code: def average(arr):
    
    if len(arr) == 0:
      sys.stderr.write("ERROR: no content in array to take average\n")
      sys.exit()
    if len(arr) == 1:  return arr[0]
    return float(sum(arr))/float(len(arr))
docs: average of the values, must have more than 0 entries.
  
    :param arr: list of numbers
    :type arr: number[] a number array
    :return: average
    :rtype: float