Re: vectorise matrix slice extraction

[Rob Mahurin]

On Feb 27, 2009, at 10:58 AM, Francesco Potorti` wrote:
> > > Is there a way to efficiently remove the for loop in this  
> > > assignment?
> > >
> > >   for jj = length(pple):-1:1
> > >     llk(:,:,:,jj) = ll(:,:,:,pple(jj),tple(jj));
> > >   endfor
> >
> > Maybe try
> >
> >         s = size(ll);
> >         s1 = 1:s(1); # etc.
> >         llk( s1,s2,s3, 1:numel(pple) ) =
> >                 reshape( ll(s1,s2,s3, pple, tple), [s(1:3), numel(pple)] );
>
> This would be equivalent to
>   llk = ll(:,:,:,pple,tple);
>
> but it does not work.  Take this as an example:


Right, so it would.  You could do the offsetting yourself, as in

        [i,j] = find(eye(2)); eye(2)(i + 2*(j-1))

which returns [1;1].  So in your case maybe

        s = size(ll);
        ll_tmp = reshape(ll, [s(1:3), prod(s(4:5))]);
        llk = ll_tmp(:,:,:, pple + s(4)*(tple-1) );
        clear ll_tmp;

?

Cheers,
Rob

--
Rob Mahurin
Department of Physics and Astronomy
University of Tennessee                 865 207 2594
Knoxville, TN 37996                     address@hidden