Indexing a matrix with another matrix?

Hi.

I have a 10000x4 matrix, Idx, where each row contains 4 indices to a logical matrix, A. I would like to set all indices in Idx to true (1) in A. This seems like something that should be possible to do very easy in one single line but I can’t quite figure it out so I’m currently doing it by looping:

A = false(10000,256);
for i = 1 : size(A,1)
    A(i,Idx(i,:)) = true;
end

which is very slow. Could someone tell me how to do this indexing without a loop?

Thanks

Answer 1

[Andrei Bobrov]

[m,n] = size(Idx);
[ii,~] = ndgrid(1:m,1:n); % or ii = repmat((1:m)',1,n);
                          %    ii = (1:m)'*ones(1,n);
A(sub2ind(size(A),ii,Idx)) = true;

[Petter Stefansson]

Thank you that was significantly faster