Find the first N non-zero elements in each row of a matrix

Question

I have a matrix in MATLAB with zeroes and I would like to get another matrix with the first N non-zero elements in each row. Let\'s say for example N = 3<

Answer 1

Usually I don't go with a for loop solution, but this is fairly intuitive:

N = 3;
[ii,jj] = find(A);
B = zeros(size(A,1),N);
for iRow = 1:size(A,1),
    nzcols = jj(ii==iRow);
    B(iRow,:) = A(iRow,nzcols(1:N));
end

Since you are guaranteed to have more than N nonzeros per row of A, that should get the job done.