C++ randomly sample k numbers from range 0:n-1 (n > k) without replacement

Question

I\'m working on porting a MATLAB simulation into C++. To do this, I am trying to replicate MATLAB\'s randsample() function. I haven\'t figured out an efficient way to do thi

Answer 1

Bob Floyd created a random sample algorithm that uses sets. The intermediate structure size is proportional to the sample size you want to take.

It works by randomly generating K numbers and adding them to a set. If a generated number happens to already exist in the set, it places the value of a counter instead which is guaranteed to have not been seen yet. Thus it is guaranteed to run in linear time and does not require a large intermediate structure. It still has pretty good random distribution properties.

This code is basically lifted from Programming Pearls with some modifications to use more modern C++.

unordered_set BobFloydAlgo(int sampleSize, int rangeUpperBound)
{
     unordered_set sample;
     default_random_engine generator;

     for(int d = rangeUpperBound - sampleSize; d < rangeUpperBound; d++)
     {
           int t = uniform_int_distribution<>(0, d)(generator);
           if (sample.find(t) == sample.end() )
               sample.insert(t);
           else
               sample.insert(d);
     }
     return sample;
}

This code has not been tested.