RANDSUBSET

A = RANDSUBSET(N, K) is equivalent to

ALLSUBSETS = NCHOOSEK(1:N,K);
A = ALLSUBSETS(RANDI(NCHOOSEK(N,K)));

% or

A = RANDPERM(N);
A = SORT(A(1:k));

This function can also be used to generate random sampling without replacement by calling RANDPERM on the output.

It works advantageously with large N (but "reasonable" K), and fast!