Count Instances in One Array Based on Another Array

Question

I am wondering if there is an efficient way to perform the following. I have two (numpy) arrays, and I would like to count the number of instances of a value occurring in one based on the criteria of another another array. For example:

a = np.array([1,-1,1,1,-1,-1])
b = np.array([.75,.35,.7,.8,.2,.6])

I would like to calculate c as the number of 1's in a that occur when b > .5, so in this case `c = 3'. My current solution is ugly and would appreciate any suggestions.

jpp · Accepted Answer · 2018-04-05 14:58:25Z

1

You can use numpy.sum for this:

a = np.array([1,-1,1,1,-1,-1])
b = np.array([.75,.35,.7,.8,.2,.6])

np.sum((a == 1) & (b > .5))  # 3

This works because bool is a subclass of int.

answered Apr 5, 2018 at 14:58

jpp

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Comments

Jaime · Accepted Answer · 2018-04-05 21:04:51Z

0

If it's only one condition you are after, try:

np.count_nonzero((a == 1) & (b > .5))

edited Apr 5, 2018 at 21:04

answered Apr 5, 2018 at 14:53

Jaime

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3 Comments

DavidG Over a year ago

Or if the desired answer is 3 then np.count_nonzero?

William Over a year ago

np.count_nonzero((a == 1) & (b > .5)) is what I was looking for. Thanks @Da

Jaime Over a year ago

Indeed! Thanks for the correction, have updated my answer.

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