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numpyboolean-operationsnumpy-einsum

einstein summation of boolean arrays in numpy

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.

Solution

  • 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))