CODEWITHSUNDEEP
c++random
user1587451
user1587451

Reputation: 1008

How to avoid same random results using std::mt19937?

My code below is influenced by thread "Does std::mt19937 require warmup?". When executed, two vectors are generated with same content what is not what I want. I've expected different content in both vectors. What am I doing wrong and how to fix it?

Output:

0.251423
0.729274
-1.43542
0.251423
0.729274
-1.43542

Code:

#include <vector>
#include <algorithm>
#include <iostream>
#include <random>
#include <array>
#include <functional>
#include <iterator>

class functor1
{
public:
  functor1()
  {
    std::random_device device;
    std::array<int, std::mt19937::state_size> seeds;
    std::generate_n(seeds.data(), seeds.size(), std::ref(device));
    std::seed_seq sequence(std::begin(seeds), std::end(seeds));
    engine.seed(sequence);
  }

  double operator()()
  {
    std::normal_distribution<double> distribution(0.0, 1.0);
    return distribution(engine);
  }

private:
  std::mt19937 engine;
};

int main()
{
    functor1 f1;

    std::vector<double> v0;
    std::vector<double> v1;

    std::generate_n(std::back_inserter(v0), 3, f1);
    std::generate_n(std::back_inserter(v1), 3, f1);

    std::ostream_iterator<double> out(std::cout, "\n");

    std::copy(v0.begin(), v0.end(), out);
    std::copy(v1.begin(), v1.end(), out);
}

Thanks

Upvotes: 2

Views: 716

Answers (1)

wilx
wilx

Reputation: 18228

This is because f1 is copied into generate_n() function invocation. Use std::ref(f1) instead and it will return different results for v0 and v1:

std::generate_n(std::back_inserter(v0), 3, std::ref(f1));
std::generate_n(std::back_inserter(v1), 3, std::ref(f1));

Upvotes: 7

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