[Numpy-discussion] Using NumPy iterators in C extension

Irwin Zaid irwin.zaid@physics.ox.ac...
Mon Jun 6 04:06:04 CDT 2011

Hi all,

I am writing a C extension for NumPy that implements a custom function.
I have a minor question about iterators, and their order of iteration,
that I would really appreciate some help with. Here goes...

My function takes a sequence of N-dimensional input arrays and a single
(N+1)-dimensional output array. For each input array, it iterates over
the N dimensions of that array and the leading N dimensions of the
output array. It stores a result in the trailing dimension of the output 
array that is cumulative for the entire sequence of input arrays.

Crucially, I need to be sure that the input and output elements always
match during an iteration, otherwise the output array becomes incorrect. 
Meaning, is the kth element visited in the output array during a single 
iteration the same for each input array?

So far, I have implemented this function using two iterators created
with NpyIter_New(), one for an N-dimensional input array and one for the 
(N+1)-dimensional output array. I call NpyIter_RemoveAxis() on the
output array iterator. I am using NPY_KEEPORDER for the NPY_ORDER flag,
which I suspect is not what I want.

How should I guarantee that I preserve order? Should I be using a
different order flag, like NPY_CORDER? Or should I be creating a
multi-array iterator with NpyIter_MultiNew for a single input array and
the output array, though I don't see how to do that in my case.

Thanks in advance for your help! I can provide more information if
something is unclear.



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