How to vectorize this nested loop for optimized performance

hello everybody, I have been trying to optimize this nested for loop which is very very slow, but with no success

    for i=1:size(Sxx,2)-1
                for j=1:size(Sxx,2)-1

                    A=Sxx(:,i).*conj(Sxx(:,j));
                    B= (ifft(A, 'symmetric'));
                    [~, T(i,j)] = max(B(1:round(end/2)));

                end
            end

any help is appreciated!

Answer 1

[Matt J]

My advice would be not to vectorize it completely (although there are ways to do so), but rather reduce it to one loop as below.

   [m,n]=size(Sxx);
   k=1:round(m/2);
   SxxConj=conj(Sxx(:,1:n-1));

     T=nan(n-1);

      for i=1:n-1

        B=ifft(Sxx(:,i).*SxxConj, [],1,'symmetric'); %If <R2016b use bsxfun

        [~,T(i,:)]=max(B(k,:),[],1);  %EDITED

      end

You could also convert this to a parfor loop if the serial performance is insufficient.

[Matt J]

Comment by Yodish:

Definitely faster (probably not enough yet though for my purposes) and more elegant, thanks Matt

PS. index of B needs to be swapped in second last line