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

Question

William 2014-3-31

评论： William 2014-3-31

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,

Answer 1

Azzi Abdelmalek 2014-3-31

编辑：Azzi Abdelmalek 2014-3-31

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)

John D'Errico 2014-3-31

编辑：John D'Errico 2014-3-31

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 2014-3-31

That's great, faster now. Many thanks