[Numpy-discussion] Array views

Dag Sverre Seljebotn d.s.seljebotn@astro.uio...
Mon Mar 28 02:34:11 CDT 2011


On 03/27/2011 08:54 PM, Sturla Molden wrote:
> Den 26.03.2011 19:31, skrev Christopher Barker:
>
>> To understand all this, you'll need to study up a bit on how numpy
>> arrays lay out and access the memory that they use: they use a concept
>> of "strided" memory. It's very powerful and flexible, but most other
>> numeric libs can't use those same data structures.
> With the ISO C bindings in Fortran 2003, we can get a Fortran pointer
> from a C pointer. This means it is actually possible to pass "strided
> memory" from NumPy to Fortran libraries in a portable way. A Fortran
> pointer is more similar to a NumPy view array than a C pointer. If I am
> not mistaken, fwrap can generate the boiler-plate Cython and Fortran code.

fwrap currently ensures that array are Fortran-contiguous. Supporting 
strided arrays has been in the back of our minds but not much thought 
has been given it really...

What would we do exactly -- pass the entire underlying buffer to Fortran 
and then re-slice it Fortran side? That's more complicated than doing a 
copy Python-side, so unless the Fortran compiler actually supports 
keeping the array strided throughout the code it's not worth it. I'm not 
aware of the rules here -- basically, if you keep an array 
assumed-shape, it can be passed around in non-contiguous form in 
Fortran? And it will be copied whenever it is passed as an 
explicit-shape array?

Dag Sverre


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