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Re: vectorization loops
Posted:
Feb 5, 2013 1:16 PM


"Christian " <muell.halde@iwu.fraunhofer.de> wrote in message news:kereqq$rnb$1@newscl01ah.mathworks.com... > 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 slord@mathworks.com To contact Technical Support use the Contact Us link on http://www.mathworks.com



