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I'm working on Python and I want to make an element-wise sum of each list within 3 lists of lists. I'll try to simplify the problem to explain better.

Input:

a = [['alpha','beta','gamma'],['delta','epsilon','zeta'],['eta','theta','iota']]
b = [['AB'],['CD'],['EF']]
c = [['1','2','3'],['4','5','6'],['7','8','9']]

The outcome I need is:

d = [['alpha','beta','gamma','AB','1','2','3'],['delta','epsilon','zeta','CD','4','5','6'],['eta','theta','iota','EF','7','8','9']]

What I tried is:

d = []
for x in a:
    y = [a[x] + b[x] + c[x]]
    d.append(y)

However I get the error "TypeError: list indices must be integers or slices, not list" because x is defined as a list equal to ['alpha','beta','gamma']

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  • d = [x + y + z for x, y, z in zip(a, b, c)] Commented Jan 12, 2023 at 23:07

3 Answers 3

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Yes, you can 'add up' the sub lists to form new lists in d:

a = [[1,2,3], [4,5,6], [7,8,9]]
b = [[11,12,13], [14,15,16], [17,18,19]]
c = [[21,22,23], [24,25,26], [27,28,29]]

d = []
for a_i,b_i,c_i in zip(a,b,c):
    d.append(a_i + b_i + c_i)

print(d)

Output as requested.

In fact, you can use the built-in sum():

d = []
for items in zip(a, b, c):
    d.append(sum(items, start=[]))

print(d)
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9 Comments

Thank you, but in both cases I get this error: TypeError: can only concatenate list (not "str") to list
@quamrana You can push it even further: d = list(map(lambda items: sum(items, start=[]), zip(a, b, c))).
@TeresaBonserio: I don't see how that can be. I tried the code first, saw the correct output and pasted it here as an answer. Are your variables a, b, c different to what you posted?
Unfortunately I still get this error: TypeError: can only concatenate list (not "str") to list
Yes, the variables I posted are just an example of what I have in reality. The lists of lists contain text, not numbers. I don't know if that could be the reason why
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Try numpy:

import numpy as np
    
a = [[1,2,3],[4,5,6],[7,8,9]]
b = [[11,12,13],[14,15,16],[17,18,19]]
c = [[21,22,23],[24,25,26],[27,28,29]]

a = np.array(a)
b = np.array(b)
c = np.array(c)
    
d = np.concatenate([a, b, c], axis=1)
    
print(d)
#[[ 1  2  3 11 12 13 21 22 23]
#[ 4  5  6 14 15 16 24 25 26]
#[ 7  8  9 17 18 19 27 28 29]]
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3 Comments

Thank you, but I get this error: ValueError: all the input arrays must have same number of dimensions, but the array at index 0 has 2 dimension(s) and the array at index 1 has 1 dimension(s)
@Teresa, try it: '''for x in range(len(a)): y = [a[x] + b[x] + c[x]] d.append(y)'''
If I try it for the original lists of lists I get this error: TypeError: can only concatenate list (not "str") to list. If I try it after converting them to arrays I get this error: UFuncTypeError: ufunc 'add' did not contain a loop with signature matching types (dtype('<U14'), dtype('<U2')) -> None
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You can you the zip function and the operator + at list it will join all the i groups

d = [x + y + z for x, y, z in zip(a, b, c)]
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