MATLAB Answers


How to add matrices dimension by dimension without using a loop?

Asked by Tintin Milou on 17 Feb 2015
Latest activity Commented on by Stephen Cobeldick on 17 Feb 2015
Hi all,
I've got this huge loop that takes a lot of time to be computed:
for i1=1:xN
for j1=1:zN
for i2=1:xN
for j2=1:zN
for j3=1:zN
J(i1,j1,i2,j2,j3) = (ldp(i2,j2,j3) - ld(i1,j1,j2))*pdf_transp(i1,j1,i2,j2,j3);
xN and zN are 300 and 7 for now, but it easily takes several minutes to compute.
I have tried to use repmat and permute and then do something like this
ld_old = permute(repmat(ld,[1 1 1 xN zN]),[1 2 4 3 5]);
ld_new = permute(repmat(ldp,[1 1 1 xN zN]),[4 5 1 2 3]);
J2 = (ld_new(:)-ld_old(:)).*pdf_transp(:);
That is much faster, but still pretty slow. Is there a more efficient way for this kind of operation?

  1 Comment

Instead of using repmat, you might be able to use bsxfun and avoid this whole business. bsxfun can save a lot memory in these kind of situations, which speed things up.

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1 Answer

Answer by James Tursa
on 17 Feb 2015
 Accepted Answer

Don't know if this will be faster than your repmat code, but try this:
J = bsxfun(@minus,reshape(ldp,1,1,i2,j2,j3),reshape(ld,i1,j1,1,j2,1)).*pdf_transp;


Very neat. Thanks, James. Just had to change the i2,j2, and so on to xN and zN.
Yes, that was a typo and good catch by you.
By the way, do you expect running the bsxfun on CUDA would improve the performance by quite a bit?

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