einstein summation of boolean arrays in numpy

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Einstein summation (numpy.einsum) of boolean arrays in numpy doesn't produce expected results. Numpy.einsum function does logical operations on boolean arrays, which is questionable in the numeric contexts. 

# summation of a boolean numpy array

x = numpy.array([True, False, True])

print(numpy.sum(x))
# output: 2

print(numpy.einsum('i->', x))
# output: True

For a boolean array x = [True, False, True], I expect that the summation of x is 2, and the result should not depend on the particular choice of the function. However, numpy.sum gave 2, and numpy.einsum gave True.

I am not sure whether I misunderstood something or there is some problem with my code. Any help is appreciated.

numpy-einsum boolean-operations numpy
1 Answers

The difference here is that sum casts the boolean into integers before summing, while einsum skips this step except if you specify it explicitly.

Try:

print(numpy.einsum('i->', x, dtype=int))
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