "start": 3443.95,
        "duration": 3.05
    },
    {
        "text": "But now that we understand what s is, we\ndon't need to use this syntactic sugar,",
        "start": 3447.0,
        "duration": 5.12
    },
    {
        "text": "as it's called.",
        "start": 3452.12,
        "duration": 0.64
    },
    {
        "text": "Any time a language has a\nfeature that's convenient to use,",
        "start": 3452.76,
        "duration": 2.92
    },
    {
        "text": "and easier to read sometimes,\nbut isn't fundamentally",
        "start": 3455.68,
        "duration": 2.69
    },
    {
        "text": "necessary to express\nyourself, it's often",
        "start": 3458.37,
        "duration": 1.75
    },
    {
        "text": "called syntactic sugar, which means\nit's just kind of a nicety to have.",
        "start": 3460.12,
        "duration": 3.89
    },
    {
        "text": "And indeed, that square\nbracket notation is just",
        "start": 3464.01,
        "duration": 2.98
    },
    {
        "text": "sugar for this more arcane, but\nperhaps more well-defined syntax now.",
        "start": 3466.99,
        "duration": 7.42
    },
    {
        "text": "The star operator in this context is\nthe dereference operator, technically.",
        "start": 3474.41,
        "duration": 4.5
    },
    {
        "text": "It's the go there operator,\nas I've been describing it.",
        "start": 3478.91,
        "duration": 2.94
    },
    {
        "text": "Go to some address.",
        "start": 3481.85,
        "duration": 1.52
    },
    {
        "text": "Well s, recall, is a string.",
        "start": 3483.37,
        "duration": 1.91
    },
    {
        "text": "But there is no string.",
        "start": 3485.28,
        "duration": 1.27
    },
    {
        "text": "Strings are just the\naddresses of characters now.",
        "start": 3486.55,
        "duration": 2.32
    },
    {
        "text": "The first in a string.",
        "start": 3488.87,
        "duration": 1.94
    },
    {
        "text": "So initially in this\nloop, what am I doing?",
        "start": 3490.81,
        "duration": 3.42
    },
    {
        "text": "s is the address of a string, the\naddress of its first character.",
        "start": 3494.23,
        "duration": 3.33
    },
    {
        "text": "And I'm saying, add to s, the value i.",
        "start": 3497.56,
        "duration": 2.7
    },
    {
        "text": "Well, i is just this variable in my\nfor loop that's initialized to 0.",
        "start": 3500.26,
        "duration": 4.27
    },
    {
        "text": "So s plus 0 is obviously just s.",
        "start": 3504.53,
        "duration": 3.19
    },
    {
        "text": "s is the address of a char.",
        "start": 3507.72,
        "duration": 1.86
    },
    {
        "text": "*s means go to s.",
        "start": 3509.58,
        "duration": 2.97
    },
    {
        "text": "What do you find when you get there?",
        "start": 3512.55,
        "duration": 1.91
    },
    {
        "text": "A character, because s is a char\nstar, the address of a character.",
        "start": 3514.46,
        "duration": 3.82
    },
    {
        "text": "And so printing out %c\n*s effectively means,",
        "start": 3518.28,
        "duration": 4.41
    },
    {
        "text": "go print that character right there.",
        "start": 3522.69,
        "duration": 2.2
    },
    {
        "text": "On the second iteration of\nthe loop, i is, of course, 1.",
        "start": 3524.89,
        "duration": 2.96
    },
    {
        "text": "So s plus 1 is 1 byte farther\nfrom the beginning of the string.",
        "start": 3527.85,
        "duration": 4.46
    },
    {
        "text": "And the star means go to that\ncharacter and print it with %c.",
        "start": 3532.31,
        "duration": 4.59
    },
    {
        "text": "One more iteration, i is now 2.",
        "start": 3536.9,
        "duration": 2.35
    },
    {
        "text": "s plus 2 is 2 bytes away\nfrom the start of the string.",
        "start": 3539.25,
        "duration": 2.35
    },
    {
        "text": "Go there and print that\ncharacter in the string.",
        "start": 3541.6,
        "duration": 3.0
    },
    {
        "text": "And so forth.",
        "start": 3544.6,
        "duration": 0.57
    },
    {
        "text": "And do this up until the\nlength of the string.",
        "start": 3545.17,
        "duration": 3.3
    },
    {
        "text": "Now this is perfectly correct.",
        "start": 3548.47,
        "duration": 2.12
    },
    {
        "text": "And if you really kind of want\nto look cool with your code",
        "start": 3550.59,
        "duration": 2.47
    },
    {
        "text": "you can use pointer arithmetic\nin this way, because all it is",
        "start": 3553.06,
        "duration": 2.541
    },
    {
        "text": "is just expressing more precisely\nwhat is going on underneath the hood.",
        "start": 3555.601,
        "duration": 3.668
    },
    {
        "text": "But it's a little more cryptic.",
        "start": 3559.269,
        "duration": 1.291
    },
    {
        "text": "It's certainly a couple more characters.",
        "start": 3560.56,
        "duration": 2.29
    },
    {
        "text": "But it is functionally\nequivalent to what",
        "start": 3562.85,
        "duration": 3.09
    },
    {
        "text": "we've been doing for weeks, which\nhas been, again, just s [i].",
        "start": 3565.94,
        "duration": 3.86
    },
    {
        "text": "So whereas some of today's\nideas are admittedly",
        "start": 3569.8,
        "duration": 2.9
    },
    {
        "text": "new-- allocating memory\nand actually looking",
        "start": 3572.7,
        "duration": 2.69
    },
    {
        "text": "underneath the hood\nat what a string is--",
        "start": 3575.39,
        "duration": 1.89
    },
    {
        "text": "we're not really getting\nnewfound capabilities",
        "start": 3577.28,
        "duration": 3.25
    },
    {
        "text": "that we didn't already have when it\ncomes to manipulating existing strings.",
        "start": 3580.53,
        "duration": 4.82
    },
    {
        "text": "So this is pointer arithmetic,\nso to speak, insofar",
        "start": 3585.35,
        "duration": 3.54
    },
    {
        "text": "as we are doing\narithmetic with pointers.",
        "start": 3588.89,
        "duration": 3.88
    },
    {
        "text": "Math with pointers.",
        "start": 3592.77,
        "duration": 2.73
    },
    {
        "text": "All right, let's take a look now\nat where things can go wrong.",
        "start": 3595.5,
        "duration": 5.77
    },
    {
        "text": "So this is a program written by our\nfriend Nick Parlante at Stanford,",
        "start": 3601.27,
        "duration": 3.14
    },
    {
        "text": "inspired by Binky, who will\nreturn in just a moment.",
        "start": 3604.41,
        "duration": 2.58
    },
    {
        "text": "And it's fundamentally broken.",
        "start": 3606.99,
        "duration": 1.56
    },
    {
        "text": "This code is incorrect.",
        "start": 3608.55,
        "duration": 1.17
    },
    {
        "text": "It also doesn't really\ndo anything useful.",
        "start": 3609.72,
        "duration": 2.26
    },
    {
        "text": "But it's meant to be demonstrative\nof things that can go wrong.",
        "start": 3611.98,
        "duration": 2.72
    },
    {
        "text": "So at the top of this program, notice\nwe are declaring two variables, x and y.",
        "start": 3614.7,
        "duration": 4.22
    },
    {
        "text": "But today those variables are not ints,\nas they might have been in weeks past,",
        "start": 3618.92,
        "duration": 3.41
    },
    {
        "text": "they are int*s, the addresses ints.",
        "start": 3622.33,
        "duration": 2.8
    },
    {
        "text": "They're not being initialized\nyet to any value, so that's fine.",
        "start": 3625.13,
        "duration": 2.9
    },
    {
        "text": "So really, this is just\ngiving us, if you will,",
        "start": 3628.03,
        "duration": 2.84
    },
    {
        "text": "like, two boxes on the screen.",
        "start": 3630.87,
        "duration": 1.85
    },
    {
        "text": "So x at this point in the story looks\nlike this, y at this point in the story",
        "start": 3632.72,
        "duration": 4.23
    },
    {
        "text": "looks like this.",
        "start": 3636.95,
        "duration": 1.07
    },
    {
        "text": "I have no idea what's inside of them.",
        "start": 3638.02,
        "duration": 1.98
    },
    {
        "text": "They have garbage value, so to speak.",
        "start": 3640.0,
        "duration": 2.06
    },
    {
        "text": "Because if we didn't\nassign them a value,",
        "start": 3642.06,
        "duration": 1.79
    },
    {
        "text": "the computer is not\ngoing to do it for us,",
        "start": 3643.85,
        "duration": 1.79
    },
    {
        "text": "so they might just have remnants\nof some past usage of that memory.",
        "start": 3645.64,
        "duration": 2.93
    },
    {
        "text": "So we don't know what's inside of them.",
        "start": 3648.57,
        "duration": 1.625
    },
    {
        "text": "But that's OK, because on this next\nline I call malloc and actually allocate",
        "start": 3650.195,
        "duration": 6.455
    },
    {
        "text": "enough memory for one int.",
        "start": 3656.65,
        "duration": 1.95
    },
    {
        "text": "Now this is kind of a silly use.",
        "start": 3658.6,
        "duration": 2.09
    },
    {
        "text": "It's not the best way\nto give yourself an int.",
        "start": 3660.69,
        "duration": 2.73
    },
    {
        "text": "We've seen for weeks\nnow how you get an int.",
        "start": 3663.42,
        "duration": 1.89
    },
    {
        "text": "You say, like, int x, or int y, or int\nz, and the computer gives you an int.",
        "start": 3665.31,
        "duration": 5.124
    },
    {
        "text": "But if we want to use malloc\nin the simplest way possible,",
        "start": 3670.434,
        "duration": 2.416
    },
    {
        "text": "we can just say, hey, malloc,\ngive me enough space for an int.",
        "start": 3672.85,
        "duration": 3.86
    },
    {
        "text": "And recall from the past that an\nint here is generally 4 bytes.",
        "start": 3676.71,
        "duration": 5.24
    },
    {
        "text": "So give me 4 bytes of\nmemory, specifically",
        "start": 3681.95,
        "duration": 2.61
    },
    {
        "text": "the address of the first of those bytes.",
        "start": 3684.56,
        "duration": 2.16
    },
    {
        "text": "That's all malloc is doing, and\nit's storing that address in x.",
        "start": 3686.72,
        "duration": 2.76
    },
    {
        "text": "For good measure I should check for\nnull, but we're not, in this case,",
        "start": 3689.48,
        "duration": 3.08
    },
    {
        "text": "per Binky.",
        "start": 3692.56,
        "duration": 1.0
    },
    {
        "text": "So what's the next line do?",
        "start": 3693.56,
        "duration": 2.03
    },
    {
        "text": "This next line means go to the address\nin x and put the number 42 there.",
        "start": 3695.59,
        "duration": 5.94
    },
    {
        "text": "That seems OK.",
        "start": 3701.53,
        "duration": 0.88
    },
    {
        "text": "Because assuming malloc\nreturns the address",
        "start": 3702.41,
        "duration": 2.8
    },
    {
        "text": "of a chunk of memory, the\nfirst address of 4 bytes,",
        "start": 3705.21,
        "duration": 3.0
    },
    {
        "text": "we can go there and put the number 42\nin binary, in 4 bytes worth of bits.",
        "start": 3708.21,
        "duration": 7.332
    },
    {
        "text": "But what about this line?",
        "start": 3715.542,
        "duration": 2.418
    },
    {
        "text": "I've flagged it in red\nbecause this program probably",
        "start": 3717.96,
        "duration": 2.81
    },
    {
        "text": "is going to go no further.",
        "start": 3720.77,
        "duration": 1.71
    },
    {
        "text": "In fact, something very,\nvery bad is about to happen.",
        "start": 3722.48,
        "duration": 4.46
    },
    {
        "text": "Why?",
        "start": 3726.94,
        "duration": 0.992
    },
    {
        "text": "Well, what is the value in y?",
        "start": 3727.932,
        "duration": 3.278
    },
    {
        "text": "Well, originally x was a garbage\nvalue until I called malloc",
        "start": 3731.21,
        "duration": 3.63
    },
    {
        "text": "and asked malloc, hey, malloc,\ngive me enough space for an int.",
        "start": 3734.84,
        "duration": 3.76
    },
    {
        "text": "So I'll draw it as a box here.",
        "start": 3738.6,
        "duration": 1.66
    },
    {
        "text": "Give me the address of\nthat chunk of memory.",
        "start": 3740.26,
        "duration": 2.21
    },
    {
        "text": "So this question mark is really now\nan arrow to that chunk of memory.",
        "start": 3742.47,
        "duration": 3.92
    },
    {
        "text": "And then I said, hey, computer, go\nahead and go there and put the value 42.",
        "start": 3746.39,
        "duration": 6.56
    },
    {
        "text": "But then my next line of code said,\nhey, computer, go to the value in y",
        "start": 3752.95,
        "duration": 4.16
    },
    {
        "text": "and put the unlucky number 13 there.",
        "start": 3757.11,
        "duration": 2.47
    },
    {
        "text": "So that's like saying, go-- I don't know\nwhere to go, because this has no value.",
        "start": 3759.58,
        "duration": 7.04
    },
    {
        "text": "And so something very\nbad is going to happen.",
        "start": 3766.62,
        "duration": 2.79
    },
    {
        "text": "Because the question mark\nimplies, this is a garbage value.",
        "start": 3769.41,
        "duration": 2.65
    },
    {
        "text": "Maybe it's 0, maybe it's 1,000, maybe\nit's some number in between or bigger.",
        "start": 3772.06,
        "duration": 3.29
    },
    {
        "text": "It's some garbage value, which\nmeans if I just go there,",
        "start": 3775.35,
        "duration": 3.27
    },
    {
        "text": "who knows where I'm going to end up?",
        "start": 3778.62,
        "duration": 1.58
    },
    {
        "text": "But odds are I'm going\nto end up somewhere",
        "start": 3780.2,
        "duration": 1.88
    },
    {
        "text": "I shouldn't, because I should not be\ntouching memory that is not my own.",
        "start": 3782.08,
        "duration": 4.33
    },
    {
        "text": "And indeed, thanks to Binky, we're about\nto see that bad things indeed happen",
        "start": 3786.41,
        "duration": 5.38
    },
    {
        "text": "when you touch memory\nthat you shouldn't.",
        "start": 3791.79,
        "duration": 2.31
    },
    {
        "text": "Let's take a look.",
        "start": 3794.1,
        "duration": 0.894
    },
    {
        "text": "[VIDEO PLAYBACK]",
        "start": 3794.994,
        "duration": 0.666
    },
    {
        "text": "-Hey, Binky, wake up.",
        "start": 3797.236,
        "duration": 2.144
    },
    {
        "text": "It's time for pointer fun.",
        "start": 3799.38,
        "duration": 2.016
    },
    {
        "text": "-What's that?",
        "start": 3801.396,
        "duration": 1.694
    },
    {
        "text": "Learn about pointers?",
        "start": 3803.09,
        "duration": 1.58
    },
    {
        "text": "Oh, goody!",
        "start": 3804.67,
        "duration": 1.77
    },
    {
        "text": "-Well, to get started, I guess we're\ngoing to need a couple pointers.",
        "start": 3806.44,
        "duration": 3.01
    },
    {
        "text": "-OK.",
        "start": 3809.45,
        "duration": 0.8
    },
    {
        "text": "This code allocates two pointers\nwhich can point to integers.",
        "start": 3810.25,
        "duration": 3.74
    },
    {
        "text": "-OK, well, I see the\ntwo pointers, but they",
        "start": 3813.99,
        "duration": 2.457
    },
    {
        "text": "don't seem to be pointing to anything.",
        "start": 3816.447,
        "duration": 1.583
    },
    {
        "text": "-That's right.",
        "start": 3818.03,
        "duration": 0.81
    },
    {
        "text": "Initially, pointers\ndon't point to anything.",
        "start": 3818.84,
        "duration": 2.12
    },
    {
        "text": "The things they point to are called\npointees, and setting them up's",
        "start": 3820.96,
        "duration": 2.98
    },
    {
        "text": "a separate step.",
        "start": 3823.94,
        "duration": 1.23
    },
    {
        "text": "-Oh, right, right, I knew that.",
        "start": 3825.17,
        "duration": 1.65
    },
    {
        "text": "The pointees are separate.",
        "start": 3826.82,
        "duration": 1.65
    },
    {
        "text": "Er, so how do you allocate a pointee?",
        "start": 3828.47,
        "duration": 2.61
    },
    {
        "text": "-OK, well, this code allocates\na new integer pointee,",
        "start": 3831.08,
        "duration": 3.78
    },
    {
        "text": "and this part sets x to point to it.",
        "start": 3834.86,
        "duration": 3.09
    },
    {
        "text": "-Hey, that looks better.",
        "start": 3837.95,
        "duration": 1.15
    },
    {
        "text": "So make it do something.",
        "start": 3839.1,
        "duration": 1.75
    },
    {
        "text": "-OK, I'll dereference the pointer x to\nstore the number 42 into its pointee.",
        "start": 3840.85,
        "duration": 5.62
    },
    {
        "text": "For this trick I'll need my\nmagic wand of dereferencing.",
