How to take union of matrix rows that are represented by another vector?

I want to take the union of some of the rows of a matrix x. The row numbers of the rows whose union has to be taken are given by vector r. Is there any built-in function in MATLAB that can do it?

x = [1  2  4  0  0; 
     3  6  5  0  0;
     7  8 10 12  9;
     2  4  6  7  0;
     3  4  5  8 12];

r = [1, 3, 5]; 

I think this will work for you - first, take the submatrix x(r,:) with the rows you want, and then find all the unique values in it:

unique(x(r,:))

ans =
     0
     1
     2
     3
     4
     5
     7
     8
     9
    10
    12