how to vecotrize this? [Matlab]

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From: Thomas on 2 Jul 2010 08:35

"Jos (10584) " <#10584(a)fileexchange.com> wrote in message <i0kl4h$n0r$1(a)fred.mathworks.com>...
> "Thomas " <steinweger(a)tu-harburg.de> wrote in message <i0kike$ajp$1(a)fred.mathworks.com>...
> > "Oleg Komarov" <oleg.komarovRemove.this(a)hotmail.it> wrote in message <i0kcin$9k0$1(a)fred.mathworks.com>...
> > > "Thomas " <steinweger(a)tu-harburg.de> wrote in message <i0kc1v$628$1(a)fred.mathworks.com>...
> > > > How can thsi be vectorized whe lamda has a size of nx3
> > > >
> > > > [l,ind] = sort(lamda,2);
> > > > for i=1:size(lamda,1)
> > > > lamda2(i,:) = lamda(i,ind(i,:));
> > > > end
> > > >
> > > > Thanks thomas
> > >
> > > isequal(l, lambda2) % 1
> > >
> > > Oleg
> >
> > Indeed, l and lamda are equal. But this doesn't matter to my question!
> > If you like treat this problem:
> >
> > lamda = rand(10,3)-0.5;
> > [l,ind] = sort(abs(lamda),2);
> > for i=1:size(lamda,1)
> > lamda2(i,:) = lamda(i,ind(i,:));
> > end
> >
> > Thomas
>
> Oleg gave you a valid question to your original question, so you really should write
>
> "but this doesn't matter to my OTHER question".
>
> Anywats, you might be interested in my (vectorized) function SORTLIND, available on the FEX:
> http://www.mathworks.com/matlabcentral/fileexchange/19443
>
> lamda = rand(10,3)-0.5;
> [unused, ind] = sortlind(abs(lamda),2);
> lamda2 = lamda(ind)
>
> Alternatively, you could pre-allocate lamda2 making the for-loop pretty fast as well:
>
> lamda2 = zeros(size(lamda)) ;
> for i=1:size(lamda,1)
> lamda2(i,:) = lamda(i,ind(i,:));
> end
>
> hth
> Jos

'sortlind' works fine!

Thans a lot, Thomas

From: Thomas on 2 Jul 2010 08:38

From: Oleg Komarov on 2 Jul 2010 09:21

"Jos (10584) "
>
> Oleg gave you a valid question to your original question, so you really should write
>
> "but this doesn't matter to my OTHER question".
>
Thanks Jos.
It was not my intention to make a fuss about the question but the level of requests on this forum vary a lot, consequently I never discard the most trivial solution (which was not per se excluded by the question itself).

Oleg

From: Matt J on 2 Jul 2010 09:49

"Thomas " <steinweger(a)tu-harburg.de> wrote in message <i0kc1v$628$1(a)fred.mathworks.com>...
> How can thsi be vectorized whe lamda has a size of nx3
>
> [l,ind] = sort(lamda,2);
> for i=1:size(lamda,1)
> lamda2(i,:) = lamda(i,ind(i,:));
> end
====================

If lamda is only nx3, I would actually expect a for-loop over columns, as opposed to rows, to be the most efficient approach, faster even than anything vectorized:

for i=1:size(lamda,2)

lamda2(:,i) = lamda(ind(:,i),i);

end

From: Bruno Luong on 2 Jul 2010 10:06

>
> If lamda is only nx3, I would actually expect a for-loop over columns, as opposed to rows, to be the most efficient approach, faster even than anything vectorized:
>
> for i=1:size(lamda,2)
>
> lamda2(:,i) = lamda(ind(:,i),i);
>
> end

Nice try, but I believe that's won't work.

Bruno

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