        "start": 3846.47,
        "duration": 3.56
    },
    {
        "text": "-Your magic wand of dereferencing?",
        "start": 3850.03,
        "duration": 2.77
    },
    {
        "text": "Uh, that's great.",
        "start": 3852.8,
        "duration": 2.38
    },
    {
        "text": "-This is what the code looks like.",
        "start": 3855.18,
        "duration": 1.78
    },
    {
        "text": "I'll just set up the number and--",
        "start": 3856.96,
        "duration": 1.555
    },
    {
        "text": "[POP]",
        "start": 3858.515,
        "duration": 0.571
    },
    {
        "text": "-Hey, look, there it goes.",
        "start": 3859.586,
        "duration": 2.434
    },
    {
        "text": "So doing a dereference on x follows\nthe arrow to access its pointee.",
        "start": 3862.02,
        "duration": 4.84
    },
    {
        "text": "In this case, to store 42 in there.",
        "start": 3866.86,
        "duration": 2.17
    },
    {
        "text": "Hey, try using it to store the number\n13 through the other pointer, y.",
        "start": 3869.03,
        "duration": 4.44
    },
    {
        "text": "-OK, I'll just go over here to\ny and get the number 13 set up,",
        "start": 3873.47,
        "duration": 5.7
    },
    {
        "text": "and then take the wand of\ndereferencing and just--",
        "start": 3879.17,
        "duration": 3.746
    },
    {
        "text": "[BUZZER]",
        "start": 3882.916,
        "duration": 0.499
    },
    {
        "text": "Oh!",
        "start": 3883.415,
        "duration": 0.825
    },
    {
        "text": "-Oh, hey, that didn't work.",
        "start": 3884.24,
        "duration": 2.71
    },
    {
        "text": "Say, Binky, I don't think the\ndereferencing y is a good idea,",
        "start": 3886.95,
        "duration": 3.64
    },
    {
        "text": "because, you know, setting up\nthe pointee is a separate step,",
        "start": 3890.59,
        "duration": 3.25
    },
    {
        "text": "and I don't think we ever did it.",
        "start": 3893.84,
        "duration": 1.77
    },
    {
        "text": "-Oh, good point.",
        "start": 3895.61,
        "duration": 1.89
    },
    {
        "text": "-Yeah, we allocated the pointer y, but\nwe never set it to point to a pointee.",
        "start": 3897.5,
        "duration": 4.61
    },
    {
        "text": "-Hm, very observant.",
        "start": 3902.11,
        "duration": 2.02
    },
    {
        "text": "-Hey, you're looking good there, Binky.",
        "start": 3904.13,
        "duration": 2.19
    },
    {
        "text": "Can you fix it so that y points\nto the same pointee as x?",
        "start": 3906.32,
        "duration": 2.96
    },
    {
        "text": "-Sure.",
        "start": 3909.28,
        "duration": 0.57
    },
    {
        "text": "I'll use my magic wand\nof pointer assignment.",
        "start": 3909.85,
        "duration": 2.81
    },
    {
        "text": "-Is that going to be a\nproblem, like before?",
        "start": 3912.66,
        "duration": 2.19
    },
    {
        "text": "-No, this doesn't touch the pointees.",
        "start": 3914.85,
        "duration": 1.8
    },
    {
        "text": "It just changes one pointer to\npoint to the same thing as another.",
        "start": 3916.65,
        "duration": 3.57
    },
    {
        "text": "-Oh, I see.",
        "start": 3920.22,
        "duration": 1.09
    },
    {
        "text": "Now y points to the same place as x.",
        "start": 3921.31,
        "duration": 2.77
    },
    {
        "text": "So wait, now y is fixed.",
        "start": 3924.08,
        "duration": 1.78
    },
    {
        "text": "It has a pointee, so you can try\nthe wand of dereferencing again",
        "start": 3925.86,
        "duration": 3.03
    },
    {
        "text": "to send the 13 over.",
        "start": 3928.89,
        "duration": 1.788
    },
    {
        "text": "-Oh, OK, here goes,",
        "start": 3930.678,
        "duration": 3.402
    },
    {
        "text": "-Hey, look at that.",
        "start": 3934.08,
        "duration": 1.12
    },
    {
        "text": "Now dereferencing works on y.",
        "start": 3935.2,
        "duration": 1.73
    },
    {
        "text": "And because the pointers are sharing\nthat one pointee, they both see the 13.",
        "start": 3936.93,
        "duration": 4.11
    },
    {
        "text": "-Yeah, sharing, whatever.",
        "start": 3941.04,
        "duration": 1.8
    },
    {
        "text": "So are we going to switch places now?",
        "start": 3942.84,
        "duration": 1.81
    },
    {
        "text": "-Oh look, we're out of time.",
        "start": 3944.65,
        "duration": 1.96
    },
    {
        "text": "-But--",
        "start": 3946.61,
        "duration": 0.84
    },
    {
        "text": "-Just remember the three pointer rules.",
        "start": 3947.45,
        "duration": 1.94
    },
    {
        "text": "Number one, the basic structure\nis that you have a pointer",
        "start": 3949.39,
        "duration": 3.05
    },
    {
        "text": "and it points over to a pointee.",
        "start": 3952.44,
        "duration": 2.11
    },
    {
        "text": "But the pointer and\npointee are separate,",
        "start": 3954.55,
        "duration": 1.96
    },
    {
        "text": "and the common error\nis to set up a pointer,",
        "start": 3956.51,
        "duration": 1.99
    },
    {
        "text": "but to forget to give it a pointee.",
        "start": 3958.5,
        "duration": 2.38
    },
    {
        "text": "Number two, pointer dereferencing\nstarts at the pointer",
        "start": 3960.88,
        "duration": 3.13
    },
    {
        "text": "and follows its arrow over\nto access its pointee.",
        "start": 3964.01,
        "duration": 2.9
    },
    {
        "text": "As we all know, this only works if there\nis a pointee, which kind of gets back",
        "start": 3966.91,
        "duration": 3.88
    },
    {
        "text": "to rule number one.",
        "start": 3970.79,
        "duration": 1.34
    },
    {
        "text": "Number three, pointer\nassignment takes one pointer",
        "start": 3972.13,
        "duration": 2.78
    },
    {
        "text": "and changes it to point to the\nsame pointee as another pointer.",
        "start": 3974.91,
        "duration": 3.31
    },
    {
        "text": "So after the assignment the two\npointers will point to the same pointee.",
        "start": 3978.22,
        "duration": 3.57
    },
    {
        "text": "Sometimes that's called sharing.",
        "start": 3981.79,
        "duration": 1.867
    },
    {
        "text": "And that's all there is to it, really.",
        "start": 3983.657,
        "duration": 1.583
    },
    {
        "text": "Bye bye, now.",
        "start": 3985.24,
        "duration": 1.52
    },
    {
        "text": "[END PLAYBACK]",
        "start": 3986.76,
        "duration": 0.936
    },
    {
        "text": "DAVID J. MALAN: So what\nelse can go wrong now",
        "start": 3987.696,
        "duration": 1.874
    },
    {
        "text": "that we have the ability\nto touch, correctly",
        "start": 3989.57,
        "duration": 2.36
    },
    {
        "text": "or incorrectly, any\nmemory that we actually",
        "start": 3991.93,
        "duration": 2.19
    },
    {
        "text": "have access to inside of our program?",
        "start": 3994.12,
        "duration": 1.85
    },
    {
        "text": "Well, memory leaks are one such problem.",
        "start": 3995.97,
        "duration": 2.31
    },
    {
        "text": "Now that you have the capability to\nask the operating system for memory",
        "start": 3998.28,
        "duration": 3.84
    },
    {
        "text": "via the malloc function, you\nhave the ability to accidentally",
        "start": 4002.12,
        "duration": 4.19
    },
    {
        "text": "not give that memory back.",
        "start": 4006.31,
        "duration": 1.72
    },
    {
        "text": "In fact, a very common mistake\nin some programming languages",
        "start": 4008.03,
        "duration": 3.05
    },
    {
        "text": "is to ask the operating\nsystem for a chunk of memory,",
        "start": 4011.08,
        "duration": 2.26
    },
    {
        "text": "use it, and then never actually free it.",
        "start": 4013.34,
        "duration": 2.52
    },
    {
        "text": "To give it back so that\nother parts of your program,",
        "start": 4015.86,
        "duration": 2.7
    },
    {
        "text": "or other programs on the computer,\ncan actually make use of that memory.",
        "start": 4018.56,
        "duration": 3.96
    },
    {
        "text": "But fortunately there exist tools\nvia which we can detect this,",
        "start": 4022.52,
        "duration": 3.26
    },
    {
        "text": "and one of them is called Valgrind.",
        "start": 4025.78,
        "duration": 1.6
    },
    {
        "text": "So this is another debugging\ntool that you'll now",
        "start": 4027.38,
        "duration": 2.041
    },
    {
        "text": "be able to use once\nyou start dynamically",
        "start": 4029.421,
        "duration": 2.679
    },
    {
        "text": "allocating memory in your program.",
        "start": 4032.1,
        "duration": 1.87
    },
    {
        "text": "So up until now you probably\nhave not used malloc,",
        "start": 4033.97,
        "duration": 2.44
    },
    {
        "text": "and therefore you have\nnot likely actually asked",
        "start": 4036.41,
        "duration": 3.2
    },
    {
        "text": "the operating system for more\nmemory in this very dynamic way.",
        "start": 4039.61,
        "duration": 3.25
    },
    {
        "text": "Instead, you have just declared\nan integer or an array,",
        "start": 4042.86,
        "duration": 3.36
    },
    {
        "text": "and asked the computer for memory\nin other ways, not using malloc.",
        "start": 4046.22,
        "duration": 4.52
    },
    {
        "text": "But suppose that we have\na program like this.",
        "start": 4050.74,
        "duration": 2.26
    },
    {
        "text": "In memory dot c I've made some mistakes.",
        "start": 4053.0,
        "duration": 3.72
    },
    {
        "text": "Deliberately, but mistakes nonetheless.",
        "start": 4056.72,
        "duration": 3.614
    },
    {
        "text": "And indeed, this draws inspiration from\nthe documentation for this very tool",
        "start": 4060.334,
        "duration": 3.166
    },
    {
        "text": "to highlight a couple of\nits most useful features.",
        "start": 4063.5,
        "duration": 2.46
    },
    {
        "text": "So let's look at this program here.",
        "start": 4065.96,
        "duration": 1.52
    },
    {
        "text": "There's no prototype\nfor this function f,",
        "start": 4067.48,
        "duration": 1.77
    },
    {
        "text": "just because the example that Valgrind,\nthis tool, gives just puts it up top,",
        "start": 4069.25,
        "duration": 3.4
    },
    {
        "text": "and that's fine.",
        "start": 4072.65,
        "duration": 0.91
    },
    {
        "text": "But let's focus on main first.",
        "start": 4073.56,
        "duration": 1.69
    },
    {
        "text": "Main takes no command line\narguments, per it's mention of void,",
        "start": 4075.25,
        "duration": 2.88
    },
    {
        "text": "and it returns an int as usual.",
        "start": 4078.13,
        "duration": 1.98
    },
    {
        "text": "The function f gets called here.",
        "start": 4080.11,
        "duration": 2.44
    },
    {
        "text": "So f open bracket, close bracket\njust means call the function f,",
        "start": 4082.55,
        "duration": 3.38
    },
    {
        "text": "but pass it no inputs, and\nthen eventually return 0.",
        "start": 4085.93,
        "duration": 3.06
    },
    {
        "text": "So let's look at f.",
        "start": 4088.99,
        "duration": 0.82
    },
    {
        "text": "What happens?",
        "start": 4089.81,
        "duration": 1.06
    },
    {
        "text": "Well, f takes no inputs,\nproduces no outputs.",
        "start": 4090.87,
        "duration": 3.05
    },
    {
        "text": "It only apparently has\nthese two lines of code.",
        "start": 4093.92,
        "duration": 2.547
    },
    {
        "text": "And let's consider what it does.",
        "start": 4096.467,
        "duration": 1.333
    },
    {
        "text": "It calls malloc, memory allocate,\n10 times the size of int.",
        "start": 4097.8,
        "duration": 4.33
    },
    {
        "text": "So that just means, hey, malloc,\ngive me enough space for 10 integers.",
        "start": 4102.13,
        "duration": 4.599
    },
    {
        "text": "Now we know on CS50 IDE that the size\nof an int recall is 4 bytes, or 32 bits.",
        "start": 4106.729,
        "duration": 5.25
    },
    {
        "text": "So this is like saying, hey,\ncomputer, give me 40 bytes total.",
        "start": 4111.979,
        "duration": 4.891
    },
    {
        "text": "4 times 10.",
        "start": 4116.87,
        "duration": 1.26
    },
    {
        "text": "Malloc recall returns the\naddress of that chunk of memory,",
        "start": 4118.13,
        "duration": 3.349
    },
    {
        "text": "and it stores it in x, which, according\nto the star, is the address of an int.",
        "start": 4121.479,
        "duration": 4.09
    },
    {
        "text": "So that's all.",
        "start": 4125.569,
        "duration": 0.811
    },
    {
        "text": "So borrowing some of the same\nideas from the Binky example.",
        "start": 4126.38,
        "duration": 2.75
    },
    {
        "text": "What is bad about this\nis this next line.",
        "start": 4129.13,
        "duration": 4.59
    },
    {
        "text": "Why is x[10]=0 a problem?",
        "start": 4133.72,
        "duration": 4.522
    },
    {
        "text": "Well, the size of this chunk of\nmemory is what, 10 ints worth.",
        "start": 4138.242,
        "duration": 3.848
    },
    {
        "text": "So 10 times 4, 40 bytes.",
        "start": 4142.09,
        "duration": 1.84
    },
    {
        "text": "But remember how chunks\nof memory are indexed",
        "start": 4143.93,
        "duration": 3.369
    },
    {
        "text": "when you use square bracket notation.",
        "start": 4147.299,
        "duration": 1.541
    },
    {
        "text": "When you use square bracket\nnotation, you better",
        "start": 4148.84,
        "duration": 2.0
    },
    {
        "text": "start counting at 0, which means\nif we have 10 ints, first one is 0,",
        "start": 4150.84,
        "duration": 4.0
    },
    {
        "text": "the last one is 9, the\n10th one does not exist.",
        "start": 4154.84,
        "duration": 4.33
    },
    {
        "text": "And so if I'm saying\nxx[10] gets any value,",
        "start": 4159.17,
        "duration": 3.149
    },
    {
        "text": "that's like saying go there\nand put the value 0 there,",
        "start": 4162.319,
        "duration": 3.281
    },
    {
        "text": "but I don't have access\nto that chunk of memory.",
        "start": 4165.6,
        "duration": 2.429
    },
    {
        "text": "I did not ask the\ncomputer for that memory.",
        "start": 4168.029,
        "duration": 2.361
    },
    {
        "text": "I asked the computer for all of this\nother memory, only 10 of these bytes.",
        "start": 4170.39,
        "duration": 4.639
    },
    {
        "text": "So this feels problematic.",
        "start": 4175.029,
        "duration": 1.551
    },
    {
        "text": "Moreover, I have asked the operating\nsystem for memory via malloc,",
        "start": 4176.58,
        "duration": 3.699
    },
    {
        "text": "hey, give me 40 bytes.",
        "start": 4180.279,
        "duration": 1.33
    },
    {
        "text": "I never handed it back.",
        "start": 4181.609,
        "duration": 2.091
    },
    {
        "text": "Thereby introducing what we\nwould call a memory leak.",
        "start": 4183.7,
        "duration": 4.22
    },
    {
        "text": "Now, as an aside, once your program\nquits, all of the memory it's allocated",
        "start": 4187.92,
        "duration": 3.63
    },
    {
        "text": "is actually automatically\ngiven back, but the problem is",
        "start": 4191.55,
        "duration": 2.72
    },
    {
        "text": "that, for long running programs,\nthings like your browsers,",
        "start": 4194.27,
        "duration": 2.54
    },
    {
        "text": "or word processing\nprograms, or any number--",
        "start": 4196.81,
        "duration": 2.521
    },
    {
        "text": "Skype-- or any number of other\nprograms that you might run locally",
        "start": 4199.331,
        "duration": 2.749
    },
    {
        "text": "on your Mac or PC, if those bigger\nprograms have memory leaks, such",
        "start": 4202.08,
        "duration": 3.94
    },
    {
        "text": "that they keep asking Mac\nOS or Windows for memory",
        "start": 4206.02,
        "duration": 2.66
    },
    {
        "text": "and never actually\nremember to give it back,",
        "start": 4208.68,
        "duration": 2.71
    },
    {
        "text": "you might experience what you might\nhave in the past of your Mac or PC",
        "start": 4211.39,
        "duration": 4.17
    },
    {
        "text": "really starting to slow\ndown and kind of crawl.",
        "start": 4215.56,
        "duration": 4.14
    },
    {
        "text": "And that can be for any number\nof reasons, but one of them",
        "start": 4219.7,
        "duration": 2.98
    },
    {
        "text": "is if your program or programs inside\nof them have some form of memory leak.",
        "start": 4222.68,
        "duration": 4.21
    },
    {
        "text": "Not your fault. It's the programmer's\nfault. But memory leaks, nonetheless.",
