How to sum columns of an array in Python

Question

How do I add up all of the values of a column in a python array? Ideally I want to do this without importing any additional libraries.

input_val = [[1, 2, 3,         


        

Answer 1

Please construct your array using the NumPy library:

import numpy as np

create the array using the array( ) function and save it in a variable:

 arr = np.array(([1, 2, 3, 4, 5],[1, 2, 3, 4, 5],[1, 2, 3, 4, 5]))

apply sum( ) function to the array specifying it for the columns by setting the axis parameter to zero:

arr.sum(axis = 0)

Answer 2

Try this:

input_val = [[1, 2, 3, 4, 5],
         [1, 2, 3, 4, 5],
         [1, 2, 3, 4, 5]]

output_val = [sum([i[b] for i in input_val]) for b in range(len(input_val[0]))]

print output_val

Answer 3

Using Numpy you can easily solve this issue in one line:

1: Input

input_val = [[1, 2, 3, 4, 5],
             [1, 2, 3, 4, 5],
             [1, 2, 3, 4, 5]]

2: Numpy does the math for you

np.sum(input_val,axis=0)

3: Then finally the results

array([ 3,  6,  9, 12, 15])

Answer 4

One-liner using list comprehensions: for each column (length of one row), make a list of all the entries in that column, and sum that list.

output_val = [sum([input_val[i][j] for i in range(len(input_val))]) \
                 for j in range(len(input_val[0]))]

Answer 5

This should work:

[sum(i) for i in zip(*input_val)]

Answer 6

zip and sum can get that done:

Code:

[sum(x) for x in zip(*input_val)]

zip takes the contents of the input list and transposes them so that each element of the contained lists is produced at the same time. This allows the sum to see the first elements of each contained list, then next iteration will get the second element of each list, etc...

Test Code:

input_val = [[1, 2, 3, 4, 5],
             [1, 2, 3, 4, 5],
             [1, 2, 3, 4, 5]]

print([sum(x) for x in zip(*input_val)])

Results:

[3, 6, 9, 12, 15]