Problem with Matrix indexing using vector

Hello, I have a problem with indexing matrix elements. I use the following code to describe my problem:

a       = blkdiag(ones(3),ones(3));
[r,c]   = find(a);
b       = zeros(size(a));
b(r,c)  = 1;

I assume that the matrix a and the matrix b are equal, but they are not. I can solve this problem with a for-loop, but for a very large matrix this solution is too slow. Are there any other solutions? Thank yoou for your help!

Answer 1

[Jan]

This is a job for sub2ind:

a        = blkdiag(ones(3),ones(3));
[r,c]    = find(a);
b        = zeros(size(a));
index    = sub2ind(size(a), r, c);
b(index) = 1;

isequal(a, b)  % TRUE

Maybe using logical indexing is more efficient than find():

mask    = (a == 1);
b       = zeros(size(a));
b(mask) = 1;