Using a 3D matrix to avoid for loops

I would like to speed up my code and avoid for loops. I want to extract smaller matrices which are placed along the diagonal of a bigger matrix and put those into an array. The code looks like this:

SC = zeros(m, n, n);
for i = 1:m
    SC(i, 1:n, 1:n) = matrix(n*(i-1)+1:i*n, n*(i-1)+1:i*n);
end
SC = SC(:,:,:);

Do I have to include the for loop or is there a way to extract the matrix all at once?

Answer 1

[Matt J]

First, your code should be faster if SC is nxnxm as opposed to mxnxn. That will make the memory accesses more contiguous. Assumig you accept this, then the following is a loop-free way to build SC.

B=repmat({sparse(true(n))},1,m);
idx=blkdiag(B{:});

SC=reshape( matrix(idx) ,n,n,[]);