Re: [Numpy-discussion] Returning numpy scalars in cython functions

18 Nov 2010

      A Thursday 18 November 2010 18:51:04 Keith Goodman escrigué:
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The cython function below returns a long int:
@cython.boundscheck(False)
def mysum(np.ndarray[np.int64_t, ndim=1] a):
    "sum of 1d numpy array with dtype=np.int64."
    cdef Py_ssize_t i
    cdef int asize = a.shape[0]
    cdef np.int64_t asum = 0
    for i in range(asize):
        asum += a[i]
    return asum

What's the best way to make it return a numpy long int, or whatever
it is called, that has dtype, ndim, size, etc. class methods? The
only thing I could come up with is changing the last line to
return np.array(asum)[()]

It works. And adds some overhead:
...
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a = np.arange(10)
timeit mysum(a)
10000000 loops, best of 3: 167 ns per loop
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timeit mysum2(a)
1000000 loops, best of 3: 984 ns per loop
And for scale:
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timeit np.sum(a)
100000 loops, best of 3: 3.3 us per loop
Perhaps the scalar constructor is your best bet:
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type(np.array(2)[()])
<type 'numpy.int64'>
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type(np.int_(2))
<type 'numpy.int64'>
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timeit np.array(2)[()]
1000000 loops, best of 3: 791 ns per loop
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timeit np.int_(2)
1000000 loops, best of 3: 234 ns per loop
-- 
Francesc Alted