        "start": 4226.89,
        "duration": 4.36
    },
    {
        "text": "So what is Valgrind good for here?",
        "start": 4231.25,
        "duration": 2.64
    },
    {
        "text": "Well, it turns out, that in Valgrind\nyou can run the command like here.",
        "start": 4233.89,
        "duration": 5.04
    },
    {
        "text": "Valgrind.",
        "start": 4238.93,
        "duration": 0.74
    },
    {
        "text": "Leak check equals full.",
        "start": 4239.67,
        "duration": 1.71
    },
    {
        "text": "Dot slash.",
        "start": 4241.38,
        "duration": 0.584
    },
    {
        "text": "And then the name of your program.",
        "start": 4241.964,
        "duration": 1.416
    },
    {
        "text": "And so let me do that.",
        "start": 4243.38,
        "duration": 1.05
    },
    {
        "text": "Let me go ahead back into CS50 IDE\nand let me go ahead and make memory.",
        "start": 4244.43,
        "duration": 5.31
    },
    {
        "text": "Compiles OK, so it doesn't appear\nto be syntactically flawed.",
        "start": 4249.74,
        "duration": 2.74
    },
    {
        "text": "Dot slash memory.",
        "start": 4252.48,
        "duration": 2.09
    },
    {
        "text": "Nothing bad seems to happen.",
        "start": 4254.57,
        "duration": 1.63
    },
    {
        "text": "So that's all fine and good.",
        "start": 4256.2,
        "duration": 1.57
    },
    {
        "text": "And you might think that, OK my\nprogram works, it's time to submit,",
        "start": 4257.77,
        "duration": 4.19
    },
    {
        "text": "all is well, but not necessarily.",
        "start": 4261.96,
        "duration": 2.27
    },
    {
        "text": "Let me go ahead and run Valgrind.",
        "start": 4264.23,
        "duration": 2.19
    },
    {
        "text": "And let me actually go\nahead-- let me go up here",
        "start": 4266.42,
        "duration": 2.314
    },
    {
        "text": "and not create a new\nfile, but a new terminal.",
        "start": 4268.734,
        "duration": 1.916
    },
    {
        "text": "Just so that we have\na much bigger window.",
        "start": 4270.65,
        "duration": 1.75
    },
    {
        "text": "And let me go ahead into this directory\nand run Valgrind, dot slash memory.",
        "start": 4272.4,
        "duration": 4.94
    },
    {
        "text": "Without any command line\narguments besides that.",
        "start": 4277.34,
        "duration": 2.65
    },
    {
        "text": "And hit Enter.",
        "start": 4279.99,
        "duration": 0.91
    },
    {
        "text": "And the downside of\nValgrind, frankly, is",
        "start": 4280.9,
        "duration": 2.36
    },
    {
        "text": "that it's just-- output is atrocious.",
        "start": 4283.26,
        "duration": 1.9
    },
    {
        "text": "Like it's just not easy to read.",
        "start": 4285.16,
        "duration": 1.68
    },
    {
        "text": "But you start to learn\nto notice patterns in it.",
        "start": 4286.84,
        "duration": 2.45
    },
    {
        "text": "So I've learned to notice when I make\nmistakes in programs that invalid right",
        "start": 4289.29,
        "duration": 3.92
    },
    {
        "text": "of size 4, not a good thing.",
        "start": 4293.21,
        "duration": 1.74
    },
    {
        "text": "That's indeed a problem of some sort.",
        "start": 4294.95,
        "duration": 2.12
    },
    {
        "text": "But what's nice about\nValgrind is that it tells me",
        "start": 4297.07,
        "duration": 2.83
    },
    {
        "text": "that the source of this invalid\nright, whatever that means,",
        "start": 4299.9,
        "duration": 3.54
    },
    {
        "text": "starts in memory.c line 13 and really\nrears its head in line 8 of memory.c.",
        "start": 4303.44,
        "duration": 7.92
    },
    {
        "text": "Well, what are those lines?",
        "start": 4311.36,
        "duration": 1.19
    },
    {
        "text": "Let's go to line 13.",
        "start": 4312.55,
        "duration": 2.12
    },
    {
        "text": "Ah, that's my call to F. And line 8.",
        "start": 4314.67,
        "duration": 2.6
    },
    {
        "text": "Oh, that's my use of x bracket 10.",
        "start": 4317.27,
        "duration": 3.79
    },
    {
        "text": "So why is Valgrind telling\nme invalid right of size 4?",
        "start": 4321.06,
        "duration": 3.89
    },
    {
        "text": "Well, size 4 sounds familiar.",
        "start": 4324.95,
        "duration": 1.32
    },
    {
        "text": "That's the size of an int.",
        "start": 4326.27,
        "duration": 1.71
    },
    {
        "text": "Invalid just means bad.",
        "start": 4327.98,
        "duration": 1.53
    },
    {
        "text": "Right means change something,\nlike mutate a value.",
        "start": 4329.51,
        "duration": 3.36
    },
    {
        "text": "So invalid right of size\n4 maybe just means this.",
        "start": 4332.87,
        "duration": 4.38
    },
    {
        "text": "I am in an invalid way\nchanging 4 bytes of memory",
        "start": 4337.25,
        "duration": 4.45
    },
    {
        "text": "by storing 0 from the right hand side\ninto x bracket 10 on the left hand",
        "start": 4341.7,
        "duration": 4.29
    },
    {
        "text": "side because, again, that x\nbracket 10 is an 11th byte, rather,",
        "start": 4345.99,
        "duration": 5.26
    },
    {
        "text": "an 11th int, that I never\nask the operating system for.",
        "start": 4351.25,
        "duration": 4.07
    },
    {
        "text": "I only asked it for 10, not 11.",
        "start": 4355.32,
        "duration": 2.98
    },
    {
        "text": "And so Valgrind is telling me, somewhat\ncryptically, that I've screwed up.",
        "start": 4358.3,
        "duration": 4.69
    },
    {
        "text": "On line 8 ultimately I've screwed\nup by touching 4 bytes of memory",
        "start": 4362.99,
        "duration": 4.23
    },
    {
        "text": "that I shouldn't have.",
        "start": 4367.22,
        "duration": 1.63
    },
    {
        "text": "Meanwhile, a little\nmore worrisome is this.",
        "start": 4368.85,
        "duration": 2.74
    },
    {
        "text": "Address such and such as\nzero bytes after a block",
        "start": 4371.59,
        "duration": 2.63
    },
    {
        "text": "of size 4 allots, da-da da-da.",
        "start": 4374.22,
        "duration": 1.94
    },
    {
        "text": "Oh, not that.",
        "start": 4376.16,
        "duration": 0.78
    },
    {
        "text": "Let's focus on-- here we go.",
        "start": 4376.94,
        "duration": 3.103
    },
    {
        "text": "Uh oh.",
        "start": 4380.043,
        "duration": 0.747
    },
    {
        "text": "Leak summary.",
        "start": 4380.79,
        "duration": 1.06
    },
    {
        "text": "Definitely lost.",
        "start": 4381.85,
        "duration": 1.12
    },
    {
        "text": "Oh, my god.",
        "start": 4382.97,
        "duration": 0.71
    },
    {
        "text": "Like I have lost 40 bytes\nof the computer's memory.",
        "start": 4383.68,
        "duration": 2.71
    },
    {
        "text": "Now what does that mean?",
        "start": 4386.39,
        "duration": 1.08
    },
    {
        "text": "This simply means that\nValgrind has noticed",
        "start": 4387.47,
        "duration": 2.01
    },
    {
        "text": "that I must have called malloc\nor some similar function,",
        "start": 4389.48,
        "duration": 2.44
    },
    {
        "text": "asked for memory, 40 bytes'\nworth, and never gave it back.",
        "start": 4391.92,
        "duration": 3.77
    },
    {
        "text": "Well, we haven't introduced the\nway of giving memory back so let",
        "start": 4395.69,
        "duration": 2.89
    },
    {
        "text": "me at least address that now.",
        "start": 4398.58,
        "duration": 1.63
    },
    {
        "text": "But let me take this advice.",
        "start": 4400.21,
        "duration": 1.45
    },
    {
        "text": "Rerun with leak check full to\nsee details of leaked memory.",
        "start": 4401.66,
        "duration": 2.999
    },
    {
        "text": "So let me go ahead and do that.",
        "start": 4404.659,
        "duration": 1.291
    },
    {
        "text": "Let me go ahead and copy\nthis so I can rerun Valgrind.",
        "start": 4405.95,
        "duration": 4.323
    },
    {
        "text": "Whoops.",
        "start": 4410.273,
        "duration": 0.967
    },
    {
        "text": "Valgrind.",
        "start": 4411.24,
        "duration": 0.857
    },
    {
        "text": "With that command line argument.",
        "start": 4412.097,
        "duration": 1.333
    },
    {
        "text": "Dot slash memory.",
        "start": 4413.43,
        "duration": 1.58
    },
    {
        "text": "And now I'll see a little\nmore detail sometimes.",
        "start": 4415.01,
        "duration": 2.87
    },
    {
        "text": "Although, it appears in this\ncase it's already pretty verbose.",
        "start": 4417.88,
        "duration": 3.36
    },
    {
        "text": "And so indeed I've still\nlost 40 bytes in one block.",
        "start": 4421.24,
        "duration": 3.69
    },
    {
        "text": "And up here again is a reiteration\nof that invalid right of size 4.",
        "start": 4424.93,
        "duration": 4.01
    },
    {
        "text": "So how do I fix this?",
        "start": 4428.94,
        "duration": 1.38
    },
    {
        "text": "Well, let me go back into my program.",
        "start": 4430.32,
        "duration": 1.74
    },
    {
        "text": "And you know what?",
        "start": 4432.06,
        "duration": 0.76
    },
    {
        "text": "The program's still pretty useless,\nbut at least let me fix that mistake.",
        "start": 4432.82,
        "duration": 3.91
    },
    {
        "text": "And go ahead now and go at x bracket 9.",
        "start": 4436.73,
        "duration": 3.43
    },
    {
        "text": "Let me remake my program.",
        "start": 4440.16,
        "duration": 1.35
    },
    {
        "text": "Make memory.",
        "start": 4441.51,
        "duration": 1.48
    },
    {
        "text": "And let me in my bigger terminal\nwindow do Valgrind dot slash memory.",
        "start": 4442.99,
        "duration": 6.9
    },
    {
        "text": "And now, oh, damn it.",
        "start": 4449.89,
        "duration": 1.12
    },
    {
        "text": "Still have definitely lost\n40 bytes in one blocks,",
        "start": 4451.01,
        "duration": 2.53
    },
    {
        "text": "but notice the output\nis much shorter now.",
        "start": 4453.54,
        "duration": 3.19
    },
    {
        "text": "I don't have a mistake that\nwas higher up here before.",
        "start": 4456.73,
        "duration": 2.57
    },
    {
        "text": "So I fixed one problem.",
        "start": 4459.3,
        "duration": 1.2
    },
    {
        "text": "Let me go ahead and\nfix that other problem.",
        "start": 4460.5,
        "duration": 1.86
    },
    {
        "text": "After using this memory,\nlet me go ahead and free x.",
        "start": 4462.36,
        "duration": 4.2
    },
    {
        "text": "So free is the opposite\nof malloc in this case.",
        "start": 4466.56,
        "duration": 3.1
    },
    {
        "text": "And if I go ahead and recompile\nmake memory and then run it",
        "start": 4469.66,
        "duration": 3.44
    },
    {
        "text": "in my bigger terminal window, nice.",
        "start": 4473.1,
        "duration": 3.4
    },
    {
        "text": "All heap blocks were freed.",
        "start": 4476.5,
        "duration": 2.3
    },
    {
        "text": "No leaks are possible.",
        "start": 4478.8,
        "duration": 2.35
    },
    {
        "text": "Now what does that mean?",
        "start": 4481.15,
        "duration": 1.19
    },
    {
        "text": "Well, it turns out, when we\nlook at our computer's memory,",
        "start": 4482.34,
        "duration": 4.124
    },
    {
        "text": "there is indeed that stack at the\nbottom that we talked about earlier.",
        "start": 4486.464,
        "duration": 2.916
    },
    {
        "text": "And the stack is where frames of\nmemory go, slices of memory go,",
        "start": 4489.38,
        "duration": 3.96
    },
    {
        "text": "that are used when\nfunctions are being called.",
        "start": 4493.34,
        "duration": 2.53
    },
    {
        "text": "And they get layered\non top of each other",
        "start": 4495.87,
        "duration": 2.18
    },
    {
        "text": "and get un-layered only\nonce the function's return.",
        "start": 4498.05,
        "duration": 2.64
    },
    {
        "text": "The heap, meanwhile, is a chunk of\nmemory above the stack, by design.",
        "start": 4500.69,
        "duration": 3.77
    },
    {
        "text": "At least as we've drawn it here.",
        "start": 4504.46,
        "duration": 1.59
    },
    {
        "text": "And in the heap you have just\na pool of available memory",
        "start": 4506.05,
        "duration": 4.44
    },
    {
        "text": "that you can draw from at any point.",
        "start": 4510.49,
        "duration": 2.76
    },
    {
        "text": "And what's powerful\nabout the heap is that,",
        "start": 4513.25,
        "duration": 2.37
    },
    {
        "text": "whereas the stack keeps growing\nup and then as functions return",
        "start": 4515.62,
        "duration": 3.28
    },
    {
        "text": "it grows back down thereby\nthrowing away, effectively",
        "start": 4518.9,
        "duration": 3.67
    },
    {
        "text": "losing track of, any memory\nthat we had on the stack,",
        "start": 4522.57,
        "duration": 2.69
    },
    {
        "text": "like x's and y's and a's and\nb's and temp variables still,",
        "start": 4525.26,
        "duration": 3.7
    },
    {
        "text": "the heap does not experience\nthat kind of disposal of memory.",
        "start": 4528.96,
        "duration": 5.44
    },
    {
        "text": "If you ask for memory via malloc, malloc\ngives you a chunk of memory over here,",
        "start": 4534.4,
        "duration": 5.38
    },
    {
        "text": "or over here, or over here, up above\nthe stack inside of the so-called heap.",
        "start": 4539.78,
        "duration": 4.92
    },
    {
        "text": "And even when your function\nreturns, whether it's f, or swap,",
        "start": 4544.7,
        "duration": 3.77
    },
    {
        "text": "or something else, that use of\nmemory persists permanently.",
        "start": 4548.47,
        "duration": 5.49
    },
    {
        "text": "Only once you call free do\nyou give that chunk of memory",
        "start": 4553.96,
        "duration": 3.11
    },
    {
        "text": "back to the operating\nsystem and allow it to reuse",
        "start": 4557.07,
        "duration": 2.68
    },
    {
        "text": "that memory for something else.",
        "start": 4559.75,
        "duration": 2.48
    },
    {
        "text": "But unfortunately this\npicture kind of suggests",
        "start": 4562.23,
        "duration": 3.23
    },
    {
        "text": "a problem and bad design in some sense.",
        "start": 4565.46,
        "duration": 2.62
    },
    {
        "text": "Although, given a finite\namount of memory, eventually",
        "start": 4568.08,
        "duration": 2.37
    },
    {
        "text": "something's got to bump\nup against something else",
        "start": 4570.45,
        "duration": 2.18
    },
    {
        "text": "if you want to keep asking\nfor and using memory.",
        "start": 4572.63,
        "duration": 2.38
    },
    {
        "text": "Those arrows kind of suggest\nthat the stack and the heap",
        "start": 4575.01,
        "duration": 3.55
    },
    {
        "text": "are really destined to collide\nwith each other if what happens?",
        "start": 4578.56,
        "duration": 3.14
    },
    {
        "text": "Well, if the stack grows too large.",
        "start": 4581.7,
        "duration": 1.907
    },
    {
        "text": "In other words, if you keep calling\nfunction after function after function",
        "start": 4583.607,
        "duration": 3.083
    },
    {
        "text": "after function and never return.",
        "start": 4586.69,
        "duration": 1.43
    },
    {
        "text": "And maybe this is accidental.",
        "start": 4588.12,
        "duration": 1.27
    },
    {
        "text": "Maybe you're using\nrecursion, per last time,",
        "start": 4589.39,
        "duration": 2.19
    },
    {
        "text": "and you accidentally\ndon't have a base case",
        "start": 4591.58,
        "duration": 2.285
    },
    {
        "text": "so you just keep calling functions.",
        "start": 4593.865,
        "duration": 1.795
    },
    {
        "text": "So much so that you\nrun out of stack space.",
        "start": 4595.66,
        "duration": 3.41
    },
    {
        "text": "Or maybe you call malloc so many\ntimes that the heap keeps growing down",
        "start": 4599.07,
        "duration": 4.53
    },
    {
        "text": "and down and hits the stack, meanwhile.",
        "start": 4603.6,
        "duration": 2.2
    },
    {
        "text": "Either of those problems can persist.",
        "start": 4605.8,
        "duration": 1.85
    },
    {
        "text": "And you might recognize\nsome familiar terms.",
        "start": 4607.65,
        "duration": 2.23
    },
    {
        "text": "The first of those scenarios\ndescribe stack overflow.",
        "start": 4609.88,
        "duration": 2.601
    },
    {
        "text": "So if any of you have ever discovered\nthe website, Stack Overflow,",
        "start": 4612.481,
        "duration": 2.749
    },
    {
