MATLAB accessing multiple elements in sparse matrix using row and column index vectors

Question

I feel there should be an easy solution but I can't find it:

I have the sparse matrices A B with the same dimension n*n. I want to create matrix C which copies values in A where B is non-zero.

This is my approach:

[r,c,v] = find(B);

% now I'd like to create an array of values using indices r and c, 
% but this doesn't work (wrong syntax)
v2 = A(r,c);

% This won't work either
idx = find(B); % linear indexing, too high-dimensional
v2 = A(idx);

% and create C
C = sparse(r,c,v2,n,n);

Here are some more details:

My matrices are very large, so the solution needs to be efficient. C(B~=0) = B(B~=0); won't do it, unfortunately.
Linear indexing won't work either as the matrices are too large (Matrix is too large to return linear indices.).

Is there really no way to use 2-dimensional indices?

Thanks for your help!

Yes, thanks a lot, this works! If you want to post it as an answer I can formally accept it :) — Lisa
Sure. I was not totally sure before you tried it on your data because we are talking about efficiency. Thanks for the credit. — Da Kuang

Da Kuang Da Kuang · Accepted Answer · 2013-06-18T16:37:42

I think C = A .* (B~=0); should work. Only non-zeros will be accessed in the entrywise multiplication of two sparse matrices so it will be fast.

MATLAB accessing multiple elements in sparse matrix using row and column index vectors

1 Answers