find non zero elements put in a matrix

i have a matrix and i want to take the row number of every non-zero element per collumn into a new matrix

Answer 1

[Wayne King]

   A = randi([-2 2],20,10);
   idx = find(A~=0);
   B = A(idx);

B contains all the nonzero elements of A, but how will you know the size of the matrix ahead of time?