std::random_device not working in g++

问题:

I'm using g++ with MinGW in Windows to compile my c++ code, which looks like this:

std::mt19937_64 rng(std::random_device{}());
std::uniform_int_distribution<int> dist(0, words.size() - 1);
std::string curr = words[dist(rng)];

as you can see, this extracts a random word from an array of std::string, but when I optput that word, its always the same, i've realised that the problem lies in std::random_device, which outputs always the same number.

why is this happening? how can I fix it?

gcc version 5.3.0

I use vs code for programming if it could help

And of course i properly include random

回答1:

You are using std::random_device on MinGW. More specifically, you are creating a new instance every time you want a random number. This usage is not supported (or rather, does not produce random numbers). See Why do I get the same sequence for every run with std::random_device with mingw gcc4.8.1? for the fact that MinGW has this behavior, but to fix your particular code, you can do something like this:

std::mt19937_64 rng(std::random_device{}());
std::uniform_int_distribution<int> dist(0, words.size() - 1);

for (int ii = 0; ii < 100; ++ii) {
    std::string curr = words[dist(rng)];
}

That is, use the same random_device repeatedly, rather than just once. And consider seeding it, or using a different RNG, if randomness is important between program runs.