Mina, try this:
% Create some sample input variable "a".
n = 1000; % Whatever you want...
% Create sample data, or use some existing vector if you want
a = 1 : n;
% Now we have sample data and we can begin.
r = 10; % Whatever you want...
% Need an n-by-r output matrix,
% so we need to pull n*r elements
% from "a" at random locations.
indexes = randi(length(a), 1, n*r);
% Pull those indexes out and reshape them
% from a 1-D vector into an n row-by-r column matrix.
out = reshape(a(indexes), n, r);
It's well commented so hopefully you can follow it but if you can't just ask.
