How to make a list of integers that is the sum of all the integers from a set of lists in a dict?

Question

Let's assume I have a created a dict that is made up of n keys. Each key is mapped to a list of integers of a consistent length. What I want to make now is a new list that represents the sum of the integers at each point in lists of the dict. To illustrate:

my_dict = {'a': [1, 2, 3, 4], 'b': [2, 3, 4, 5], 'c': [3, 4, 5, 6]}

total_sum_list = []

for key in my_dict.keys():
    total_sum_list += ###some way of adding the numbers together

Expected output:

total_sum_list = [6,9,12,15]

As demonstrated above, I am not sure how to set up this for loop so that I can create a list like total_sum_list. I have tried putting together a list comprehension, but my efforts have not been successful thus far. Any suggestions?

Answer 1

What you need is to transpose the lists so you can sum the columns. So use zip on the dictionary values (keys can be ignored) and sum in list comprehension:

in one line:

total_sum_list = [sum(x) for x in zip(*my_dict.values())]

result:

[6, 9, 12, 15]

How it works:

zip interleaves the values. I'm using argument unpacking to pass the dict values are arguments to zip (like zip(a,b,c)). So when you do:

for x in zip(*my_dict.values()):
    print(x)

you get (as tuple):

(1, 3, 2)
(2, 4, 3)
(3, 5, 4)
(4, 6, 5)

data are ready to be summed (even in different order, but we don't care since addition is commutative :))