How to sum elements inside a nested list?

Question

So, I have a list that looks something like this

[[[1, 2], [3, 4], [5, 6]],
 [[7, 8], [9, 10], [11, 12]],
 [[13, 14], [15, 16], [17, 18]],
 [[19, 20], [21, 22], [23, 24]]]

And I want it to look like this

[[3, 7, 11],
 [15, 19, 23],
 [27, 31, 35],
 [39, 43, 27]]

that is 3 = sum([1, 2]), 7 = sum([3, 4]), ....

I have tried nesting for loops but I haven't found anything that got the desired result, does anyone know how I could do this?

Please show your attempt with nested loops so we can explain why it didn't give the expected output — roganjosh, Sep 24 '18 at 12:44

ruohola · Answer 1 · 2018-09-24T12:53:07.217

This will do the job quite nicely and imo is more readable than list comprehensions.

lists = [[[1, 2], [3, 4], [5, 6]],
        [[7, 8], [9, 10], [11, 12]],
        [[13, 14], [15, 16], [17, 18]],
        [[19, 20], [21, 22], [23, 24]]]

new_lists = []
for nested in lists:
    new_ls = []
    for ls in nested:
        new_ls.append(sum(ls))
    new_lists.append(new_ls)


>>> new_lists
[[3, 7, 11], [15, 19, 23], [27, 31, 35], [39, 43, 47]]

d_kennetz · Answer 2 · 2018-09-24T14:27:14.523

You could also use list comprehensions:

[[sum(x) for x in triple] for triple in lists]

In the above list comprehension, triple will be your list of three doubles so the first for loop will be covering these. x will then be each list of doubles, inside of the triple so we are summing it, while keeping it inside the original triple by using this bracket around:

[sum(x) for x in triple]

output:

[[3, 7, 11], [15, 19, 23], [27, 31, 35], [39, 43, 47]]

score 0 · Answer 3 · answered Sep 24 '18 at 12:48

If you are happy to use a 3rd party library, you can use NumPy and sum along a single dimension:

import numpy as np

A = np.array([[[1, 2], [3, 4], [5, 6]],
              [[7, 8], [9, 10], [11, 12]],
              [[13, 14], [15, 16], [17, 18]],
              [[19, 20], [21, 22], [23, 24]]])

res = A.sum(2)

Result:

array([[ 3,  7, 11],
       [15, 19, 23],
       [27, 31, 35],
       [39, 43, 47]])

See also: What are the advantages of NumPy over regular Python lists?

How to sum elements inside a nested list?

3 Answers3