MATLAB Central Newsreader - Avoiding for loop

Avoiding for loop

Pete sherer — Mon, 09 Mar 2009 22:15:03 -0400

Are there anyway I can avoid using FOR loop for the case shown below?

oriX = [1 2 3 4 5
10 20 30 40 50
100 200 300 400 500
1000 2000 3000 4000 5000];
rowindx = [2 4 1 3 3];
newX= zeros( 1, length(rowindx));

for ii = 1: 5
newX(ii) = oriX( rowindx(ii), ii);
end

newX =

10 2000 3 400 500

Thanks a lot in advance

Re: Avoiding for loop

Matt Fig — Mon, 09 Mar 2009 22:23:01 -0400

[r,c] = size(oriX)
oriX(sub2ind([r,c],rowindx,1:c))

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Re: Avoiding for loop

Nasser Abbasi — Tue, 10 Mar 2009 04:47:01 -0400

"Pete sherer" <tsh@abg.com> wrote in message
news:gp44d7$bgd$1@fred.mathworks.com...
> Are there anyway I can avoid using FOR loop for the case shown below?
>
> oriX = [1 2 3 4 5
> 10 20 30 40 50
> 100 200 300 400 500
> 1000 2000 3000 4000 5000];
> rowindx = [2 4 1 3 3];
> newX= zeros( 1, length(rowindx));
>
> for ii = 1: 5
> newX(ii) = oriX( rowindx(ii), ii);
> end
>
>
> newX =
>
> 10 2000 3 400 500
>
> Thanks a lot in advance

Matt just gave you a solution.

But, I just want to say something here.

Unless there is a huge performance difference (which might well be the
case), I would prefer to see the loop solution.

The loop solution is more clear, and someone who is not a matlab expert can
understand it more easily.

Also, in the future, in the unforeseen event that one wants to 'port' the
code to a different language, it is much easier to convert standard loop
construct as those are standard in many languages.

All what I am saying is, unless performance is a big issue, stick with the
more easy to understand constructs for the reader, even thought it might
take more key strokes to write it.

--Nasser

Re: Avoiding for loop

per isakson — Tue, 10 Mar 2009 06:55:03 -0400

"Nasser Abbasi" <nma@12000.org> wrote in message <ehmtl.15384$as4.12507@nlpi069.nbdc.sbc.com>...
>
> "Pete sherer" <tsh@abg.com> wrote in message
> news:gp44d7$bgd$1@fred.mathworks.com...
> > Are there anyway I can avoid using FOR loop for the case shown below?
> >
> > oriX = [1 2 3 4 5
> > 10 20 30 40 50
> > 100 200 300 400 500
> > 1000 2000 3000 4000 5000];
> > rowindx = [2 4 1 3 3];
> > newX= zeros( 1, length(rowindx));
> >
> > for ii = 1: 5
> > newX(ii) = oriX( rowindx(ii), ii);
> > end
> >
> >
> > newX =
> >
> > 10 2000 3 400 500
> >
> > Thanks a lot in advance
>
> Matt just gave you a solution.
>
> But, I just want to say something here.
>
> Unless there is a huge performance difference (which might well be the
> case), I would prefer to see the loop solution.
>
> The loop solution is more clear, and someone who is not a matlab expert can
> understand it more easily.
>
> Also, in the future, in the unforeseen event that one wants to 'port' the
> code to a different language, it is much easier to convert standard loop
> construct as those are standard in many languages.
>
> All what I am saying is, unless performance is a big issue, stick with the
> more easy to understand constructs for the reader, even thought it might
> take more key strokes to write it.
>
> --Nasser
>

The loop is an order of magnitude fastern than the proposed one-liner on my PC
/per

Re: Avoiding for loop

Matt Fig — Wed, 11 Mar 2009 03:04:02 -0400

"per isakson" <poi.nospam@bimDOTkthDOT.se> wrote in message
> The loop is an order of magnitude fastern than the proposed one-liner on my PC
> /per

As is so often the case. In this particular case the for loop is hidden in sub2ind mcode. So there is the function call overhead and error checking overhead, etc.

I wonder how many releases of Matlab it will take until there are not so many requests to get rid of the 'evil' For loops. It seems to me that there is still a lingering perception that For loops in Matlab MUST be slow. This, of course was understandable prior to 6.5. Since then, maybe not so much...

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