        "text": "which is a wonderful\nplace to go for advice",
        "start": 4615.23,
        "duration": 3.632
    },
    {
        "text": "and tips and tricks with\nprogramming, more generally.",
        "start": 4618.862,
        "duration": 2.208
    },
    {
        "text": "stackoverflow.com draws its name\nfrom exactly that programming",
        "start": 4621.07,
        "duration": 4.87
    },
    {
        "text": "risk and problem.",
        "start": 4625.94,
        "duration": 1.5
    },
    {
        "text": "Heap overflow is similar in spirit.",
        "start": 4627.44,
        "duration": 1.922
    },
    {
        "text": "It's just the opposite.",
        "start": 4629.362,
        "duration": 0.958
    },
    {
        "text": "When the heap grows down\ntoo far, as we've drawn it.",
        "start": 4630.32,
        "duration": 2.76
    },
    {
        "text": "And they're both\nexamples, more generally,",
        "start": 4633.08,
        "duration": 1.99
    },
    {
        "text": "of what you might call buffer overflows.",
        "start": 4635.07,
        "duration": 2.25
    },
    {
        "text": "A buffer is a chunk of space that's\ntypically finite in some form",
        "start": 4637.32,
        "duration": 4.31
    },
    {
        "text": "and you can eventually overflow it\nby trying to use more memory then",
        "start": 4641.63,
        "duration": 3.47
    },
    {
        "text": "you should have.",
        "start": 4645.1,
        "duration": 0.69
    },
    {
        "text": "And a buffer overflows\ntypically actually relates",
        "start": 4645.79,
        "duration": 2.53
    },
    {
        "text": "to arrays and chunks of memory.",
        "start": 4648.32,
        "duration": 2.17
    },
    {
        "text": "And, in fact, a great example\nfrom Wikipedia is this one here.",
        "start": 4650.49,
        "duration": 3.17
    },
    {
        "text": "The code is a little cryptic too because\nit doesn't really do anything useful,",
        "start": 4653.66,
        "duration": 4.04
    },
    {
        "text": "but it's fundamentally\nflawed in the following way.",
        "start": 4657.7,
        "duration": 2.35
    },
    {
        "text": "Let's consider exactly what happens\nwhen you have a stack overflow.",
        "start": 4660.05,
        "duration": 4.04
    },
    {
        "text": "It turns out that this is a bad\nenough problem that you can actually",
        "start": 4664.09,
        "duration": 3.39
    },
    {
        "text": "lose control over your computer itself.",
        "start": 4667.48,
        "duration": 2.77
    },
    {
        "text": "It can be hacked into if\nan adversary, a bad guy,",
        "start": 4670.25,
        "duration": 3.21
    },
    {
        "text": "has the ability somehow to inject his\nor her own adversarial code, code that",
        "start": 4673.46,
        "duration": 5.41
    },
    {
        "text": "deletes your files, or encrypt your\nfiles, or does something malicious,",
        "start": 4678.87,
        "duration": 3.79
    },
    {
        "text": "into your program because of a\nmemory related mistake you've made.",
        "start": 4682.66,
        "duration": 3.13
    },
    {
        "text": "Let's consider this example.",
        "start": 4685.79,
        "duration": 1.3
    },
    {
        "text": "Here's the main function.",
        "start": 4687.09,
        "duration": 1.15
    },
    {
        "text": "In this particular example, it takes\ncommand line arguments arg-c and arg-v",
        "start": 4688.24,
        "duration": 4.07
    },
    {
        "text": "and just apparently calls this function\nfoo, passing in the second word",
        "start": 4692.31,
        "duration": 5.0
    },
    {
        "text": "that the user typed in.",
        "start": 4697.31,
        "duration": 1.02
    },
    {
        "text": "So the first command line argument\nthat the user ran at the prompt.",
        "start": 4698.33,
        "duration": 2.83
    },
    {
        "text": "And it just blindly does this.",
        "start": 4701.16,
        "duration": 1.25
    },
    {
        "text": "It doesn't check arg-c, it doesn't check\nfor null, it doesn't check anything.",
        "start": 4702.41,
        "duration": 3.53
    },
    {
        "text": "It just blindly passes\nin arg-v bracket 1.",
        "start": 4705.94,
        "duration": 2.714
    },
    {
        "text": "And that's generally bad practice.",
        "start": 4708.654,
        "duration": 1.416
    },
    {
        "text": "Right?",
        "start": 4710.07,
        "duration": 0.5
    },
    {
        "text": "If you're not error checking, bad things\nare likely going to happen eventually.",
        "start": 4710.57,
        "duration": 3.57
    },
    {
        "text": "Well, what does foo do?",
        "start": 4714.14,
        "duration": 1.76
    },
    {
        "text": "Foo takes a char star as inputs.",
        "start": 4715.9,
        "duration": 2.99
    },
    {
        "text": "Now, a week ago, we would\nhave said string bar,",
        "start": 4718.89,
        "duration": 2.794
    },
    {
        "text": "but we don't need to\nhide that detail anymore.",
        "start": 4721.684,
        "duration": 1.916
    },
    {
        "text": "Bar is just the address of a character.",
        "start": 4723.6,
        "duration": 3.34
    },
    {
        "text": "It's a string.",
        "start": 4726.94,
        "duration": 1.19
    },
    {
        "text": "Foo returns nothing, but\nhas two lines of code.",
        "start": 4728.13,
        "duration": 2.53
    },
    {
        "text": "First, it declares an array\ncalled c-- just because.",
        "start": 4730.66,
        "duration": 3.88
    },
    {
        "text": "This is kind of a\ncontrived example online,",
        "start": 4734.54,
        "duration": 3.28
    },
    {
        "text": "but the pictures will\ntell it all-- of size 12.",
        "start": 4737.82,
        "duration": 3.126
    },
    {
        "text": "So this is saying, hey, computer.",
        "start": 4740.946,
        "duration": 1.374
    },
    {
        "text": "Give me an array of 12 characters.",
        "start": 4742.32,
        "duration": 2.16
    },
    {
        "text": "Or give me a chunk of memory of size\n12 that I plan to put characters into.",
        "start": 4744.48,
        "duration": 4.265
    },
    {
        "text": "And then, mem copy, you\nmight not have seen before,",
        "start": 4748.745,
        "duration": 2.125
    },
    {
        "text": "but it essentially does this it\ncopies into this chunk of memory",
        "start": 4750.87,
        "duration": 4.8
    },
    {
        "text": "whatever is at that chunk of\nmemory up to this many bytes.",
        "start": 4755.67,
        "duration": 4.59
    },
    {
        "text": "So mem copy, as the names\nsuggests, copies memory from here",
        "start": 4760.26,
        "duration": 3.58
    },
    {
        "text": "to here a total number\nof this many bytes.",
        "start": 4763.84,
        "duration": 4.01
    },
    {
        "text": "So why is this worrisome?",
        "start": 4767.85,
        "duration": 2.13
    },
    {
        "text": "Well, if for whatever reason\nyou have only allocated 12 bytes",
        "start": 4769.98,
        "duration": 4.12
    },
    {
        "text": "and the user has typed\na word at the prompt",
        "start": 4774.1,
        "duration": 2.82
    },
    {
        "text": "when running your program\nthat is more than 12 bytes,",
        "start": 4776.92,
        "duration": 3.78
    },
    {
        "text": "it would seem that the user\nis able to touch memory",
        "start": 4780.7,
        "duration": 6.7
    },
    {
        "text": "that you the programmer never intended.",
        "start": 4787.4,
        "duration": 1.81
    },
    {
        "text": "Why?",
        "start": 4789.21,
        "duration": 0.78
    },
    {
        "text": "Well, your use of mem copy is\nchecking the length of something,",
        "start": 4789.99,
        "duration": 3.64
    },
    {
        "text": "but it's checking the length of bar.",
        "start": 4793.63,
        "duration": 1.92
    },
    {
        "text": "So if the user types in not 12\ncharacters, but 20 characters,",
        "start": 4795.55,
        "duration": 3.71
    },
    {
        "text": "strleng of bar is going to be 20.",
        "start": 4799.26,
        "duration": 2.66
    },
    {
        "text": "The user's input is 20 characters long.",
        "start": 4801.92,
        "duration": 1.78
    },
    {
        "text": "So your code is saying copy\n20 characters from bar into c.",
        "start": 4803.7,
        "duration": 5.5
    },
    {
        "text": "Unfortunately, c is only of size 12 and\nso that's 8 bytes you end up copying",
        "start": 4809.2,
        "duration": 5.36
    },
    {
        "text": "that you shouldn't.",
        "start": 4814.56,
        "duration": 1.35
    },
    {
        "text": "So if you were given a chunk of\nmemory here that's only of size 8",
        "start": 4815.91,
        "duration": 3.8
    },
    {
        "text": "and you're just 12, you're touching\n8 more bytes to the right of it,",
        "start": 4819.71,
        "duration": 3.93
    },
    {
        "text": "so to speak, that don't\nactually belong to you.",
        "start": 4823.64,
        "duration": 3.26
    },
    {
        "text": "Now, at best, nothing's going to happen.",
        "start": 4826.9,
        "duration": 2.056
    },
    {
        "text": "Your computer, your program\nis not going to notice.",
        "start": 4828.956,
        "duration": 2.124
    },
    {
        "text": "Things are just going to hum along\nand all is going to seem fine.",
        "start": 4831.08,
        "duration": 3.39
    },
    {
        "text": "Much like my memory\nprogram a moment ago.",
        "start": 4834.47,
        "duration": 1.94
    },
    {
        "text": "Seemed fine, but Valgrind\nthought otherwise.",
        "start": 4836.41,
        "duration": 2.79
    },
    {
        "text": "In this case, really\nbad things can happen.",
        "start": 4839.2,
        "duration": 3.32
    },
    {
        "text": "Because if what the human adversary,\nthe bad guy, has typed at the prompt",
        "start": 4842.52,
        "duration": 4.85
    },
    {
        "text": "is not just some word like foo,\nor bar, or David, or Zamyla,",
        "start": 4847.37,
        "duration": 3.98
    },
    {
        "text": "or any number of sort\nof innocuous strings,",
        "start": 4851.35,
        "duration": 3.1
    },
    {
        "text": "but is actually the ASCII equivalent,\nso the textual equivalent,",
        "start": 4854.45,
        "duration": 3.93
    },
    {
        "text": "of some malicious code-- so if he or\nshe actually somehow typed at the prompt",
        "start": 4858.38,
        "duration": 4.87
    },
    {
        "text": "a pattern of characters that really\nrepresent a pattern of bits that",
        "start": 4863.25,
        "duration": 3.9
    },
    {
        "text": "do something bad, like\ndelete all your files,",
        "start": 4867.15,
        "duration": 2.17
    },
    {
        "text": "or spam everyone in your address\nbook, or any number of things.",
        "start": 4869.32,
        "duration": 3.61
    },
    {
        "text": "If they provide a string that's\nlonger than 12 characters,",
        "start": 4872.93,
        "duration": 3.74
    },
    {
        "text": "the overflow is going to\nend up somewhere in memory.",
        "start": 4876.67,
        "duration": 3.25
    },
    {
        "text": "And where is that?",
        "start": 4879.92,
        "duration": 1.17
    },
    {
        "text": "Well, let's zoom in a little lower level\nto what your computer's memory looks",
        "start": 4881.09,
        "duration": 4.52
    },
    {
        "text": "like on the stack.",
        "start": 4885.61,
        "duration": 1.78
    },
    {
        "text": "So it's a little more detail.",
        "start": 4887.39,
        "duration": 1.52
    },
    {
        "text": "So let's absorb this for just a moment.",
        "start": 4888.91,
        "duration": 1.96
    },
    {
        "text": "Think of this now as the bottom of\nthe stack, parent routines stack.",
        "start": 4890.87,
        "duration": 4.67
    },
    {
        "text": "So you can think of this as main, or\nfoo, or some function on the stack.",
        "start": 4895.54,
        "duration": 3.64
    },
    {
        "text": "It turns out-- and I didn't\nmention this earlier--",
        "start": 4899.18,
        "duration": 2.19
    },
    {
        "text": "that besides there being arguments\nor parameters on the stack",
        "start": 4901.37,
        "duration": 3.12
    },
    {
        "text": "and local variables on the stack,\nit turns out that the computer also",
        "start": 4904.49,
        "duration": 4.21
    },
    {
        "text": "uses the stack just to kind of store\na reminder for itself of the address",
        "start": 4908.7,
        "duration": 5.68
    },
    {
        "text": "in memory of the\nfunction that called it.",
        "start": 4914.38,
        "duration": 3.23
    },
    {
        "text": "So if this picture represents foo, the\nfunction right now, who called foo?",
        "start": 4917.61,
        "duration": 5.82
    },
    {
        "text": "Main called foo and so what foo\ndoes is, on its own stack, it",
        "start": 4923.43,
        "duration": 4.36
    },
    {
        "text": "just kind of jots down a note to\nitself here in pink, where it says,",
        "start": 4927.79,
        "duration": 3.0
    },
    {
        "text": "return address, when I'm done,\nreturn to this address of main.",
        "start": 4930.79,
        "duration": 3.24
    },
    {
        "text": "The computer does that for itself.",
        "start": 4934.03,
        "duration": 2.56
    },
    {
        "text": "So when foo is done executing,\npreviously, I just wipe the screen",
        "start": 4936.59,
        "duration": 3.27
    },
    {
        "text": "and remove foo.",
        "start": 4939.86,
        "duration": 1.04
    },
    {
        "text": "But how does the program know where\nto go back to after foo is done?",
        "start": 4940.9,
        "duration": 3.424
    },
    {
        "text": "It's because it put a little\nbread crumb for itself.",
        "start": 4944.324,
        "duration": 2.166
    },
    {
        "text": "The address of main, itself.",
        "start": 4946.49,
        "duration": 2.112
    },
    {
        "text": "Not the address of variables, the\naddress of the function itself,",
        "start": 4948.602,
        "duration": 2.708
    },
    {
        "text": "which, recall, is in the text\nsegment of your computer's memory.",
        "start": 4951.31,
        "duration": 3.46
    },
    {
        "text": "But never mind that for now.",
        "start": 4954.77,
        "duration": 1.95
    },
    {
        "text": "So now, let's look at what the stack\nframe for foo is actually like.",
        "start": 4956.72,
        "duration": 5.441
    },
    {
        "text": "Here is that array of size 12.",
        "start": 4962.161,
        "duration": 1.919
    },
    {
        "text": "And it's drawn as a rectangle.",
        "start": 4964.08,
        "duration": 1.42
    },
    {
        "text": "If we made it wider, we could just\ndo 0, 1, 2, all the way through C 11,",
        "start": 4965.5,
        "duration": 4.1
    },
    {
        "text": "but the author of this graphic has just\ndrawn things a little more square like.",
        "start": 4969.6,
        "duration": 4.63
    },
    {
        "text": "So this is the first byte in C and\nthis is the last byte in C, 0 and 11,",
        "start": 4974.23,
        "duration": 4.52
    },
    {
        "text": "respectively.",
        "start": 4978.75,
        "duration": 0.85
    },
    {
        "text": "And then bar, recall, is the\nonly argument to the foo function",
        "start": 4979.6,
        "duration": 3.08
    },
    {
        "text": "and that, as I did mention before,\nbelongs on the stack as well.",
        "start": 4982.68,
        "duration": 4.44
    },
    {
        "text": "But here's the problem.",
        "start": 4987.12,
        "duration": 1.33
    },
    {
        "text": "And this is by design.",
        "start": 4988.45,
        "duration": 1.44
    },
    {
        "text": "It appears that whoever\ndesigned the stack,",
        "start": 4989.89,
        "duration": 4.0
    },
    {
        "text": "has it growing upwards,\nas in this picture.",
        "start": 4993.89,
        "duration": 2.66
    },
    {
        "text": "And that's how I described it earlier.",
        "start": 4996.55,
        "duration": 1.9
    },
    {
        "text": "So the stack itself grows up.",
        "start": 4998.45,
        "duration": 1.84
    },
    {
        "text": "Frames go up and up and up and up.",
        "start": 5000.29,
        "duration": 2.39
    },
    {
        "text": "But within a frame, notice what happens.",
        "start": 5002.68,
        "duration": 2.38
    },
    {
        "text": "Within a frame, if you've got a buffer,\nan array of memory, a chunk of memory,",
        "start": 5005.06,
        "duration": 4.83
    },
    {
        "text": "and you write to it, you start\nwriting to that memory, by design,",
        "start": 5009.89,
        "duration": 4.54
    },
    {
        "text": "at the top left corner, so to\nspeak, to the right, and then down,",
        "start": 5014.43,
        "duration": 4.29
    },
    {
        "text": "and then down.",
        "start": 5018.72,
        "duration": 1.1
    },
    {
        "text": "So top left to bottom\nright, so to speak.",
        "start": 5019.82,
        "duration": 2.76
    },
    {
        "text": "Pictorially.",
        "start": 5022.58,
        "duration": 1.5
    },
    {
        "text": "This was not necessarily a good\nthing, at least in this case.",
        "start": 5024.08,
