How to count the number of true elements in a NumPy bool array
You have multiple options. Two options are the following.
boolarr.sum()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)>>> boolarrarray([[False, False, True], [ True, False, True], [ True, False, True]], dtype=bool)>>> boolarr.sum()5
Of course, that is a bool
-specific answer. More generally, you can use numpy.count_nonzero
.
>>> np.count_nonzero(boolarr)5
That question solved a quite similar question for me and I thought I should share :
In raw python you can use sum()
to count True
values in a list
:
>>> sum([True,True,True,False,False])3
But this won't work :
>>> sum([[False, False, True], [True, False, True]])TypeError...
In terms of comparing two numpy arrays and counting the number of matches (e.g. correct class prediction in machine learning), I found the below example for two dimensions useful:
import numpy as npresult = np.random.randint(3,size=(5,2)) # 5x2 random integer arraytarget = np.random.randint(3,size=(5,2)) # 5x2 random integer arrayres = np.equal(result,target)print resultprint targetprint np.sum(res[:,0])print np.sum(res[:,1])
which can be extended to D dimensions.
The results are:
Prediction:
[[1 2] [2 0] [2 0] [1 2] [1 2]]
Target:
[[0 1] [1 0] [2 0] [0 0] [2 1]]
Count of correct prediction for D=1: 1
Count of correct prediction for D=2: 2