Eliminating a FOR loop

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Scott
Scott le 2 Sep 2014
Modifié(e) : Andrei Bobrov le 2 Sep 2014
I am trying to eliminate a FOR loop to improve speed. The problem I am working with involves evaluating each element of an array with each element of a matrix. Below is a simplified example of what I am trying to do.
I have two matrices:
A = [a11 a12 a13;a21 a22 a23; a31 a32 a33];
B = [b1 b2 b3 b4];
What I would like to do is evaluate each element of B against each element of A. For example
for i=1:1:4
[I,J] = find(B(i)==A(:,:));
end
Is there a way to do this without a FOR loop.
Thanks in advance,
Scott
  1 commentaire
Guillaume
Guillaume le 2 Sep 2014
Modifié(e) : Guillaume le 2 Sep 2014
Your example overwrites I and J on each pass of the loop, so it's not clear what you want out. Did you mean:
[I{i} J{i}] = find(B(i)==A);

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Matt J
Matt J le 2 Sep 2014
Modifié(e) : Matt J le 2 Sep 2014
L=bsxfun(@eq,A(:),B(:).');
L=reshape(L,[size(A),length(B)]);
[I,J,~]=ind2sub(size(L),find(L));
  1 commentaire
Scott
Scott le 2 Sep 2014
Matt,
Thank you very much. Your answer was exactly what I was looking for.

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Plus de réponses (4)

Iain
Iain le 2 Sep 2014
The function you might be looking for is probably
intersect
Pierre Benoit
Pierre Benoit le 2 Sep 2014
Modifié(e) : Pierre Benoit le 2 Sep 2014
[~,idx] = ismember(B,A);
[I,J] = ind2sub(size(A),idx);
Guillaume
Guillaume le 2 Sep 2014
You can use arrayfun or bsxfun to replace a loop. It's not necessary faster but may make the intent of the code clearer. In this case, assuming you want I and J as cell arrays, not so:
[I, J] = arrayfun(@(b) ind2sub(size(A), find(b==A)), B, 'uni', false);
Andrei Bobrov
Andrei Bobrov le 2 Sep 2014
Modifié(e) : Andrei Bobrov le 2 Sep 2014
A = randi(10,5,6);
B = randi(10,4,1);
[i1,j1] = find(bsxfun(@eq,A,reshape(B,1,1,[])));
out_ij = [i1, rem(j1 - 1,size(A,2)) + 1];

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