Fortran comparison Re: [SciPy-dev] scipy.weave versus simple C++.

Pearu Peterson pearu at cens.ioc.ee
Sun Jan 13 18:42:35 CST 2002


On Sun, 13 Jan 2002, Prabhu Ramachandran wrote:

> Anyway, my question is u(i,j) as referred to in either Python (u[i,j])
> or in blitz or f2py -- do they mean the same thing?  Or is it that
> when I write a loop in fortran that uses a numeric array I must make
> sure that u(i,j) == u[j,i] ??

I am currently reconsidering the Fortran/C array issues. It is looking
promising what I have worked out: u[i,j] in Python and u(i,j) in Fortran
will be the same. And one will not need to think about how matrices
are stored in memory, as a first instance at least. However, if you are
aware about the fact that Fortran and C arrays have different storage
orders, then you can be clever by creating arrays with a proper storage
order that will be good for performance. The approach will be similar to
the one in Pyfort but I think it can be more efficient.

Let me say that with this new approach there will be no performance hit
compared to the current f2py approach. Just the appearance of Fortran and
C multidimensional arrays in Python will be unified and that is a very
important step as then matrices in mathematical sense will coincide.

However, to code this approach efficiently I need a
PyArray_TransposedContiguousFromObject function that does not
exists in Numeric.
If an Object is PyArray already, then I can easily write that
part. However, if an Object is a List or any other Sequence
then one needs to repeat lots of code in Numeric. Basically,
Array_FromSequence needs to be rewritten to TransposedArray_FromSequence
which requires quite good knowledge of Numeric internals which I lack.
A good thing is that PyArrayObject needs not to be changed at all.

If someone has already coded all that in above or at least parts of it, I
would appreciate code examples very much.

Pearu




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