Merge nested list items based on a repeating value

Question

Although poorly written, this code:

marker_array = [[\'hard\',\'2\',\'soft\'],[\'heavy\',\'2\',\'light\'],[\'rock\',\'2\',\'feather\'],[\'fast\',\'3\'], [\'turtl         


        

Answer 1

A different solution based on itertools.groupby:

from itertools import groupby

# normalizes the list of markers so all markers have 3 elements
def normalized(markers):
    for marker in markers:
        yield marker + [""] * (3 - len(marker))

def concatenated(markers):
  # use groupby to iterator over lists of markers sharing the same key
  for key, markers_in_category in groupby(normalized(markers), lambda m: m[1]):
    # get separate lists of left and right words
    lefts, rights = zip(*[(m[0],m[2]) for m in markers_in_category])
    # remove empty strings from both lists
    lefts, rights = filter(bool, lefts), filter(bool, rights)
    # yield the concatenated entry for this key (also removing the empty string at the end, if necessary)
    yield filter(bool, [" ".join(lefts), key, " ".join(rights)])

The generator concatenated(markers) will yield the results. This code correctly handles the ['fast', '3'] case and doesn't return an additional third element in such cases.