I have a NumPy array 'boolarr' of boolean type. I want to count the number of elements whose values are True. Is there a NumPy or Python routine dedicated for this task? Or, do I need to iterate over the elements in my script?

解決方案

You have multiple options. Two options are the following.

numpy.sum(boolarr)

numpy.count_nonzero(boolarr)

Here's an example:

>>> import numpy as np

>>> boolarr = np.array([[0, 0, 1], [1, 0, 1], [1, 0, 1]], dtype=np.bool)

>>> boolarr

array([[False, False, True],

[ True, False, True],

[ True, False, True]], dtype=bool)

>>> np.sum(boolarr)

5

Of course, that is a bool-specific answer. More generally, you can use numpy.count_nonzero.

>>> np.count_nonzero(boolarr)

5

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