How does randperm with 2 arguments work internally?

Hello, I need to generate an efficient c code where I need to use randperm with 2 arguments. I could figure out from other answer how randperm with single argument works but randperm with 2 arguments seems to be more complex. At some places it uses more than 2 random numbers per value. My guess was that it first randomly chooses an element from the array and then places it in a random place in output, but I was wrong. I didn't get the same output on same seed.

Answer 1

[Roger Stafford]

   [~,p] = sort(rand(n,1));
   p = p(1:k);

where p contains k unique values chosen from 1:n. Of course Mathworks may well use a different method by now.

[Anurag Agarwal]

Hey, thanks for reply but no this is not the method MATLAB uses. It is not even efficient. For any k it will take nlog(n) time to compute the permutation.

[Walter Roberson]

This is the method that MATLAB used until recently; the source for randperm was completely open and most of the regular volunteers looked at it. We do not know what routine is used now, as Mathworks went to some undocumented internal implementation.