        "duration": 3.92
    },
    {
        "text": "Because if the stack is growing up\nand this is the top of foo's frame",
        "start": 5028.0,
        "duration": 4.47
    },
    {
        "text": "and it's use of the array\nc is by design going down,",
        "start": 5032.47,
        "duration": 3.91
    },
    {
        "text": "that's all fine if the user only\nprovides a string of length 12 or less,",
        "start": 5036.38,
        "duration": 4.38
    },
    {
        "text": "12 or shorter, but in this case if\nthe adversary types in a 20 character",
        "start": 5040.76,
        "duration": 5.04
    },
    {
        "text": "string or 200 character\nstring, the computer",
        "start": 5045.8,
        "duration": 2.65
    },
    {
        "text": "is not just going to stop there.",
        "start": 5048.45,
        "duration": 1.44
    },
    {
        "text": "Unless you wrote code\nwith an if condition",
        "start": 5049.89,
        "duration": 1.86
    },
    {
        "text": "to check the length that the user\nis trying to pass in and protect",
        "start": 5051.75,
        "duration": 3.92
    },
    {
        "text": "against that, it's\ngoing to overwrite all",
        "start": 5055.67,
        "duration": 2.3
    },
    {
        "text": "of these bytes, all of these bytes,\nwhatever this is, all of these bytes,",
        "start": 5057.97,
        "duration": 4.51
    },
    {
        "text": "and this is what's worrisome.",
        "start": 5062.48,
        "duration": 1.64
    },
    {
        "text": "If you have a really smart\nadversary up against you,",
        "start": 5064.12,
        "duration": 3.46
    },
    {
        "text": "trying to hack into your system, and he\nor she is smart enough or lucky enough",
        "start": 5067.58,
        "duration": 4.58
    },
    {
        "text": "to figure out how to inject a pattern\nof bits into your program in this way,",
        "start": 5072.16,
        "duration": 5.61
    },
    {
        "text": "such that he or she overwrites\nthis return address,",
        "start": 5077.77,
        "duration": 3.45
    },
    {
        "text": "a really clever adversary\ncan trick your program",
        "start": 5081.22,
        "duration": 3.46
    },
    {
        "text": "into returning to, not the\nfunction that called foo, main,",
        "start": 5084.68,
        "duration": 4.99
    },
    {
        "text": "but returning to the address\nof the adversary's own function",
        "start": 5089.67,
        "duration": 4.32
    },
    {
        "text": "that he or she has somehow injected\nby way of typing input at the prompt,",
        "start": 5093.99,
        "duration": 5.5
    },
    {
        "text": "in this case.",
        "start": 5099.49,
        "duration": 1.11
    },
    {
        "text": "In other words, if the user types in\njust, hello, h-e-l-l-o, backslash, 0,",
        "start": 5100.6,
        "duration": 4.62
    },
    {
        "text": "all is fine and good and it\nfits perfectly in that frame.",
        "start": 5105.22,
        "duration": 3.05
    },
    {
        "text": "But because we didn't\nhave good error checking",
        "start": 5108.27,
        "duration": 1.94
    },
    {
        "text": "on the length of the\nmemory we're copying,",
        "start": 5110.21,
        "duration": 2.79
    },
    {
        "text": "suppose the adversary includes a\nwhole bunch of adversarial code, which",
        "start": 5113.0,
        "duration": 3.49
    },
    {
        "text": "the author here abstracted away as a,\na, a, adversary, or attack, attack,",
        "start": 5116.49,
        "duration": 4.14
    },
    {
        "text": "attack, what might actually happen here?",
        "start": 5120.63,
        "duration": 2.79
    },
    {
        "text": "Well, if that adversary gets\nlucky enough or is smart enough,",
        "start": 5123.42,
        "duration": 4.28
    },
    {
        "text": "he or she might be able to\noverwrite these 4 bytes here",
        "start": 5127.7,
        "duration": 5.16
    },
    {
        "text": "with the address, brilliantly,\nof his or her own attack code,",
        "start": 5132.86,
        "duration": 5.87
    },
    {
        "text": "which is the bits or the characters\nthat he or she typed at the prompt.",
        "start": 5138.73,
        "duration": 3.51
    },
    {
        "text": "So again, a lot of this is luck.",
        "start": 5142.24,
        "duration": 1.46
    },
    {
        "text": "Or a lot of this is trial\nand error sometimes,",
        "start": 5143.7,
        "duration": 2.08
    },
    {
        "text": "to attack a program in\nthis way, but if you",
        "start": 5145.78,
        "duration": 2.28
    },
    {
        "text": "can trick the computer into\njumping to an address that",
        "start": 5148.06,
        "duration": 3.82
    },
    {
        "text": "happens to point at data you\ninjected into the program,",
        "start": 5151.88,
        "duration": 4.39
    },
    {
        "text": "you have effectively taken\nover that user's program,",
        "start": 5156.27,
        "duration": 2.83
    },
    {
        "text": "tricking the program into running\nany and all code that in this case",
        "start": 5159.1,
        "duration": 5.04
    },
    {
        "text": "you provided by a arg-v. So if you've\never heard of a server getting hacked,",
        "start": 5164.14,
        "duration": 3.59
    },
    {
        "text": "or in the future you will hear\nof a server getting hacked,",
        "start": 5167.73,
        "duration": 2.48
    },
    {
        "text": "or a program getting compromised,\ncould mean any number of things,",
        "start": 5170.21,
        "duration": 3.1
    },
    {
        "text": "but one of them, and an\nall too common approach,",
        "start": 5173.31,
        "duration": 2.44
    },
    {
        "text": "is that the programmer who wrote\nthe software used a buffer,",
        "start": 5175.75,
        "duration": 4.68
    },
    {
        "text": "used an array, a chunk\nof memory, and he or she",
        "start": 5180.43,
        "duration": 2.24
    },
    {
        "text": "did not check the boundaries\nof that array sufficiently.",
        "start": 5182.67,
        "duration": 4.56
    },
    {
        "text": "Did not make sure that\nhis or her own code",
        "start": 5187.23,
        "duration": 1.79
    },
    {
        "text": "was not going too far as was\nthis particular example here.",
        "start": 5189.02,
        "duration": 5.64
    },
    {
        "text": "So now that we have this\npower of going anywhere",
        "start": 5194.66,
        "duration": 2.77
    },
    {
        "text": "we want in memory, it's incredibly easy\nto accidentally or maliciously use,",
        "start": 5197.43,
        "duration": 5.36
    },
    {
        "text": "but that's it.",
        "start": 5202.79,
        "duration": 1.03
    },
    {
        "text": "That boils down to just\nthis basic understanding",
        "start": 5203.82,
        "duration": 2.01
    },
    {
        "text": "of what's going on underneath\nthe hood and what you can now",
        "start": 5205.83,
        "duration": 2.458
    },
    {
        "text": "do with the computer's memory.",
        "start": 5208.288,
        "duration": 2.322
    },
    {
        "text": "And now, let's transition to the\nsecond of our real world demands.",
        "start": 5213.57,
        "duration": 4.81
    },
    {
        "text": "Now that we have the ability\nto talk about memory addresses",
        "start": 5218.38,
        "duration": 3.1
    },
    {
        "text": "and actually do things at\nthis lower level, turns out,",
        "start": 5221.48,
        "duration": 3.44
    },
    {
        "text": "we can start solving some really\ninteresting problems, among them",
        "start": 5224.92,
        "duration": 3.0
    },
    {
        "text": "related to images and forensics, the art\nand science of recovering information.",
        "start": 5227.92,
        "duration": 5.69
    },
    {
        "text": "In fact, you might recall from various\nTV shows or movies, that it's all",
        "start": 5233.61,
        "duration": 3.28
    },
    {
        "text": "too common for the good guys\nin the movies and the TV",
        "start": 5236.89,
        "duration": 3.382
    },
    {
        "text": "shows to be looking over\nthe shoulder of a tech",
        "start": 5240.272,
        "duration": 1.958
    },
    {
        "text": "and seeing some footage of a burglary\nor some other crime that's committed",
        "start": 5242.23,
        "duration": 3.38
    },
    {
        "text": "and say, hey, can you clean that up?",
        "start": 5245.61,
        "duration": 1.74
    },
    {
        "text": "Or can you enhance that?",
        "start": 5247.35,
        "duration": 1.77
    },
    {
        "text": "And, indeed, let's take a look at\none such clip from the real world",
        "start": 5249.12,
        "duration": 3.56
    },
    {
        "text": "and see how it relates\nto the actual world",
        "start": 5252.68,
        "duration": 2.41
    },
    {
        "text": "and how we can leverage some of this\nnew found savvy with memory addresses",
        "start": 5255.09,
        "duration": 3.17
    },
    {
        "text": "and computing to actually solve\nsome of these problems for real.",
        "start": 5258.26,
        "duration": 4.804
    },
    {
        "text": "[VIDEO PLAYBACK]",
        "start": 5263.064,
        "duration": 0.666
    },
    {
        "text": "-He's lying.",
        "start": 5263.73,
        "duration": 1.85
    },
    {
        "text": "-About what?",
        "start": 5265.58,
        "duration": 0.66
    },
    {
        "text": "-I don't know.",
        "start": 5266.24,
        "duration": 2.892
    },
    {
        "text": "-So what do we know?",
        "start": 5269.132,
        "duration": 2.742
    },
    {
        "text": "-That at 9:15 Ray\nSantoya was at the ATM.",
        "start": 5271.874,
        "duration": 3.269
    },
    {
        "text": "-OK, so the question is,\nwhat was he doing at 9:16?",
        "start": 5275.143,
        "duration": 3.967
    },
    {
        "text": "-Shooting the 9 mm at something.",
        "start": 5279.11,
        "duration": 2.81
    },
    {
        "text": "Maybe he saw the sniper.",
        "start": 5281.92,
        "duration": 1.63
    },
    {
        "text": "-Or was working with him.",
        "start": 5283.55,
        "duration": 1.772
    },
    {
        "text": "-Wait.",
        "start": 5285.322,
        "duration": 1.5
    },
    {
        "text": "Go back one.",
        "start": 5286.822,
        "duration": 0.972
    },
    {
        "text": "-What do you see?",
        "start": 5287.794,
        "duration": 0.972
    },
    {
        "text": "-Bring his face up.",
        "start": 5296.056,
        "duration": 1.004
    },
    {
        "text": "Full screen.",
        "start": 5297.06,
        "duration": 3.1
    },
    {
        "text": "-His glasses.",
        "start": 5300.16,
        "duration": 0.66
    },
    {
        "text": "-There's a reflection.",
        "start": 5300.82,
        "duration": 1.5
    },
    {
        "text": "-That's the Nuevita's baseball team.",
        "start": 5312.56,
        "duration": 1.51
    },
    {
        "text": "That's their logo.",
        "start": 5314.07,
        "duration": 1.38
    },
    {
        "text": "-And he's talking to\nwhoever's wearing that jacket.",
        "start": 5315.45,
        "duration": 2.217
    },
    {
        "text": "[END PLAYBACK]",
        "start": 5317.667,
        "duration": 0.583
    },
    {
        "text": "DAVID J. MALAN: OK.",
        "start": 5318.25,
        "duration": 0.8
    },
    {
        "text": "So this is just nonsense.",
        "start": 5319.05,
        "duration": 1.26
    },
    {
        "text": "Like, you can't just\nclean up an image, or look",
        "start": 5320.31,
        "duration": 2.11
    },
    {
        "text": "at something that's very\npixelated, so to speak,",
        "start": 5322.42,
        "duration": 2.0
    },
    {
        "text": "where pixels are dots on the\nscreen, and just kind of zoom in",
        "start": 5324.42,
        "duration": 2.67
    },
    {
        "text": "and clean that up and enhance it.",
        "start": 5327.09,
        "duration": 1.944
    },
    {
        "text": "And, indeed, that's kind\nof this running joke.",
        "start": 5329.034,
        "duration": 1.916
    },
    {
        "text": "Enhance doesn't really\ngenerally mean enhance when you",
        "start": 5330.95,
        "duration": 3.41
    },
    {
        "text": "have a finite amount of information.",
        "start": 5334.36,
        "duration": 1.58
    },
    {
        "text": "For instance, here is a\nfinite amount of information.",
        "start": 5335.94,
        "duration": 2.58
    },
    {
        "text": "A photograph of Zamyla that\nlooks to be of very high quality.",
        "start": 5338.52,
        "duration": 3.53
    },
    {
        "text": "You can see lots of detail.",
        "start": 5342.05,
        "duration": 1.57
    },
    {
        "text": "And so you might think that we can\nmaybe see the reflection of someone",
        "start": 5343.62,
        "duration": 3.47
    },
    {
        "text": "in her eye if we really just\nzoom in and enhance, but no.",
        "start": 5347.09,
        "duration": 4.06
    },
    {
        "text": "This is what you see, and it's actually\nkind of creepy at this distance.",
        "start": 5351.15,
        "duration": 3.94
    },
    {
        "text": "This is what you see when you zoom\nin on that exact same photo of Zamyla",
        "start": 5355.09,
        "duration": 5.06
    },
    {
        "text": "to find that glint in her eye of the\nbad guy, or the badge on his shoulder,",
        "start": 5360.15,
        "duration": 3.4
    },
    {
        "text": "or whatever it was.",
        "start": 5363.55,
        "duration": 1.27
    },
    {
        "text": "There is only a finite\namount of information here.",
        "start": 5364.82,
        "duration": 2.57
    },
    {
        "text": "There is literally only a finite\nnumber of rows and columns",
        "start": 5367.39,
        "duration": 3.39
    },
    {
        "text": "and therefore really big pixels\nfrom which to glean information.",
        "start": 5370.78,
        "duration": 3.5
    },
    {
        "text": "Now, there do exist algorithms and\nsoftware that can smooth this out.",
        "start": 5374.28,
        "duration": 4.26
    },
    {
        "text": "You can enhance the image in the\nsense that you can kind of maybe",
        "start": 5378.54,
        "duration": 3.38
    },
    {
        "text": "tweak the color so that it looks a\nlittle more gradual, the color changes,",
        "start": 5381.92,
        "duration": 3.91
    },
    {
        "text": "and a little less jarring, but\nyou can't just put pixels there,",
        "start": 5385.83,
        "duration": 3.91
    },
    {
        "text": "put bits there that are not\nexistent in the first place.",
        "start": 5389.74,
        "duration": 4.4
    },
    {
        "text": "And so, with this, invites the\nquestion well, then what is an image,",
        "start": 5394.14,
        "duration": 4.64
    },
    {
        "text": "how do you represent it, and what\ncan you actually glean from it",
        "start": 5398.78,
        "duration": 3.17
    },
    {
        "text": "and do from it?",
        "start": 5401.95,
        "duration": 0.84
    },
    {
        "text": "Well, let's consider in the simplest\ncase what an image might be.",
        "start": 5402.79,
        "duration": 3.1
    },
    {
        "text": "Here is an image of a\nwonderfully simple smiley face.",
        "start": 5405.89,
        "duration": 2.88
    },
    {
        "text": "It's black and white.",
        "start": 5408.77,
        "duration": 1.3
    },
    {
        "text": "And if it's black and white, frankly, we\ncan get away with just ones and zeros.",
        "start": 5410.07,
        "duration": 4.05
    },
    {
        "text": "I might arbitrarily say that the number\n1 shall represent the color white",
        "start": 5414.12,
        "duration": 5.22
    },
    {
        "text": "and the number 0 shall represent\nthe color black and, as such,",
        "start": 5419.34,
        "duration": 3.42
    },
    {
        "text": "if I just have a bunch of patterns\nof 8 bits, ones and zeros,",
        "start": 5422.76,
        "duration": 3.92
    },
    {
        "text": "I can effectively think\nof them as a grid.",
        "start": 5426.68,
        "duration": 2.34
    },
    {
        "text": "Rows and columns.",
        "start": 5429.02,
        "duration": 1.36
    },
    {
        "text": "And if I see a 0 pixel, assume it's\na black dot, if I see a 1 pixel,",
        "start": 5430.38,
        "duration": 4.51
    },
    {
        "text": "it's a white dot.",
        "start": 5434.89,
        "duration": 2.03
    },
    {
        "text": "And, as such, I can\ncreate a bitmapped image.",
        "start": 5436.92,
        "duration": 3.63
    },
    {
        "text": "Bitmap, it's like a map, x's and\ny's, of bits, bitmapped image.",
        "start": 5440.55,
        "duration": 4.47
    },
    {
        "text": "There is the simplest, perhaps,\nsmiley face we can draw.",
        "start": 5445.02,
        "duration": 2.994
    },
    {
        "text": "Now it's not all that interesting.",
        "start": 5448.014,
        "duration": 1.416
    },
    {
        "text": "It would be a pretty useless\nphotograph if you only",
        "start": 5449.43,
        "duration": 2.98
    },
    {
        "text": "had dots of that level of detail.",
        "start": 5452.41,
        "duration": 1.599
    },
    {
        "text": "And indeed we saw, when you\nreally zoom in on Zumilah's eyes,",
        "start": 5454.009,
        "duration": 2.541
    },
    {
