matrix multiplication

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

[James Tursa]

Ummm ... you already did. I presume you really have a more general question in mind? Like a larger size variable? Or ...?

Answer 1

[James Tursa]

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.

[Behrouz]

Thanks.

Answer 2

[Andrei Bobrov]

B = reshape(bsxfun(@times,permute(A,[1 3 2]),permute(A,[3 1 2])),size(A,1),[]).'