Take two list of lists and returns a new list of lists which is the sum of the values at the matching index positions?

Question

I'm trying to make a function that takes two list of lists and returns the sum of their values.

Let's say I have 2 lists:

list1 = [[7,8], []]
list2 = [[3,4,2],[6,9]]

Expected output:

[[10,12,2],[6,9]]

Or another 2 lists:

list1 = [[], []]
list2 = [[3,4,2],[6,9]]

Expected output:

[[3,4,2],[6,9]]

I think using list comprehension would be the most effective way to accomplish this, but I'm not as familiar with list comprehension as I'd like to be.

I've looked on Stack Overflow, and the closest answer I can find to this question is only with 2 lists that don't contain another list.

Have you made any progress to solving this without loat comprehensions? If so, please show it and any associated errors. — roganjosh, Nov 01 '18 at 19:10
Possible duplicate of [Add SUM of values of two LISTS into new LIST](https://stackoverflow.com/questions/14050824/add-sum-of-values-of-two-lists-into-new-list) — BenG, Nov 01 '18 at 19:10
@BenG-TW, that duplicate does not consider the unequal sizes of the lists — sacuL, Nov 01 '18 at 19:13

score 3 · Accepted Answer · edited Nov 01 '18 at 19:14

You can use nested list comprehensions with zip and itertools.zip_longest:

import itertools
list1 = [[7,8], []]
list2 = [[3,4,2],[6,9]]

>>> [[x+y for x,y in itertools.zip_longest(i1,i2,fillvalue=0)] for i1,i2 in zip(list1,list2)]
[[10, 12, 2], [6, 9]]

list1 = [[], []]
list2 = [[3,4,2],[6,9]]

>>>[[x+y for x,y in itertools.zip_longest(i1,i2,fillvalue=0)] for i1,i2 in zip(list1,list2)]
[[3, 4, 2], [6, 9]]

Take two list of lists and returns a new list of lists which is the sum of the values at the matching index positions?

1 Answers1