        "text": "you have really big grids.",
        "start": 5456.55,
        "duration": 1.084
    },
    {
        "text": "And those were colorful, but\nthere isn't that much information.",
        "start": 5457.634,
        "duration": 2.624
    },
    {
        "text": "We need more resolution.",
        "start": 5460.258,
        "duration": 1.082
    },
    {
        "text": "We need more dots,\nmore pixels, more bits.",
        "start": 5461.34,
        "duration": 2.82
    },
    {
        "text": "And so the general case is to use\nnot something as simple as this,",
        "start": 5464.16,
        "duration": 3.3
    },
    {
        "text": "but a more standard format like JPEG.",
        "start": 5467.46,
        "duration": 1.97
    },
    {
        "text": "JPEG is what you see on Facebook\nand on your cameras, most likely.",
        "start": 5469.43,
        "duration": 3.27
    },
    {
        "text": "It's an image format that\nphotographs are commonly stored in,",
        "start": 5472.7,
        "duration": 4.27
    },
    {
        "text": "because it stores millions of\ncolors-- potentially for photographs--",
        "start": 5476.97,
        "duration": 3.7
    },
    {
        "text": "and it also allows you to compress them.",
        "start": 5480.67,
        "duration": 3.19
    },
    {
        "text": "You can compress JPEGs in\nsuch a way that you throw away",
        "start": 5483.86,
        "duration": 3.09
    },
    {
        "text": "some of the 0's and 1's,\nthereby degrading the image.",
        "start": 5486.95,
        "duration": 2.92
    },
    {
        "text": "So it's a little blurrier,\nor a little splotchy,",
        "start": 5489.87,
        "duration": 2.76
    },
    {
        "text": "but at least it's much, much smaller\nand can be emailed or texted or stored",
        "start": 5492.63,
        "duration": 3.83
    },
    {
        "text": "with far less space involved.",
        "start": 5496.46,
        "duration": 2.17
    },
    {
        "text": "And it turns out, if we\nput on our forensics hats",
        "start": 5498.63,
        "duration": 3.41
    },
    {
        "text": "now-- it turns out that JPEGs\nall share something in common.",
        "start": 5502.04,
        "duration": 5.76
    },
    {
        "text": "Any of the JPEGs you see on the\ninternet or have on your hard drive",
        "start": 5507.8,
        "duration": 3.42
    },
    {
        "text": "or on your phone start with the\nsame three bytes, typically.",
        "start": 5511.22,
        "duration": 4.99
    },
    {
        "text": "Specifically, the first three bytes\nare the numbers 255, 216, 255.",
        "start": 5516.21,
        "duration": 7.44
    },
    {
        "text": "Why that?",
        "start": 5523.65,
        "duration": 0.52
    },
    {
        "text": "It's just the standard that was\nadopted some time ago for JPEGs.",
        "start": 5524.17,
        "duration": 4.15
    },
    {
        "text": "It's like a magic number, if you\nwill, at the start of the image.",
        "start": 5528.32,
        "duration": 3.18
    },
    {
        "text": "Now it turns out these\nare decimal numbers.",
        "start": 5531.5,
        "duration": 1.83
    },
    {
        "text": "And frankly, in the world\nof computing, and really",
        "start": 5533.33,
        "duration": 2.97
    },
    {
        "text": "the world of file inputs and\noutput-- file management--",
        "start": 5536.3,
        "duration": 3.147
    },
    {
        "text": "we don't really talk in decimal.",
        "start": 5539.447,
        "duration": 1.333
    },
    {
        "text": "It's just not commonly done.",
        "start": 5540.78,
        "duration": 1.29
    },
    {
        "text": "More commonly done is to talk,\nnot in decimal, not in binary",
        "start": 5542.07,
        "duration": 3.58
    },
    {
        "text": "as we did in week 0, but in hexadecimal.",
        "start": 5545.65,
        "duration": 2.91
    },
    {
        "text": "So it looks a little spooky\nat first, but it really",
        "start": 5548.56,
        "duration": 3.71
    },
    {
        "text": "is an alphabet of 16 characters\ninstead of 10, instead of 2.",
        "start": 5552.27,
        "duration": 4.904
    },
    {
        "text": "And you start counting at 0, as the real\nworld does-- 0, 1, 2, 3, 4, 5, 6, 7, 8,",
        "start": 5557.174,
        "duration": 3.333
    },
    {
        "text": "9-- but there is no 10, because\nthese are single digits.",
        "start": 5560.507,
        "duration": 4.643
    },
    {
        "text": "So after 9 comes A, B, C,\nD, E. So with hexadecimal,",
        "start": 5565.15,
        "duration": 5.13
    },
    {
        "text": "you can effectively count\nfrom 0 to 15 so long",
        "start": 5570.28,
        "duration": 3.95
    },
    {
        "text": "as you borrow some\nletters of the alphabet",
        "start": 5574.23,
        "duration": 1.85
    },
    {
        "text": "to do 10, 11, 12, 13, 14, and 15.",
        "start": 5576.08,
        "duration": 3.83
    },
    {
        "text": "So let's map that onto\nthis pattern of numbers",
        "start": 5579.91,
        "duration": 3.56
    },
    {
        "text": "that demarcates the start of a JPEG.",
        "start": 5583.47,
        "duration": 2.03
    },
    {
        "text": "These numbers, if we do them\nout in binary, equals this.",
        "start": 5585.5,
        "duration": 3.5
    },
    {
        "text": "And we can leave this\nas an exercise at home",
        "start": 5589.0,
        "duration": 2.4
    },
    {
        "text": "if you'd like, but trust\nme, per week 0, that these",
        "start": 5591.4,
        "duration": 4.36
    },
    {
        "text": "are the patterns of bits that are\nequivalent to 255, 216, and 255.",
        "start": 5595.76,
        "duration": 4.74
    },
    {
        "text": "Those are the light bulbs you\nwould need to turn on and off.",
        "start": 5600.5,
        "duration": 2.99
    },
    {
        "text": "Meanwhile, we can add some\nspace there-- so before, after--",
        "start": 5603.49,
        "duration": 5.42
    },
    {
        "text": "and I did this deliberately.",
        "start": 5608.91,
        "duration": 1.78
    },
    {
        "text": "Because it turns out that hexadecimal,\ninsofar as you can count as high as 15",
        "start": 5610.69,
        "duration": 6.91
    },
    {
        "text": "for 16 total characters-- it turns\nout that hexadecimal is really useful.",
        "start": 5617.6,
        "duration": 3.97
    },
    {
        "text": "Because with four bits,\nfour 0's and 1's, you",
        "start": 5621.57,
        "duration": 3.71
    },
    {
        "text": "can count from zero\nall the way up to what?",
        "start": 5625.28,
        "duration": 4.5
    },
    {
        "text": "All the way up to 15, which is perfect,\nbecause if you have four 1 bits,",
        "start": 5629.78,
        "duration": 4.78
    },
    {
        "text": "that's 15.",
        "start": 5634.56,
        "duration": 0.77
    },
    {
        "text": "If you have four 0 bits, that's 0.",
        "start": 5635.33,
        "duration": 2.05
    },
    {
        "text": "And it turns out that patterns\nof 4 bits-- so half of a byte--",
        "start": 5637.38,
        "duration": 4.22
    },
    {
        "text": "map perfectly to hexadecimal characters.",
        "start": 5641.6,
        "duration": 2.47
    },
    {
        "text": "You can express chunks of 4 bits with\nhexadecimal characters perfectly.",
        "start": 5644.07,
        "duration": 4.11
    },
    {
        "text": "As such, it turns out that 15,\nagain, is F. So we have FF.",
        "start": 5648.18,
        "duration": 5.16
    },
    {
        "text": "And it turns out that\n1101 and 1000 from 216.",
        "start": 5653.34,
        "duration": 5.419
    },
    {
        "text": "If you take those 8 bits\nand just spread them out",
        "start": 5658.759,
        "duration": 2.041
    },
    {
        "text": "slightly into two groups of four, is the\nletter D in hexadecimal and the number",
        "start": 5660.8,
        "duration": 4.21
    },
    {
        "text": "8 in hexadecimal.",
        "start": 5665.01,
        "duration": 1.24
    },
    {
        "text": "And then we have another couple\nof 15's-- two sets of four 1 bits.",
        "start": 5666.25,
        "duration": 4.41
    },
    {
        "text": "So that's FF.",
        "start": 5670.66,
        "duration": 1.18
    },
    {
        "text": "Which is to say that the first\nthree bytes at the start of any JPEG",
        "start": 5671.84,
        "duration": 5.23
    },
    {
        "text": "happen to be FF D8 FF.",
        "start": 5677.07,
        "duration": 2.81
    },
    {
        "text": "And it's human convention to prefix\nhexadecimal digits for clarity",
        "start": 5679.88,
        "duration": 4.03
    },
    {
        "text": "in the real world with just 0x.",
        "start": 5683.91,
        "duration": 1.76
    },
    {
        "text": "It means nothing fundamentally\nother than, hey world, here comes",
        "start": 5685.67,
        "duration": 3.17
    },
    {
        "text": "a hexadecimal number, so that the\nworld just knows what it's looking at",
        "start": 5688.84,
        "duration": 3.83
    },
    {
        "text": "and doesn't confuse it for some\nother numeric or base system.",
        "start": 5692.67,
        "duration": 3.2
    },
    {
        "text": "So 0x just means here comes a\nseries of hexadecimal digits.",
        "start": 5695.87,
        "duration": 3.94
    },
    {
        "text": "So that is what is at\nthe beginning of a JPEG.",
        "start": 5699.81,
        "duration": 2.88
    },
    {
        "text": "And in fact, one of the problems\nwe put before you in CS50 each year",
        "start": 5702.69,
        "duration": 4.33
    },
    {
        "text": "is to recover some JPEGs.",
        "start": 5707.02,
        "duration": 2.81
    },
    {
        "text": "In fact, it's all too common,\nunfortunately, to lose photographs",
        "start": 5709.83,
        "duration": 4.01
    },
    {
        "text": "from a memory card or to accidentally\ndelete files from your computer.",
        "start": 5713.84,
        "duration": 3.21
    },
    {
        "text": "And typically, you might\nfreak out or be worried",
        "start": 5717.05,
        "duration": 2.4
    },
    {
        "text": "that, damn, I didn't mean to delete\nthose photos or delete those files.",
        "start": 5719.45,
        "duration": 3.86
    },
    {
        "text": "And sometimes they are gone for good.",
        "start": 5723.31,
        "duration": 2.12
    },
    {
        "text": "But it turns out that\ncomputers and even phones",
        "start": 5725.43,
        "duration": 2.34
    },
    {
        "text": "don't necessarily delete\nphotos right away.",
        "start": 5727.77,
        "duration": 1.96
    },
    {
        "text": "They forget where they are, but\nthey don't outright delete them.",
        "start": 5729.73,
        "duration": 2.915
    },
    {
        "text": "They don't change the 0's to 1's and\nthe 1's to 0's unless you start taking",
        "start": 5732.645,
        "duration": 3.125
    },
    {
        "text": "more and more photos immediately.",
        "start": 5735.77,
        "duration": 1.64
    },
    {
        "text": "But if you have accidentally formatted\nor corrupted your memory card",
        "start": 5737.41,
        "duration": 3.56
    },
    {
        "text": "in a phone or a computer, such that\nyou think you've now lost your photos,",
        "start": 5740.97,
        "duration": 4.42
    },
    {
        "text": "what if you did this?",
        "start": 5745.39,
        "duration": 1.14
    },
    {
        "text": "What if you wrote software that iterated\nfrom the start of your memory card",
        "start": 5746.53,
        "duration": 4.49
    },
    {
        "text": "or your phone all the way to the end of\nyour phone or your memory card's memory",
        "start": 5751.02,
        "duration": 4.55
    },
    {
        "text": "looking for this pattern of\nbits, this pattern of numbers",
        "start": 5755.57,
        "duration": 3.9
    },
    {
        "text": "or hexadecimal digits, this pattern\nof 24 bits-- 8 plus 8 plus 8?",
        "start": 5759.47,
        "duration": 4.94
    },
    {
        "text": "With high probability,\nif, upon iterating",
        "start": 5764.41,
        "duration": 2.66
    },
    {
        "text": "through your phone's memory or\ncamera's memory, you see FF D8 and FF,",
        "start": 5767.07,
        "duration": 5.13
    },
    {
        "text": "you can perhaps infer that the following\nmegabytes-- 2 megabytes, however",
        "start": 5772.2,
        "duration": 5.16
    },
    {
        "text": "big the photograph is-- is\nthe entirety of a photo.",
        "start": 5777.36,
        "duration": 3.64
    },
    {
        "text": "And if you just grab those\nbits and save a copy,",
        "start": 5781.0,
        "duration": 2.0
    },
    {
        "text": "maybe you can recover\nall or some of the photo.",
        "start": 5783.0,
        "duration": 2.35
    },
    {
        "text": "And indeed, one of the challenges\nyou'll soon see in our problems",
        "start": 5785.35,
        "duration": 2.904
    },
    {
        "text": "that focus on forensics is\ngoing to be exactly this.",
        "start": 5788.254,
        "duration": 2.166
    },
    {
        "text": "We will accidentally format a memory\ncard from a camera or a phone.",
        "start": 5790.42,
        "duration": 4.47
    },
    {
        "text": "We will give you a forensic image\nof that memory card, so to speak,",
        "start": 5794.89,
        "duration": 3.41
    },
    {
        "text": "a perfect copy of all of the 0's and\n1's therein, give it to you as a file,",
        "start": 5798.3,
        "duration": 4.67
    },
    {
        "text": "and challenge you with writing software\nthat iterates over that file's bytes,",
        "start": 5802.97,
        "duration": 3.79
    },
    {
        "text": "from the 0th byte all the way to the end\nof the file, looking for this pattern.",
        "start": 5806.76,
        "duration": 3.665
    },
    {
        "text": "And any time you see this\npattern, you will need to say,",
        "start": 5810.425,
        "duration": 3.675
    },
    {
        "text": "this is probably a JPEG.",
        "start": 5814.1,
        "duration": 1.51
    },
    {
        "text": "Let me start copying these bytes into\na separate file name-- something.jpg.",
        "start": 5815.61,
        "duration": 5.29
    },
    {
        "text": "And indeed, if you get the code right,\nsuddenly out of this forensic image",
        "start": 5820.9,
        "duration": 4.74
    },
    {
        "text": "pops any number of photographs\nthat we thought were lost forever.",
        "start": 5825.64,
        "duration": 5.48
    },
    {
        "text": "Bitmap is another type of\nimage file-- older, perhaps,",
        "start": 5831.12,
        "duration": 4.89
    },
    {
        "text": "than JPEG and simpler--\ntruly a bitmapped image",
        "start": 5836.01,
        "duration": 2.87
    },
    {
        "text": "with slightly less fancy\nalgorithms for compression.",
        "start": 5838.88,
        "duration": 2.82
    },
    {
        "text": "But if you remember this\nworld, this is perhaps",
        "start": 5841.7,
        "duration": 3.56
    },
    {
        "text": "a perfect example of a bitmap image\nfrom Windows XP back in the day.",
        "start": 5845.26,
        "duration": 4.45
    },
    {
        "text": "The wallpaper or desktop image that many\nof you might have had on your computers",
        "start": 5849.71,
        "duration": 3.69
    },
    {
        "text": "looked like this.",
        "start": 5853.4,
        "duration": 0.87
    },
    {
        "text": "As an aside, if you go poking\naround I think on Wikipedia,",
        "start": 5854.27,
        "duration": 2.5
    },
    {
        "text": "someone wonderfully went\nto this same spot, found it",
        "start": 5856.77,
        "duration": 2.73
    },
    {
        "text": "where the artist had taken\nthis photograph for Microsoft,",
        "start": 5859.5,
        "duration": 3.156
    },
    {
        "text": "and it now looks like this.",
        "start": 5862.656,
        "duration": 1.124
    },
    {
        "text": "So it's not quite the\nbeautiful place it once was,",
        "start": 5863.78,
        "duration": 3.61
    },
    {
        "text": "but that, indeed, is the same hill.",
        "start": 5867.39,
        "duration": 1.494
    },
    {
        "text": "And this is really just\nan opportunity to talk",
        "start": 5868.884,
        "duration": 1.916
    },
    {
        "text": "about this picture,\nwhich is what's really",
        "start": 5870.8,
        "duration": 2.6
    },
    {
        "text": "underneath the hood of that\nbeautiful grassy meadow.",
        "start": 5873.4,
        "duration": 3.6
    },
    {
        "text": "It's this kind of file header.",
        "start": 5877.0,
        "duration": 2.47
    },
    {
        "text": "We very quickly can make nature\nseem very boring quickly with this.",
        "start": 5879.47,
        "duration": 3.34
    },
    {
        "text": "But this is a very complicated,\nconvoluted looking way",
        "start": 5882.81,
        "duration": 3.33
    },
    {
        "text": "of just formalizing\nwhat we mean by a file.",
        "start": 5886.14,
        "duration": 4.22
    },
    {
        "text": "Like, what is a file?",
        "start": 5890.36,
        "duration": 1.09
    },
    {
        "text": "A file is just a bunch of bits, 0's and\n1's, stored somewhere on your laptop,",
        "start": 5891.45,
        "duration": 3.87
    },
    {
        "text": "or desktop, or phone, or wherever.",
        "start": 5895.32,
