void random_permute (arma::uvec& p, int n)
{
  //n = last - first;
  srand(time(0));
  default_random_engine generator;
  uniform_int_distribution<int> distribution(0, n-1);

  for (size_t i = n - 1; i > 0; --i)
  {
    //generate random number between [0, n - 1].
    int t = distribution(generator);

    int temp = p(i);
    p(i) = p(t);
    p(t) = temp;
  }
}