[Numpy-discussion] A minor clarification no why count_nonzero is faster for boolean arrays

Raghav R V ragvrv at gmail.com
Thu Dec 17 12:52:15 EST 2015


I was just playing with `count_nonzero` and found it to be significantly
faster for boolean arrays compared to integer arrays


    >>> a = np.random.randint(0, 2, (100, 5))
    >>> a_bool = a.astype(bool)

    >>> %timeit np.sum(a)
    100000 loops, best of 3: 5.64 µs per loop

    >>> %timeit np.count_nonzero(a)
    1000000 loops, best of 3: 1.42 us per loop

    >>> %timeit np.count_nonzero(a_bool)
    1000000 loops, best of 3: 279 ns per loop (but why?)

I tried looking into the code and dug my way through to this line
<https://github.com/numpy/numpy/blob/c0e48cfbbdef9cca954b0c4edd0052e1ec8a30aa/numpy/core/src/multiarray/item_selection.c#L2172>.
I am unable to dig further.

I know this is probably a trivial question, but was wondering if anyone
could provide insight on why this is so?

Thanks

R
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