How to count the number of true elements in a NumPy bool array

Question

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

User · Answer

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...

User · Answer

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 np result   np random randint 3 size  5 2     5x2 random integer array target   np random randint 3 size  5 2     5x2 random integer array  res   np equal result target  print result print target print 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

User · Answer

You have multiple options  Two options are the following   numpy sum boolarr  numpy count nonzero boolarr    Here s an example    gt  gt  gt  import numpy as np  gt  gt  gt  boolarr   np array   0  0  1    1  0  1    1  0  1    dtype np bool   gt  gt  gt  boolarr array   False  False   True            True  False   True            True  False   True    dtype bool    gt  gt  gt  np sum boolarr  5   Of course  that is a bool-specific answer  More generally  you can use numpy count nonzero    gt  gt  gt  np count nonzero boolarr  5

User · Answer

boolarr sum axis 1 or axis 0   axis   1 will output number of trues in a row and axis   0 will count number of trues in columns so boolarr  true true true   false false true   print boolarr sum axis 1    will be  3 1

[python] How to count the number of true elements in a NumPy bool array

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