# Elementwise Matrix multiplication with a Vector to get a higher dimension Matrix

21 views (last 30 days)
Kilian Helfenbein on 10 Dec 2019
Commented: deepu kurian on 24 May 2022
Hey everyone :)
I would like to create a 3D Matrix out of a 2D Matrix and a Vector. The first page of the 3D Matrix should be equal to the product of the 2D Matrix times the first element of the Vector and so on.
At the moment my Code works but looks very inefficient to me.
clear variables
A1 = [1 2 3; 4 5 6; 7 8 9];
Asize = size(A1);
V1 = (1:1:3);
lengthV1 = length(V1);
A2 = zeros(Asize(1), Asize(2), lengthV1);
idxi = 1;
for idxV1 = V1
for idxA11 = 1:Asize(1)
for idxA12 = 1:Asize(2)
E1 = A1(idxA11, idxA12);
E2 = E1 * idxV1;
A2(idxA11, idxA12, idxi) = E2;
end
end
idxi = idxi+1;
end
The result is exactly what I want. The problem is that in my real code I get a huge 5D Matrix, which gives me a memory error. How can I make my Code more efficient? I tried to apply Vectorization to this example but failed.
Thank you in advance!
##### 1 CommentShowHide None
Adam on 10 Dec 2019
"The problem is that in my real code I get a huge 5D Matrix"
What does this mean? It seems like a fundamental aspect of the problem, but you tag it on the end as a passing comment. If you are trying to multiply a 2d array by a 1d array to expand it to a 3d array where do 5d arrays come in? If you mean you have a 5d array (note: a matrix is 2d by definition in Matlab) to multiply by a 1d array, then I suggest you make this the focus of the question as answers people may give to multiply a 2d by a 1d will not necessarily scale up to 5d.

ME on 10 Dec 2019
I think this should do the trick.
A1 = [1 2 3; 4 5 6; 7 8 9];
Asize = size(A1);
V1 = (1:1:3);
lengthV1 = length(V1);
A2 = zeros(Asize(1), Asize(2), lengthV1);
for i=1:lengthV1
A2(:,:,i) = A1(:,:)*V1(i);
end
##### 1 CommentShowHide None
Kilian Helfenbein on 10 Dec 2019
Works fine :) Thanks :)

### More Answers (1)

Stephen23 on 10 Dec 2019
Edited: Stephen23 on 10 Dec 2019
"How can I make my Code more efficient?"
A2 = bsxfun(@times,A1,reshape(V1,1,1,[])) % for versions >= R2007a
A2 = A1 .* reshape(V1,1,1,[]) % for versions >= R2016b
##### 2 CommentsShowHide 1 older comment
deepu kurian on 24 May 2022
Thanks:) really useful

R2019b

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!