Re: [Help-gsl] Why is Negative Stride in GSL not allowed?

[Stephan Petzchen]

GREAT. YOU WON THE GAME!

Thanks, so much.

best,

Stephan

On Mon, Nov 19, 2012 at 12:01 PM, Michael Lehn <address@hidden>wrote:

> Am 19.11.2012 um 11:52 schrieb Stephan Petzchen:
>
> > But there is no such code in ATLAS. I did look up
> > It is not only taking the absolut value of incX also the reference
> starting point should be the end of the vector.
> >
> >
> > In all there testcases they set the incX value to absolute (i.e.  Mabs(
> INCX )), which simply follow the rule in one of your former messages: *(X +
> abs(incX)*i) which is not considering the BLAS standard with starting point
> at the end of the vector: X(1 + (n-i) * abs(INCX)).
> >
> > So, my opinion now is: they do not comply with the BLAS standard
> document for negative stride.
> >
> > I hope you or someone other can show me that I am wrong. PLEASE...   ;-)
> >
>
> I just downloaded ATLAS 3.10.  Have a look into
> ATLAS/interfaces/blas/C/src/cblas_daxpy.c.  There
> you find
>
>
> //---- SNIP
> ---------------------------------------------------------------------
>
> void cblas_daxpy(const int N, const double alpha, const double *X,
>                  const int incX, double *Y, const int incY)
> {
>    if (N > 0)
>    {
>       if (incX < 0)
>       {
>          if (incY < 0) ATL_daxpy(N, alpha, X, -incX, Y, -incY);
>          else ATL_daxpy(N, alpha, X+(1-N)*incX, incX, Y, incY);
>       }
>       else if (incY < 0) ATL_daxpy(N, alpha, X+(N-1)*incX, -incX, Y,
> -incY);
>       else ATL_daxpy(N, alpha, X, incX, Y, incY);
>    }
> }
>
> //---- SNAP
> ---------------------------------------------------------------------
>
>
> Cheers,
>
> Michael
>



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