        "duration": 2.367
    },
    {
        "text": "But what does it mean\nto be a Microsoft Word",
        "start": 5897.687,
        "duration": 1.833
    },
    {
        "text": "file versus a JPEG versus\na bitmap versus an Excel",
        "start": 5899.52,
        "duration": 6.0
    },
    {
        "text": "file versus any number\nof other file formats",
        "start": 5905.52,
        "duration": 2.25
    },
    {
        "text": "that you might have on your computer?",
        "start": 5907.77,
        "duration": 1.79
    },
    {
        "text": "MPEG4, or MP3, or AAC, or any number\nof media formats as well-- well really,",
        "start": 5909.56,
        "duration": 5.63
    },
    {
        "text": "what it means to be a file, is to be\nfilled with 0's and 1's-- that's true--",
        "start": 5915.19,
        "duration": 5.14
    },
    {
        "text": "but to follow a certain pattern of 0's\nand 1's such that the first several",
        "start": 5920.33,
        "duration": 5.08
    },
    {
        "text": "bits or first many bits of a\nfile typically follow a pattern.",
        "start": 5925.41,
        "duration": 4.57
    },
    {
        "text": "And indeed, JPEGs have\na pretty simple pattern.",
        "start": 5929.98,
        "duration": 2.13
    },
    {
        "text": "Those first three bytes,\nthose first 24 bits,",
        "start": 5932.11,
        "duration": 2.14
    },
    {
        "text": "follow a certain pattern\nof 255, 216, 255.",
        "start": 5934.25,
        "duration": 3.86
    },
    {
        "text": "And that demarcates the start of a JPEG.",
        "start": 5938.11,
        "duration": 1.889
    },
    {
        "text": "And there's some more\ncomplexity therein, too.",
        "start": 5939.999,
        "duration": 1.916
    },
    {
        "text": "In a bitmap, this looks\noverwhelming at first glance,",
        "start": 5941.915,
        "duration": 3.135
    },
    {
        "text": "but this is how the world\ndecided to standardize",
        "start": 5945.05,
        "duration": 2.47
    },
    {
        "text": "what's inside of the first\nfew bytes of a bitmap",
        "start": 5947.52,
        "duration": 3.13
    },
    {
        "text": "file like that grassy\nmeadow from Windows XP.",
        "start": 5950.65,
        "duration": 2.66
    },
    {
        "text": "And we'll come back to this in more\ndetail and the problem set before long.",
        "start": 5953.31,
        "duration": 3.69
    },
    {
        "text": "But you'll notice some identifiers\nhere that might jump out at you.",
        "start": 5957.0,
        "duration": 3.39
    },
    {
        "text": "So width, height, size, compression--\nso there are some key words in there",
        "start": 5960.39,
        "duration": 5.549
    },
    {
        "text": "that might make some intuitive sense.",
        "start": 5965.939,
        "duration": 1.541
    },
    {
        "text": "And indeed, somewhere inside of a\nbitmap image, like that grassy meadow,",
        "start": 5967.48,
        "duration": 3.96
    },
    {
        "text": "is information stored, like\nwhat is the width of that image?",
        "start": 5971.44,
        "duration": 2.5
    },
    {
        "text": "What is the height of that image?",
        "start": 5973.94,
        "duration": 1.374
    },
    {
        "text": "Is it compressed, and how?",
        "start": 5975.314,
        "duration": 1.116
    },
    {
        "text": "And then more interestingly,\nat the bottom of this picture",
        "start": 5976.43,
        "duration": 3.64
    },
    {
        "text": "is the beginning of what bitmap calls\nRGB triples-- Red, Green, Blue triples.",
        "start": 5980.07,
        "duration": 5.99
    },
    {
        "text": "It turns out with\npatterns of three bytes,",
        "start": 5986.06,
        "duration": 5.42
    },
    {
        "text": "you can represent, top to bottom,\nleft to right, essentially, the color",
        "start": 5991.48,
        "duration": 4.26
    },
    {
        "text": "of every one of the dots in an image.",
        "start": 5995.74,
        "duration": 2.92
    },
    {
        "text": "So when we enhanced Zamyla's image, and\nyou literally saw those really big dots",
        "start": 5998.66,
        "duration": 3.68
    },
    {
        "text": "or pixels.",
        "start": 6002.34,
        "duration": 0.61
    },
    {
        "text": "Each of those dots or pixels\nhas 24 bits representing",
        "start": 6002.95,
        "duration": 4.1
    },
    {
        "text": "its color-- how much red,\nhow much green, how much blue",
        "start": 6007.05,
        "duration": 2.54
    },
    {
        "text": "per our discussion of colors in week 0.",
        "start": 6009.59,
        "duration": 2.7
    },
    {
        "text": "And so that's what most\nof a bitmap file is here.",
        "start": 6012.29,
        "duration": 2.43
    },
    {
        "text": "The dot dot dot just means these\ncan go on for quite some time.",
        "start": 6014.72,
        "duration": 3.362
    },
    {
        "text": "What color is this dot?",
        "start": 6018.082,
        "duration": 0.958
    },
    {
        "text": "What color is this dot?",
        "start": 6019.04,
        "duration": 0.958
    },
    {
        "text": "What color is this dot?",
        "start": 6019.998,
        "duration": 0.972
    },
    {
        "text": "All of that is standardized\ninside of the file.",
        "start": 6020.97,
        "duration": 3.42
    },
    {
        "text": "And it turns out in C, we can express\nand represent that kind of structure",
        "start": 6024.39,
        "duration": 5.78
    },
    {
        "text": "using one last keyword for this week.",
        "start": 6030.17,
        "duration": 2.62
    },
    {
        "text": "In C, there is also a keyword\ncalled struct-- structure--",
        "start": 6032.79,
        "duration": 5.12
    },
    {
        "text": "that allows you to create\nan actual structure",
        "start": 6037.91,
        "duration": 2.64
    },
    {
        "text": "inside of which you store information.",
        "start": 6040.55,
        "duration": 2.02
    },
    {
        "text": "For instance, the programming\nlanguage C does not",
        "start": 6042.57,
        "duration": 2.73
    },
    {
        "text": "come with a data type\nthat represents students.",
        "start": 6045.3,
        "duration": 2.14
    },
    {
        "text": "It comes with ints, and chars,\nand floats-- heck, it doesn't even",
        "start": 6047.44,
        "duration": 2.98
    },
    {
        "text": "come with string.",
        "start": 6050.42,
        "duration": 1.08
    },
    {
        "text": "It certainly doesn't come with student.",
        "start": 6051.5,
        "duration": 2.0
    },
    {
        "text": "But I would generally\nthink of a student as being",
        "start": 6053.5,
        "duration": 2.647
    },
    {
        "text": "the combination of a few things.",
        "start": 6056.147,
        "duration": 1.333
    },
    {
        "text": "And let's keep it simple.",
        "start": 6057.48,
        "duration": 1.041
    },
    {
        "text": "A student is a combination of the\nstudent's name and their dorm.",
        "start": 6058.521,
        "duration": 3.759
    },
    {
        "text": "And bunches of other detail, but for\nnow, let's just focus on name and dorm.",
        "start": 6062.28,
        "duration": 4.1
    },
    {
        "text": "Now the syntax is a\nlittle new for us here,",
        "start": 6066.38,
        "duration": 1.81
    },
    {
        "text": "but all this is saying is, hey,\nC, define a type for me that",
        "start": 6068.19,
        "duration": 5.64
    },
    {
        "text": "is a structure containing two\nfields inside of it, name and dorm,",
        "start": 6073.83,
        "duration": 4.64
    },
    {
        "text": "and call this new type students.",
        "start": 6078.47,
        "duration": 1.984
    },
    {
        "text": "So it's a little more complicated\nthan our typedef for a string,",
        "start": 6080.454,
        "duration": 2.666
    },
    {
        "text": "because a string was just\na synonym for char star.",
        "start": 6083.12,
        "duration": 2.55
    },
    {
        "text": "This case, a student, is a synonym\nfor this container of things.",
        "start": 6085.67,
        "duration": 4.77
    },
    {
        "text": "It's like encapsulating two\npieces of data-- name and dorm.",
        "start": 6090.44,
        "duration": 4.04
    },
    {
        "text": "But this is really useful as follows.",
        "start": 6094.48,
        "duration": 3.5
    },
    {
        "text": "With this kind of C code, can\nwe now do things like this?",
        "start": 6097.98,
        "duration": 4.83
    },
    {
        "text": "Let me go ahead and open up structs.h.",
        "start": 6102.81,
        "duration": 3.501
    },
    {
        "text": "And you'll see a little\nheader file I whipped up",
        "start": 6106.311,
        "duration": 1.999
    },
    {
        "text": "here that contains very\nlittle information,",
        "start": 6108.31,
        "duration": 3.27
    },
    {
        "text": "just that same typedef\nfor struct, but I've also",
        "start": 6111.58,
        "duration": 4.0
    },
    {
        "text": "included CS50.h in my own header file.",
        "start": 6115.58,
        "duration": 3.02
    },
    {
        "text": "Why?",
        "start": 6118.6,
        "duration": 0.552
    },
    {
        "text": "Well, just for today's\npurposes, I wanted",
        "start": 6119.152,
        "duration": 1.708
    },
    {
        "text": "to relax our discussion of char star\nfor a moment and talk about string.",
        "start": 6120.86,
        "duration": 4.68
    },
    {
        "text": "But technically, I could just do this.",
        "start": 6125.54,
        "duration": 2.87
    },
    {
        "text": "And technically I could do this.",
        "start": 6128.41,
        "duration": 1.74
    },
    {
        "text": "And then I could get rid of that,\nbecause now this is true C code.",
        "start": 6130.15,
        "duration": 3.81
    },
    {
        "text": "There is no need for the CS50 library.",
        "start": 6133.96,
        "duration": 1.73
    },
    {
        "text": "But either is equivalent, and whatever\nyou're more comfortable with for now",
        "start": 6135.69,
        "duration": 3.125
    },
    {
        "text": "is certainly fine.",
        "start": 6138.815,
        "duration": 2.395
    },
    {
        "text": "But let me now open up structs-0.c\nand do something with this.",
        "start": 6141.21,
        "duration": 5.32
    },
    {
        "text": "So in this program, notice\nthat the top of this file, I",
        "start": 6146.53,
        "duration": 2.53
    },
    {
        "text": "first define a value\ncalled students to be 3.",
        "start": 6149.06,
        "duration": 4.47
    },
    {
        "text": "This is an example of a constant.",
        "start": 6153.53,
        "duration": 1.47
    },
    {
        "text": "Sharp define or pound define here\nsays, define, not a variable,",
        "start": 6155.0,
        "duration": 5.06
    },
    {
        "text": "but a constant-- and the\nconvention in programming",
        "start": 6160.06,
        "duration": 2.77
    },
    {
        "text": "is to use all capital letters--\ndefine a constant equal to some value.",
        "start": 6162.83,
        "duration": 3.37
    },
    {
        "text": "This is not a variable\nwhose value can change.",
        "start": 6166.2,
        "duration": 2.46
    },
    {
        "text": "This is just a keyword,\nSTUDENTS, in all caps,",
        "start": 6168.66,
        "duration": 2.69
    },
    {
        "text": "that is fixed now on that value 3.",
        "start": 6171.35,
        "duration": 2.47
    },
    {
        "text": "And it's one way to define a constant\nin C, a value that does not change.",
        "start": 6173.82,
        "duration": 3.47
    },
    {
        "text": "What am I now going\nto do with this value?",
        "start": 6177.29,
        "duration": 1.89
    },
    {
        "text": "I first, here, declare a variable called\nstudents containing three students.",
        "start": 6179.18,
        "duration": 5.334
    },
    {
        "text": "So it's a little confusing that we've\ngot students, STUDENTS, student,",
        "start": 6184.514,
        "duration": 2.916
    },
    {
        "text": "but they all mean different things.",
        "start": 6187.43,
        "duration": 1.458
    },
    {
        "text": "students is just going\nby the name of my array.",
        "start": 6188.888,
        "duration": 2.552
    },
    {
        "text": "STUDENTS in all caps is the value\n3 from that constant up above.",
        "start": 6191.44,
        "duration": 4.12
    },
    {
        "text": "And then student is the type of this\narray per the header file, structs.h.",
        "start": 6195.56,
        "duration": 6.63
    },
    {
        "text": "So this is just really a\nsuccinct way of saying,",
        "start": 6202.19,
        "duration": 2.33
    },
    {
        "text": "hey, computer, give me an array of size\n3 inside of which are three students,",
        "start": 6204.52,
        "duration": 6.1
    },
    {
        "text": "ultimately.",
        "start": 6210.62,
        "duration": 1.07
    },
    {
        "text": "Now here's an array that\niterates from i up to three.",
        "start": 6211.69,
        "duration": 3.06
    },
    {
        "text": "I print out name colon, prompting\nthe user with getstring for a name.",
        "start": 6214.75,
        "duration": 4.14
    },
    {
        "text": "And then notice this new operator dot.",
        "start": 6218.89,
        "duration": 3.6
    },
    {
        "text": "Because struct is being used, and\nstudents bracket i is a structure, not",
        "start": 6222.49,
        "duration": 6.62
    },
    {
        "text": "an int, or a float, or a string itself,\nwe have to go inside of that structure",
        "start": 6229.11,
        "duration": 4.42
    },
    {
        "text": "to get its field called name.",
        "start": 6233.53,
        "duration": 1.79
    },
    {
        "text": "So dot name says, here's the\nstructure student-- go inside,",
        "start": 6235.32,
        "duration": 3.35
    },
    {
        "text": "and get at the name field, and\nput at that name field whatever",
        "start": 6238.67,
        "duration": 3.25
    },
    {
        "text": "the return value of getstring is.",
        "start": 6241.92,
        "duration": 1.549
    },
    {
        "text": "And do the same thing for dorm.",
        "start": 6243.469,
        "duration": 1.291
    },
    {
        "text": "Go into the i-th student's structure,\nand store at the dorm field",
        "start": 6244.76,
        "duration": 5.35
    },
    {
        "text": "the string that has been typed in next.",
        "start": 6250.11,
        "duration": 2.28
    },
    {
        "text": "Now let's scroll down a\nlittle lower and take a look",
        "start": 6252.39,
        "duration": 2.2
    },
    {
        "text": "at what this program proceeds to do.",
        "start": 6254.59,
        "duration": 2.25
    },
    {
        "text": "It's a very simple loop that sort\nof, just for demonstration's sake,",
        "start": 6256.84,
        "duration": 3.64
    },
    {
        "text": "says so-and-so is in such-and-such,\nprinting out the student's name",
        "start": 6260.48,
        "duration": 4.47
    },
    {
        "text": "and dorm respectively.",
        "start": 6264.95,
        "duration": 1.66
    },
    {
        "text": "In other words, if I go down here\nand compile structs-0, and then",
        "start": 6266.61,
        "duration": 3.9
    },
    {
        "text": "run structs-0-- I'm going\nto go ahead and type",
        "start": 6270.51,
        "duration": 2.56
    },
    {
        "text": "in David Mather and Rob and\nKirkland and Zumilah and Courier,",
        "start": 6273.07,
        "duration": 8.7
    },
    {
        "text": "and you see literally a\nregurgitation of what I wrote.",
        "start": 6281.77,
        "duration": 3.04
    },
    {
        "text": "But what's key here is the\nsyntax that I've introduced.",
        "start": 6284.81,
        "duration": 2.59
    },
    {
        "text": "I'm using a struct, and I'm\naccessing its properties or fields",
        "start": 6287.4,
        "duration": 3.39
    },
    {
        "text": "within by way of this dot operator.",
        "start": 6290.79,
        "duration": 2.06
    },
    {
        "text": "Just means go inside the\nstruct and get that value.",
        "start": 6292.85,
        "duration": 2.56
    },
    {
        "text": "But more cool than that is that\nnow that we understand files,",
        "start": 6295.41,
        "duration": 5.38
    },
    {
        "text": "and now that we have the ability to\nstore arbitrary structures that did not",
        "start": 6300.79,
        "duration": 3.51
    },
    {
        "text": "come with C, so to speak,\nnotice what else we can do.",
        "start": 6304.3,
        "duration": 4.24
    },
    {
        "text": "In this version of the program,\nI'm adding one final feature,",
        "start": 6308.54,
        "duration": 4.54
    },
    {
        "text": "combining today's discussion\nof pointers with this most",
        "start": 6313.08,
        "duration": 3.9
    },
    {
        "text": "recent discussion of files.",
        "start": 6316.98,
        "duration": 2.42
    },
    {
        "text": "It turns out, in C-- and\nit's a little inconsistent--",
        "start": 6319.4,
        "duration": 2.65
    },
    {
        "text": "there is a data type called FILE in all\ncapital letters, for whatever reason.",
        "start": 6322.05,
        "duration": 4.544
    },
    {
