std::random_device not working in g++

Question

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

@Mocoma [edit] the question to include necessary information — Passer By, Aug 19 '17 at 09:08
Are you constructing a new `random_device` each time, or do you construct a single one and then use it repeatedly? — John Zwinck, Aug 19 '17 at 09:11
@JohnZwinck construct a new one beacuse the program itself only uses it one time. For checking that the random device generates a new number every time, i created it another time — Mocoma, Aug 19 '17 at 09:28
An implementation of `random_device` in MinGW: https://xkcd.com/221/ — Revolver_Ocelot, Aug 19 '17 at 10:06
Possible duplicate of [Why do I get the same sequence for every run with std::random\_device with mingw gcc4.8.1?](https://stackoverflow.com/questions/18880654/why-do-i-get-the-same-sequence-for-every-run-with-stdrandom-device-with-mingw) — Revolver_Ocelot, Aug 19 '17 at 10:08

score 2 · Accepted Answer · answered Aug 20 '17 at 11:13

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.

std::random_device not working in g++

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