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Computing A(:, m)*B(m, :) without a for loop

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Nicolas Perez
Nicolas Perez on 12 Nov 2018
Commented: Nicolas Perez on 12 Nov 2018
Provided I have two matrices A and B of dimensions NxM and MxN I want to compute a matrix C of dimensions NxNxM such that each NxN slice is the result of the prduct A(:, m)*B(m, :). I know this can be obtained in the following way:
for m=1:M
C(:, :, m) = A(:, m)*B(m, :);
end
Is there a way of obtaining the same result without using a for loop (with the aim of making the computation faster)?

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Accepted Answer

Matt J
Matt J on 12 Nov 2018
Ar=reshape(A,N,1,[]);
Br=reshape(B.',1,N,[]);
C=Ar.*Br;

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More Answers (1)

madhan ravi
madhan ravi on 12 Nov 2018
C(:, :, 1:M) = A(:, 1:M)*B(1:M, :);

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Nicolas Perez
Nicolas Perez on 12 Nov 2018
The output of A(:, 1:M)*B(1:M, :); is the sum of C(:, :, 1:M) along the third dimension (an NxN matrix). Thus, you get the "Assignment has fewer non-singleton rhs dimensions than non-singleton subscripts" error.

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