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

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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?

Answers (2)

James Tursa
James Tursa on 28 Jul 2017
Edited: James Tursa on 28 Jul 2017
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;

Andrei Bobrov
Andrei Bobrov on 28 Jul 2017
Edited: Andrei Bobrov on 28 Jul 2017
Let A - your matrix [M x N]
out = bsxfun(@times,permute(A,[1,3,2]),permute(A,[3,1,2]));

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