matrix multiplication
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Assume we have the following matrix A = [ 1 2; 3 4; 5 6]; and we want to do the following multiplication
[ A(:,1)*A(:,1)';A(:,2)*A(:,2)'] without loop. So the result should be like this B =
1 3 5
3 9 15
5 15 25
4 8 12
8 16 24
12 24 36
Could you please help me with this problem.
Thanks
1 个评论
James Tursa
2012-1-28
Ummm ... you already did. I presume you really have a more general question in mind? Like a larger size variable? Or ...?
采纳的回答
James Tursa
2012-1-28
Another way:
[m n] = size(A);
Ar = reshape(A,m,1,n);
B = reshape(mtimesx(Ar,Ar,'t'),m,m*n).'
You can find mtimesx on the FEX here:
href=""<http://www.mathworks.com/matlabcentral/fileexchange/25977-mtimesx-fast-matrix-multiply-with-multi-dimensional-support</a>>
Is there a reason why you need the result stacked vertically as you have it? The last transpose is moving all of the data around in memory. You could avoid it if you stacked the results horizontally instead.
更多回答（1 个）
Andrei Bobrov
2012-1-28
B = reshape(bsxfun(@times,permute(A,[1 3 2]),permute(A,[3 1 2])),size(A,1),[]).'
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