code: public void mapRows() {
              int index = 0;
              for (String a_phrase : filtered_phrases) {
                  String[] curr = a_phrase.split(":");
                  String A = curr[0];
                  String B = curr[1];
                  matrix_row_map.put(new Integer(index), A + ":" + B);
                  index++;
                  //add reverse pair as well
                  matrix_row_map.put(new Integer(index), B + ":" + A);
                  index++;
              }
      }
docs: Maps a list of phrases to the rows of the sparse matrix.
  Takes an ArrayList containing the filtered phrases (originals and alternates) and maps them to the sparse matrix.
  
  @param phrases an ArrayList containing the filtered phrases
  @return void