        "text": "In lower case here, I'm just saying\nhey, computer, give me a variable called",
        "start": 6326.594,
        "duration": 3.166
    },
    {
        "text": "file of type file star.",
        "start": 6329.76,
        "duration": 2.22
    },
    {
        "text": "That is, give me a\nvariable that will store",
        "start": 6331.98,
        "duration": 2.54
    },
    {
        "text": "a pointer to a file, the\naddress of a file, if you will.",
        "start": 6334.52,
        "duration": 4.04
    },
    {
        "text": "fopen is a new function that\nwe've not used before that says,",
        "start": 6338.56,
        "duration": 4.04
    },
    {
        "text": "hey, browser, open up the\nfile called students.csv",
        "start": 6342.6,
        "duration": 3.86
    },
    {
        "text": "in write mode. w means write mode.",
        "start": 6346.46,
        "duration": 2.41
    },
    {
        "text": "So I want to save a file, not open and\nread a file, but write and save a file.",
        "start": 6348.87,
        "duration": 4.75
    },
    {
        "text": "Sanity check-- is file\nnot equal to null?",
        "start": 6353.62,
        "duration": 2.021
    },
    {
        "text": "If so, we're going to proceed.",
        "start": 6355.641,
        "duration": 1.249
    },
    {
        "text": "I just want to make\nsure nothing went wrong.",
        "start": 6356.89,
        "duration": 1.42
    },
    {
        "text": "And something might go wrong, if maybe\nI don't have permission on the computer",
        "start": 6358.31,
        "duration": 3.18
    },
    {
        "text": "to create files, maybe the computer's\nout of space or something-- something",
        "start": 6361.49,
        "duration": 3.125
    },
    {
        "text": "could go wrong and null is returned per\nthe documentation for fopen or its man",
        "start": 6364.615,
        "duration": 4.545
    },
    {
        "text": "page.",
        "start": 6369.16,
        "duration": 1.01
    },
    {
        "text": "So here I have a loop.",
        "start": 6370.17,
        "duration": 1.22
    },
    {
        "text": "For i from 0 to 3-- so do\nthis 3 times-- fprintf.",
        "start": 6371.39,
        "duration": 3.99
    },
    {
        "text": "So fprintf is identical\nto printf that you've",
        "start": 6375.38,
        "duration": 2.63
    },
    {
        "text": "come to get to know in recent\nweeks, but fprintf is file printf.",
        "start": 6378.01,
        "duration": 4.12
    },
    {
        "text": "And it allows you to print strings,\nnot to the screen, but to a file.",
        "start": 6382.13,
        "duration": 4.72
    },
    {
        "text": "So its first argument\nis the name of the file",
        "start": 6386.85,
        "duration": 2.09
    },
    {
        "text": "that you've opened that\nyou want to print to.",
        "start": 6388.94,
        "duration": 1.95
    },
    {
        "text": "Second argument-- and all the others--\nare just the same as printf as always--",
        "start": 6390.89,
        "duration": 2.94
    },
    {
        "text": "a string, maybe with some\nformat codes, and then some",
        "start": 6393.83,
        "duration": 2.24
    },
    {
        "text": "values that you want to\nplug in here and here.",
        "start": 6396.07,
        "duration": 3.2
    },
    {
        "text": "So if you've ever seen a CSV file in the\nreal world-- a Comma Separated Values,",
        "start": 6399.27,
        "duration": 5.14
    },
    {
        "text": "these are very simple\nMicrosoft Excel files",
        "start": 6404.41,
        "duration": 3.47
    },
    {
        "text": "or Apple Numbers files\nor Google Spreadsheets",
        "start": 6407.88,
        "duration": 2.14
    },
    {
        "text": "can export these as well-- they are just\ntext files whereby all of the fields",
        "start": 6410.02,
        "duration": 4.19
    },
    {
        "text": "are separated by commas.",
        "start": 6414.21,
        "duration": 1.9
    },
    {
        "text": "And so we now, today, have the ability\nnot only to understand what's going on",
        "start": 6416.11,
        "duration": 3.56
    },
    {
        "text": "underneath the hood\nmemory-wise, but we now",
        "start": 6419.67,
        "duration": 2.11
    },
    {
        "text": "have the ability to use the star\noperator-- and therefore pointers more",
        "start": 6421.78,
        "duration": 4.2
    },
    {
        "text": "generally, which are required\nin order to manipulate files",
        "start": 6425.98,
        "duration": 3.39
    },
    {
        "text": "with fopen and this file data type-- and\nwith fprintf can I write information?",
        "start": 6429.37,
        "duration": 6.38
    },
    {
        "text": "Can I print information to those files?",
        "start": 6435.75,
        "duration": 2.17
    },
    {
        "text": "So for the first time ever,\nI have written a program",
        "start": 6437.92,
        "duration": 2.785
    },
    {
        "text": "that, when I run in just a moment,\nis going to have the ability",
        "start": 6440.705,
        "duration": 2.625
    },
    {
        "text": "to save information.",
        "start": 6443.33,
        "duration": 1.89
    },
    {
        "text": "I have effectively implemented the\nequivalent of the File Save menu",
        "start": 6445.22,
        "duration": 2.84
    },
    {
        "text": "for the first time.",
        "start": 6448.06,
        "duration": 0.97
    },
    {
        "text": "Every other program\nwe've written so far just",
        "start": 6449.03,
        "duration": 1.875
    },
    {
        "text": "throws its information and its\nmemory away, but not this time.",
        "start": 6450.905,
        "duration": 3.565
    },
    {
        "text": "Let me go ahead and do\nmake structs-1, and then",
        "start": 6454.47,
        "duration": 8.21
    },
    {
        "text": "dot slash structs-1, and let me go ahead\nand type in David and Mather and Rob",
        "start": 6462.68,
        "duration": 7.8
    },
    {
        "text": "and Kirkland and Zumilah and Courier.",
        "start": 6470.48,
        "duration": 4.5
    },
    {
        "text": "Enter.",
        "start": 6474.98,
        "duration": 1.08
    },
    {
        "text": "Nothing seems to happen.",
        "start": 6476.06,
        "duration": 1.62
    },
    {
        "text": "But let me go to my file browser in the\nIDE, and notice this-- students.csv.",
        "start": 6477.68,
        "duration": 6.25
    },
    {
        "text": "If I open that, I have created this\nfile-- David comma Mather, Rob comma",
        "start": 6483.93,
        "duration": 5.13
    },
    {
        "text": "Kirkland, Zamyla comma Courier.",
        "start": 6489.06,
        "duration": 1.63
    },
    {
        "text": "And if I downloaded this\nfile, double-clicked it",
        "start": 6490.69,
        "duration": 2.54
    },
    {
        "text": "as you can now if you'd like,\nand open it on your Mac or PC,",
        "start": 6493.23,
        "duration": 3.29
    },
    {
        "text": "if you have Numbers installed or Excel\ninstalled or some comparable program.",
        "start": 6496.52,
        "duration": 3.66
    },
    {
        "text": "That should open that program, because\nit will recognize the .csv extension.",
        "start": 6500.18,
        "duration": 3.65
    },
    {
        "text": "And it will display all of those\nnames and all of those dorms",
        "start": 6503.83,
        "duration": 3.44
    },
    {
        "text": "or houses in separate columns.",
        "start": 6507.27,
        "duration": 2.39
    },
    {
        "text": "Because the world decided some\ntime ago that the format known",
        "start": 6509.66,
        "duration": 3.13
    },
    {
        "text": "as CSV, Comma Separated Values,\nis just simple text file",
        "start": 6512.79,
        "duration": 3.34
    },
    {
        "text": "with commas separating values.",
        "start": 6516.13,
        "duration": 3.37
    },
    {
        "text": "And so we now have the ability\nto express all of this and more.",
        "start": 6519.5,
        "duration": 4.65
    },
    {
        "text": "So on the horizon is to solve some\nof these same kinds of problems",
        "start": 6524.15,
        "duration": 4.25
    },
    {
        "text": "and to actually implement for yourself\ncode that writes files and reads files.",
        "start": 6528.4,
        "duration": 4.24
    },
    {
        "text": "But to solve actual problems\nmotivated from the real world domain,",
        "start": 6532.64,
        "duration": 2.75
    },
    {
        "text": "to recover information\nthat's been deliberately",
        "start": 6535.39,
        "duration": 2.07
    },
    {
        "text": "scrambled or obscured,\nto recover information",
        "start": 6537.46,
        "duration": 2.43
    },
    {
        "text": "that's been accidentally\nor maliciously deleted,",
        "start": 6539.89,
        "duration": 2.23
    },
    {
        "text": "to recover things, photographs,\nmemories that you actually",
        "start": 6542.12,
        "duration": 2.85
    },
    {
        "text": "care about-- all with a\nfundamental understanding of what's",
        "start": 6544.97,
        "duration": 3.92
    },
    {
        "text": "going on inside of your own computer.",
        "start": 6548.89,
        "duration": 2.24
    },
    {
        "text": "But until then, keep an eye out for\nsuch TV shows and movies as these.",
        "start": 6551.13,
        "duration": 4.992
    },
    {
        "text": "[VIDEO PLAYBACK]",
        "start": 6556.122,
        "duration": 0.666
    },
    {
        "text": "-OK.",
        "start": 6556.788,
        "duration": 1.482
    },
    {
        "text": "Now let's get a good look at you.",
        "start": 6558.27,
        "duration": 1.976
    },
    {
        "text": "-Hold it.",
        "start": 6567.162,
        "duration": 0.988
    },
    {
        "text": "Run that back.",
        "start": 6568.15,
        "duration": 0.988
    },
    {
        "text": "-Wait a minute.",
        "start": 6569.138,
        "duration": 0.988
    },
    {
        "text": "Go right.",
        "start": 6570.126,
        "duration": 1.814
    },
    {
        "text": "-There.",
        "start": 6571.94,
        "duration": 0.5
    },
    {
        "text": "Freeze that.",
        "start": 6572.44,
        "duration": 0.5
    },
    {
        "text": "-Full screen.",
        "start": 6572.94,
        "duration": 0.792
    },
    {
        "text": "-OK, freeze that.",
        "start": 6573.732,
        "duration": 0.718
    },
    {
        "text": "-Tighten up on that, will you?",
        "start": 6574.45,
        "duration": 1.25
    },
    {
        "text": "-Vector in on that guy\nby the back of you.",
        "start": 6575.7,
        "duration": 1.93
    },
    {
        "text": "-Zoom in right here on this spot.",
        "start": 6577.63,
        "duration": 2.64
    },
    {
        "text": "-With the right equipment, the\nimage can be enlarged and sharpened.",
        "start": 6580.27,
        "duration": 3.86
    },
    {
        "text": "-What's that?",
        "start": 6584.13,
        "duration": 0.703
    },
    {
        "text": "-It's an enhancement program.",
        "start": 6584.833,
        "duration": 1.222
    },
    {
        "text": "-Can you clear that up any?",
        "start": 6586.055,
        "duration": 1.625
    },
    {
        "text": "-I don't know.",
        "start": 6587.68,
        "duration": 1.3
    },
    {
        "text": "Let's enhance it.",
        "start": 6588.98,
        "duration": 1.02
    },
    {
        "text": "-Enhance section A6.",
        "start": 6590.0,
        "duration": 2.52
    },
    {
        "text": "-I enhanced the detail and--",
        "start": 6592.52,
        "duration": 1.302
    },
    {
        "text": "-I think there's enough to enhance.",
        "start": 6593.822,
        "duration": 1.458
    },
    {
        "text": "Release it to my screen.",
        "start": 6595.28,
        "duration": 0.999
    },
    {
        "text": "-Enhance the reflection in her eye.",
        "start": 6596.279,
        "duration": 1.872
    },
    {
        "text": "-Let's run this through\nvideo enhancement.",
        "start": 6598.151,
        "duration": 1.749
    },
    {
        "text": "-Edgar, can you enhance this?",
        "start": 6599.9,
        "duration": 1.208
    },
    {
        "text": "-Hang on.",
        "start": 6601.108,
        "duration": 1.682
    },
    {
        "text": "-I've been working on this reflection.",
        "start": 6602.79,
        "duration": 2.05
    },
    {
        "text": "-Someone's reflection.",
        "start": 6604.84,
        "duration": 1.01
    },
    {
        "text": "-Reflection.",
        "start": 6605.85,
        "duration": 0.954
    },
    {
        "text": "-There's a reflection of the man's face.",
        "start": 6606.804,
        "duration": 1.666
    },
    {
        "text": "-The reflection.",
        "start": 6608.47,
        "duration": 0.666
    },
    {
        "text": "-There's a reflection.",
        "start": 6609.136,
        "duration": 1.084
    },
    {
        "text": "-Zoom in on the mirror.",
        "start": 6610.22,
        "duration": 1.35
    },
    {
        "text": "-You can see a reflection.",
        "start": 6611.57,
        "duration": 2.069
    },
    {
        "text": "-Can you enhance the image from here?",
        "start": 6613.639,
        "duration": 1.541
    },
    {
        "text": "-Can you enhance it right here?",
        "start": 6615.18,
        "duration": 1.04
    },
    {
        "text": "-Can you enhance it?",
        "start": 6616.22,
        "duration": 0.833
    },
    {
        "text": "-Can you enhance it?",
        "start": 6617.053,
        "duration": 0.907
    },
    {
        "text": "-Can we enhance this?",
        "start": 6617.96,
        "duration": 0.77
    },
    {
        "text": "-Can you enhance it?",
        "start": 6618.73,
        "duration": 0.75
    },
    {
        "text": "-Hold on a second.",
        "start": 6619.48,
        "duration": 0.75
    },
    {
        "text": "I'll enhance.",
        "start": 6620.23,
        "duration": 0.59
    },
    {
        "text": "-Zoom in on the door.",
        "start": 6620.82,
        "duration": 1.11
    },
    {
        "text": "-Times 10.",
        "start": 6621.93,
        "duration": 0.984
    },
    {
        "text": "-Zoom.",
        "start": 6622.914,
        "duration": 0.5
    },
    {
        "text": "-Move in.",
        "start": 6623.414,
        "duration": 0.956
    },
    {
        "text": "More.",
        "start": 6624.37,
        "duration": 0.67
    },
    {
        "text": "-Wait, stop.",
        "start": 6625.04,
        "duration": 0.72
    },
    {
        "text": "-Stop.",
        "start": 6625.76,
        "duration": 0.5
    },
    {
        "text": "-Pause it.",
        "start": 6626.26,
        "duration": 0.57
    },
    {
        "text": "-Rotate it 75 degrees\naround the vertical, please.",
        "start": 6626.83,
        "duration": 3.48
    },
    {
        "text": "-Stop.",
        "start": 6630.31,
        "duration": 1.326
    },
    {
        "text": "And go back to the part\nabove the doors again.",
        "start": 6631.636,
        "duration": 3.18
    },
    {
        "text": "-Got an image enhancer that can bitmap.",
        "start": 6634.816,
        "duration": 1.624
    },
    {
        "text": "-Maybe we can use the Pradeep\nSen method see into the windows.",
        "start": 6636.44,
        "duration": 3.182
    },
    {
        "text": "-This software is state of the art.",
        "start": 6639.622,
        "duration": 1.458
    },
    {
        "text": "-The eigenvalue is off.",
        "start": 6641.08,
        "duration": 1.69
    },
    {
        "text": "-With the right\ncombination of algorithms--",
        "start": 6642.77,
        "duration": 2.39
    },
    {
        "text": "-He's taken illumination\nalgorithms to the next level,",
        "start": 6645.16,
        "duration": 2.28
    },
    {
        "text": "and I can use them to\nenhance this photograph.",
        "start": 6647.44,
        "duration": 2.2
    },
    {
        "text": "-Lock on and enlarge the z-axis.",
        "start": 6649.64,
        "duration": 2.63
    },
    {
        "text": "-Enhance.",
        "start": 6652.27,
        "duration": 1.01
    },
    {
        "text": "-Enhance.",
        "start": 6653.28,
        "duration": 0.57
    },
    {
        "text": "-Enhance.",
        "start": 6653.85,
        "duration": 0.6
    },
    {
        "text": "-Freeze and enhance.",
        "start": 6654.45,
        "duration": 1.023
    },
    {
        "text": "[END PLAYBACK]",
        "start": 6655.473,
        "duration": 0.583
    },
    {
        "text": "DAVID J. MALAN: All right.",
        "start": 6656.056,
        "duration": 1.083
    },
    {
        "text": "That's it for CS50.",
        "start": 6657.139,
        "duration": 1.491
    },
    {
        "text": "We will see you next time.",
        "start": 6658.63,
        "duration": 1.737
    },
    {
        "text": "[VIDEO PLAYBACK]",
        "start": 6664.023,
        "duration": 2.567
    },
    {
        "text": "-Everyone knows you went to Yale,\nbut nobody knows what happened.",
        "start": 6666.59,
        "duration": 3.78
    },
    {
        "text": "What can you tell me about that weekend?",
        "start": 6670.37,
        "duration": 1.81
    },
    {
        "text": "What can you tell me about Rosebud?",
        "start": 6672.18,
        "duration": 2.81
    },
    {
        "text": "[DRAMATIC CHORD]",
        "start": 6674.99,
        "duration": 2.4
    },
    {
        "text": "[DRAMATIC MUSIC PLAYING]",
        "start": 6677.39,
        "duration": 24.9
    },
    {
        "text": "[END PLAYBACK]",
        "start": 6702.29,
        "duration": 1.25
    }
]