Numpy Summation for Index in Boolean Array

Question

In Numpy I have a boolean array of equal length of a matrix. I want to run a calculation on the matrix elements that correspond to the boolean array. How do I do this?

a: [true, false, true]
b: [[1,1,1],[2,2,2],[3,3,3]]

Say the function was to sum the elements of the sub arrays

index 0 is True: thus I add 3 to the summation (Starts at zero)

index 1 is False: thus summation remains at 3

index 2 is True: thus I add 9 to the summation for a total of 12

How do I do this (the boolean and summation part; I don't need how to add up each individual sub array)?

ali_m · Accepted Answer · 2015-03-17 01:33:28Z

2

You can simply use your boolean array a to index into the rows of b, then take the sum of the resulting (2, 3) array:

import numpy as np

a = np.array([True, False, True])
b = np.array([[1,1,1],[2,2,2],[3,3,3]])

# index rows of b where a is True (i.e. the first and third row of b)
print(b[a])
# [[1 1 1]
#  [3 3 3]]

# take the sum over all elements in these rows
print(b[a].sum())
# 12

It sounds like you would benefit from reading the numpy documentation on array indexing.

answered Mar 17, 2015 at 1:33

ali_m

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