[Numpy-discussion] Right way of looping throught ndarrays using Cython
Lisandro Dalcin
dalcinl@gmail....
Fri Jun 20 16:26:40 CDT 2008
On 6/20/08, Gael Varoquaux <gael.varoquaux@normalesup.org> wrote:
> I am trying to figure the right way of looping throught ndarrays using
> Cython, currently. Things seem to have evolved a bit compared to what
> some documents on the web claim (eg "for i from 0<=i<n" does not seem
> faster than "for i in range(n)").
Regarding for loops, Cython recently gained an optimization (perhaps
you already know this, but just in case). If the 'i' variable is a
'cdef' one, then Cython does not actually use 'range', but a plain C
for loop. See yourself:
cdef void foo():
cdef int i, j=0
for i in range(5,20,3):
j += i
The generated C code is (comments stripped):
static void __pyx_f_7fortest_foo(void) {
int __pyx_v_i;
int __pyx_v_j;
__pyx_v_j = 0;
for (__pyx_v_i = 5; __pyx_v_i < 20; __pyx_v_i+=3) {
__pyx_v_j += __pyx_v_i;
}
}
Really, really nice!!!, doesn't it? You can just forget about the 'for
i from ...' form (at least for the 99% of the cases, I think).
Additionally, 'for' loops can now also be written like this (without 'from'):
for 0<= i < n:
do_stuff()
but IMHO, the 'range' optimization is just a big win!
--
Lisandro Dalcín
---------------
Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
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