How to reset a random number engine?

Question

I am trying to use the C++11 approach to random number generation:

#include <random>
#include <functional>
#include <iostream>

int main(int argc, char *argv[])
{    
    std::normal_distribution<double> normal(0, 1);
    std::mt19937 engine; // Mersenne twister MT19937
    auto generator = std::bind(normal, engine);

    int size = 2;

    engine.seed(0);
    normal.reset();
    for (int i = 0; i < size; ++i)
        std::cout << generator() << std::endl;

    std::cout << std::endl;

    engine.seed(1);
    normal.reset();
    for (int i = 0; i < size; ++i)
        std::cout << generator() << std::endl;

    std::cout << std::endl;

    engine.seed(0);
    normal.reset();
    for (int i = 0; i < size; ++i)
        std::cout << generator() << std::endl;

    return 0;
}

The output is:

0.13453
-0.146382

0.46065
-1.87138

0.163712
-0.214253

That means that the first and the third sequence are not identical even if they are seeded with the same number. Please, what am I doing wrong? Is the

std::normal_distribution<double>

Just a function in the mathematical sense (produces y out of x deterministically) or am I missing something? If it is just a function, what does the reset method actually do?

Answer 1

You are binding the engine and the distribution, such the following calls on reset won't affect the bound function.

The solution is to bind references to the engine and the distro

auto generator = std::bind(std::ref(normal), std::ref(engine));

Answer 2

The problem you are having is with std::bind. std::bind makes copies of its arguments. The reason std::bind makes copies is because the function will be called at some unknown point in the future when the arguments may no longer exist. This means your calls to engine.seed(), etc. are useless. Using std::ref you can bind your arguments by reference which will give you the expected output.

auto generator = std::bind(std::ref(normal), std::ref(engine));