Difference between revisions of "cpp/numeric/random/random device"

Revision as of 13:49, 24 January 2022

std::random_device is a uniformly-distributed integer random number generator that produces non-deterministic random numbers.

std::random_device may be implemented in terms of an implementation-defined pseudo-random number engine if a non-deterministic source (e.g. a hardware device) is not available to the implementation. In this case each std::random_device object may generate the same number sequence.

Member types

Member functions

Notes

A notable implementation where std::random_device is deterministic is old versions of MinGW (bug 338, fixed since GCC 9.2). The latest MinGW versions can be downloaded from GCC with the MCF thread model.

Example

#include <iostream>
#include <string>
#include <map>
#include <random>
 
int main()
{
    std::random_device rd;
    std::map<int, int> hist;
    std::uniform_int_distribution<int> dist(0, 9);
    for (int n = 0; n < 20000; ++n) {
        ++hist[dist(rd)]; // note: demo only: the performance of many 
                          // implementations of random_device degrades sharply
                          // once the entropy pool is exhausted. For practical use
                          // random_device is generally only used to seed 
                          // a PRNG such as mt19937
    }
    for (auto p : hist) {
        std::cout << p.first << " : " << std::string(p.second/100, '*') << '\n';
    }
}

0 : ********************
1 : *******************
2 : ********************
3 : ********************
4 : ********************
5 : *******************
6 : ********************
7 : ********************
8 : *******************
9 : ********************