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Elemental matrix multiplication [NxN] and [1xn] into [NxNxn]

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William on 31 Mar 2014
Commented: William on 31 Mar 2014
Hi,
I want to multiply every element in A[NxN] by every value in B[1xn] to get C[NxNxn]
Current code:
for i = 1:n
c(:,:,i) = b(i) * a(:,:)
end
Is it possible to do this without the for loop to make it run faster?
Many thanks,

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Accepted Answer

Azzi Abdelmalek on 31 Mar 2014
Edited: Azzi Abdelmalek on 31 Mar 2014
a=rand(4,3)
b=rand(1,6)
[n,m]=size(a)
p=numel(b);
c=reshape(bsxfun(@times, repmat(a(:)',numel(b),1),b')',n,m,p)

2 Comments

John D'Errico on 31 Mar 2014
That seems overly difficult the way you did it. Cleaner is just to reshape B first, THEN use bsxfun. No need at all to use repmat!
C = bsxfun(@times,A,reshape(B,[1,1,numel(B)]));
The idea is that BSXFUN will implicitly replicate as necessary those dimensions that are singleton (i.e., have a 1 in them.)
William on 31 Mar 2014
That's great, faster now. Many thanks

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