OpenMP in C++, a toy example

#include <iostream>
#include <thread>
#include <chrono>
#include <omp.h>
#include <vector>
#include <stdlib.h> 

int f(int i) {
  std::this_thread::sleep_for (std::chrono::seconds(1));
  return i;
}

int main(int argc, char* argv[])
{
  int n_threads = atoi(argv[1]);
  std::cout << n_threads << " threads ..." << std::endl;
  std::vector<int> M(25);
#pragma omp parallel for num_threads(n_threads)
  for (int i=0; i<25; i++)
    M[i] = f(i);
  return 0;
}