Applying sparsity pattern to other matrix in matlab-CodePudding

I'm trying to apply a sparsity pattern to another matrix. At the moment I'm just using assignment after using spalloc. The Matlab alerts that this is slow, but doesn't give a solution to make it faster. How do I apply the sparsity pattern of A to the matrix FWtrans? Meaning that the zero values of A have to be zero values in FWtrans. Only indices in A that have a value can have the value of FWtrans in X.

Wtrans = transpose(W);
As = sparse(A);
FWtrans = F * Wtrans;
[m,n] = size(FWtrans);
k = find(As);
X = spalloc(m,n, nnz(A));
% optimalize assignment too speed up
for i = k
    X(k) = FWtrans(k);
end

CodePudding user response：

Instead of allocating the sparse array and then copying values to it, you can use sparse to allocate the array and assign the values all at once.

Wtrans = transpose(W);
As = sparse(A);
FWtrans = F * Wtrans;
[m,n] = size(FWtrans);

% unfortunately, sparse takes only row/column coordinates...
[r,c] = find(As);
% but it's more convenient to get the values using indices
v = FWtrans(find(As));

% let sparse allocate a m*n array and
% assign the values v to the proper locations
X = sparse(r, c, v, m, n);

It might be a bit quicker to use sub2ind after find in order to find the row/column coordinates instead of calling find twice, but I have my doubts.