replacing entries in matrices

I begin with a nx2 matrix (coordinate positions of n particles in a 2d plane), call this R. Initially, all the elements in R are zero.

R = zeros(n,2);

I have identified that after some conditions, the 10th and 50th particles will have to change their positions to (10,10) and (20,20).

So I want to put the coordinates (10,10) and (20,20) into the 10th and 50th row, respectively, while keeping all the others zero. How do I do that?

Thanks

Answer 1

[Thorsten]

 R(10,:) = [10, 10];
 R(50,:) = [20, 20];