sum columns of part of 2D array Python

Question

INPUT:

M = [[1,2,3],
    [1,2,3],
    [1,2,3]]

how take the sum of the columns in the two first rows and implement it in the array M

OUTPUT:

M = [[2,4,6],
    [1,2,3]]

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– Daniel Bickler, Commented Jun 16, 2017 at 20:04

akuiper · Accepted Answer · 2017-06-16 15:43:50Z

1

Use numpy.add.reduceat:

import numpy as np
M = [[1,2,3],
    [1,2,3],
    [1,2,3]]

np.add.reduceat(M, [0, 2])     
# indices [0,2] splits the list into [0,1] and [2] to add them separately, 
# you can see help(np.add.reduceat) for more

# array([[2, 4, 6],
#        [1, 2, 3]])

answered Jun 16, 2017 at 15:43

akuiper

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Alex Hall · Accepted Answer · 2017-06-16 15:44:31Z

1

M = [[1, 2, 3],
     [4, 5, 6],
     [7, 8, 9]]

M[:2] = [[a + b for a, b in zip(M[0], M[1])]]

print(M)  # [[5, 7, 9], [7, 8, 9]]

Things to google to understand this:

M[:2] =: python slice assignment
[... for .. in ...]: python list comprehension
for a, b in ...: python tuple unpacking loop

answered Jun 16, 2017 at 15:44

Alex Hall

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