Multiply each column in MxN matrix with it's transpose to create MxMxN matrix

Hello,

I have a MxN matrix, and I need to multiply each column vector that is of size Mx1 with it's transpose. This gives me a MxM matrix for each column. I can do this in a for loop, to create this MxM matrix for each column and to save it in a MxMxN matrix. But I would like to speed this up because the for loop is taking too long. Any ideas or hints?

Thanks!

Answer 1

[James Tursa]

X = your MxN matrix
[M,N] = size(X);
Xcols = reshape(X,M,1,N);
Xrows = reshape(X,1,M,N);
result = bsxfun(@times,Xcols,Xrows);

Or if you have R2016b or later, that last line can be

result = Xcols .* Xrows;

Answer 2

[Andrei Bobrov]

Let A - your matrix [M x N]

out = bsxfun(@times,permute(A,[1,3,2]),permute(A,[3,1,2]));