Fast matrix multiplication in loop

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Batuhan
Batuhan 2014-2-7
编辑： Matt J 2014-2-7
Dear All,
I have two matrices with dimensions 3x3 and E6x3. I need to multiply each row of the latter with the former. It's like
a=rand(3,3);
b=(1000000,3);
for i=1:size(b,1)
c=a*b(i,:)';
end
However, this step takes hours to be done. I wonder if there is any faster way to do this.
Cheers.

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采纳的回答

Azzi Abdelmalek
Azzi Abdelmalek 2014-2-7
编辑：Azzi Abdelmalek 2014-2-7
a=rand(3,3);
b=rand(100,3);
n=size(a,2);
m=size(b,1);
c=zeros(m,n);
for i=1:size(b,1)
c(i,:)=a*b(i,:)';
end
%or simply
c=(a*b')'

更多回答（2 个）

Jos (10584)
Jos (10584) 2014-2-7
Two options:
1. pre-allocate C to avoid memory allocation in each iteration
C = zeros(N, ..) % pre-allocation
for k = 1:N,
C(k,:) = ..
end
2. Use BSXFUN
  1 个评论
Batuhan
Batuhan 2014-2-7
Thank you. But I'm not sure if bsxfun is the right one, since
C=bsxfun(@times,a*b) results in
Error using bsxfun Non-singleton dimensions of the two input arrays must match each other.

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Batuhan
Batuhan 2014-2-7
Thank you all. Really, preallocation was the issue and fixes it.
  1 个评论
Matt J
Matt J 2014-2-7
编辑：Matt J 2014-2-7
No, it's crazy to do this with a loop, pre-allocated or otherwise. Just do c=b*a', as Azzi noted.

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