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

You may also use sum with zip within the map function:

# In Python 3.x 
>>> list(map(sum, zip(*input_val)))
[3, 6, 9, 12, 15]
# explicitly type-cast it to list as map returns generator expression

# In Python 2.x, explicit type-casting to list is not needed as `map` returns list
>>> map(sum, zip(*input_val))
[3, 6, 9, 12, 15]

Answer 2

I guess you can use:

import numpy as np
new_list = sum(map(np.array, input_val))

Answer 3

output_val=input_val.sum(axis=0)

this would make the code even simpler I guess

Answer 4

In case you decide to use any library, numpy easily does this:

np.sum(input_val,axis=0)

Answer 5

Try this code. This will make output_val end up as [3, 6, 9, 12, 15] given your input_val:

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

vals_length = len(input_val[0])
output_val = [0] * vals_length # init empty output array with 0's
for i in range(vals_length): # iterate for each index in the inputs
    for vals in input_val:
        output_val[i] += vals[i] # add to the same index

print(output_val) # [3, 6, 9, 12, 15]

Answer 6

I think this is the most pythonic way of doing this

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