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Topic: vectorization loops
Replies: 3   Last Post: Feb 5, 2013 2:35 PM

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Steven Lord

Posts: 18,038
Registered: 12/7/04
Re: vectorization loops
Posted: Feb 5, 2013 1:16 PM
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"Christian " <> wrote in message
> Hello,
> does there exist something like an approach for vectorization loops (2, 3
> or 4 nested loops)?

Be more specific, please. There are many different techniques you can use to
write MATLAB code; loops and vectorization are two such techniques.

> I mean using ndgrid, reshape, repmat, sub2ind, ... or whatever.
> I already have read a lot here in the newsgroup and the MATLAB help. Bruno
> show some very good examples here :) but mostly without some explanation.
> So I still encounter difficulties.
> Example for discussion:
> n=3;
> a=rand(n,n);
> for i=1:n
> for j=1:n
> b = b + a(i,j)*a(j,i); % command
> end
> end
> b

So you want to take each element of a, multiply it by its reflection in the
main diagonal, and add up all the resulting values?

eachTerm = a.*a.';
b = sum(eachTerm(:));

The first line performs all the operations a(i, j)*a(j, i) at once using
elementwise multiplication and the nonconjugate transpose (in case a
contains some complex values in your real problem, of which this is a
simplified example.) The second line performs the summation.

Steve Lord
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