Sum elements of list using condition on indexes

Question

I have a list of length L, with some random values:

list1 = [3, 1, 1, 6, 8, 3, 7, 4, 8, 4]

and another list of the same length, with boolean values

list2 = [0, 0, 0, 1, 1, 1, 0, 0, 0, 0]

I want to sum the values of list1 where the corresponding element of list2 is 1. (in this case 6+8+3)

is there an easy way to make it in Python?

What have you done? Did you make any attempts to solve the problem?. Also possible Duplicate of [How can I add the corresponding elements of several lists of numbers?](http://stackoverflow.com/questions/11280536/how-can-i-add-the-corresponding-elements-of-several-lists-of-numbers) — cyberbemon, Sep 02 '14 at 15:58

score 4 · Answer 1 · answered Sep 02 '14 at 15:55

4

You can use itertools.compress with sum:

>>> import itertools
>>> list1 = [3, 1, 1, 6, 8, 3, 7, 4, 8, 4]
>>> list2 = [0, 0, 0, 1, 1, 1, 0, 0, 0, 0]
>>> list(itertools.compress(list1, list2))
[6, 8, 3]
>>> sum(itertools.compress(list1, list2))
17

answered Sep 02 '14 at 15:55

falsetru

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score 2 · Accepted Answer · answered Sep 02 '14 at 15:55

Yes, using sum and itertools.compress:

>>> from itertools import compress
>>> sum(compress(list1, list2))
17

Explanation: compress() takes two iterables, and yields elements from the first if the corresponding element of the second is truthy. sum() ... well, you can probably guess.

score 0 · Answer 3 · answered May 29 '17 at 15:10

You can also do a conditional comprehension with zip:

>>> list1 = [3, 1, 1, 6, 8, 3, 7, 4, 8, 4]
>>> list2 = [0, 0, 0, 1, 1, 1, 0, 0, 0, 0]

>>> sum(item for item, flag in zip(list1, list2) if flag)  
17

The if flag checks if the corresponding item is not-zero. If you want an explicit check for 1:

>>> sum(item for item, flag in zip(list1, list2) if flag == 1)  
17

Sum elements of list using condition on indexes

3 Answers3