randomly shuffle entries of a vector

shuffle_vec.cc

#include <vector>
#include <random>
#include <algorithm>
#include <iostream>

#include "TRandom3.h"

// g++ -std=c++11 shuffle_vec.cc $(root-config --libs --cflags)

// for a non-c++11 compliant version this function can be used instead of the lambda defined below in main
// for std::random_shuffle (std::shuffle has been introduced in c++11 only)
inline int rootGetRandom(const int N)
{
  return gRandom->Integer(N);
}

int main()
{
  std::vector<int> vec;
  vec.reserve(100);
  for (int i = 0; i < 100; ++i) { vec.push_back(i); }

  std::vector<int> vec2{vec};

  // using std::shuffle
  std::mt19937 rng(std::random_device{}());
  std::shuffle(vec.begin(), vec.end(), rng);

  // using std::random_shuffle and the mersenne twister from ROOT
  if (gRandom) delete gRandom;
  gRandom = new TRandom3(std::random_device{}());
  auto rootRNG = [&gRandom](const int N) { return gRandom->Integer(N); };
  std::random_shuffle(vec2.begin(), vec2.end(), rootRNG);
  
  for (const auto& v : vec) std::cout << v << " ";
  std::cout << "\n";

  for (const auto& v : vec2) std::cout << v << " ";
  std::cout << "\n";

  return 0;
}