[Numpy-discussion] Detect if array has been transposed

Nathaniel Smith njs at pobox.com
Sun Oct 12 19:18:37 EDT 2014

On Mon, Oct 13, 2014 at 12:07 AM, Pauli Virtanen <pav at iki.fi> wrote:
> 12.10.2014, 22:16, Eric Firing kirjoitti:
>> On 2014/10/12, 8:29 AM, Pauli Virtanen wrote:
>>> 12.10.2014, 20:19, Mads Ipsen kirjoitti:
>>>> Is there any way for me to detect (on the Python side) that transpose()
>>>> has been invoked on the matrix, and thereby only do the copy operation
>>>> when it really is needed?
>>> The correct way to do this is to, either:
>>> In your C code check PyArray_IS_C_CONTIGUOUS(obj) and raise an error if
>>> it is not. In addition, on the Python side, check for
>>> `a.flags.c_contiguous` and make a copy if it is not.
>>> OR
>>> In your C code, get an handle to the array using PyArray_FromANY (or
>>> PyArray_FromOTF) with NPY_ARRAY_C_CONTIGUOUS requirement set so that it
>>> makes a copy when necessary.
>> or let numpy handle it on the python side:
>> foo(numpy.ascontiguousarray(a))
> Yes, but the C code really should check that the input array is
> C-contiguous, if it only works for C-contiguous inputs.

I.e. your original instructions were correct, but instead of checking
a.flags.c_contiguous by hand etc. the OP should just call
ascontiguousarray which takes care of that part.

Nathaniel J. Smith
Postdoctoral researcher - Informatics - University of Edinburgh

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