[Numpy-discussion] Returning numpy scalars in cython functions

[Francesc Alted]

A Thursday 18 November 2010 18:51:04 Keith Goodman escrigué:
> 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:
> >> a = np.arange(10)
> >> timeit mysum(a)
> 
> 10000000 loops, best of 3: 167 ns per loop
> 
> >> timeit mysum2(a)
> 
> 1000000 loops, best of 3: 984 ns per loop
> 
> And for scale:
> >> timeit np.sum(a)
> 
> 100000 loops, best of 3: 3.3 us per loop

Perhaps the scalar constructor is your best bet:

>>> type(np.array(2)[()])
<type 'numpy.int64'>
>>> type(np.int_(2))
<type 'numpy.int64'>
>>> timeit np.array(2)[()]
1000000 loops, best of 3: 791 ns per loop
>>> timeit np.int_(2)
1000000 loops, best of 3: 234 ns per loop


-- 
Francesc Alted