Replacing arrays within a matrix

Really quick question.

How do I replace in one command for instance, in a matrix of 4x4, the first two rows by a vector v, and the other two rows by a vector u?

Answer 1

[KSSV]

As you are replacing all the elements of the matrix..why you want to go for replace? You create a different matrix itself.....

u = rand(1,4) ; 
v = rand(1,4) ;
iwant = [u ; u ;v ;v ]