Multiply two 3D arrays that result would be 4D

Hello to All,

I am trying to work and understand 4D arrays. Lets say I have two 3D gpuArrays a(100x5x5) and b(5x5x5). I would like to multiply them that it would result in 4D array c(100x5x5x5), and example of multiplication:

For j=1:5
For i=1:5
c(:,:,i,j)=a(:,i,j)*b(i,:,j)
end
end

What would be the way to achieve this. I am trying to go for 4D arrays, because I would like to avoid for loops.

[James Tursa]

a(:,i,j) is a 100x1. b(i,:,j) is a 1x5. So the result of a matrix multiply is going to be a 100x5. This would mean you need c(:,:,i,j) instead of c(:,i,j) on the lhs. Is this what you want to do?

[Mantas Vaitonis]

@James Tursa, yes you are correct, c suppose to be c(:,:,i,j), I fixed my comment. Thank You for noticing.

Answer 1

[James Tursa]

Assuming I understand what you are trying to do,

result = permute(a,[1 4 2 3]) .* permute(b,[4 2 1 3]);

or in earlier versions of MATLAB

result = bsxfun(@times,permute(a,[1 4 2 3]) , permute(b,[4 2 1 3]));

[Mantas Vaitonis]

Yes this is exactly what I need. Thank You and you guys always give such great and fast responses. Now I can move forward.

Answer 2

[Matt J]

c=a.*permute(b,[4,1,3,2]);