Help with mt19937 random generator

Language Help C++
1

Hi! i’ve been learning c++ now for a bit and wanted to explore better (more random) random number generators in c++. Came across the mt19937 generator and after reading about its standard practice use i wanted to try some DND rng test code. However after running my code in vsc, it kept printing the same output - 12, 6, 3 respectively - no matter how i change the code. Is my code wrong? Am i missing some pathing? Do i need to download a specific extension for c++ vsc? Code below.
cout << Thanks!;

#include
#include
#include
#include
using namespace std;

int main() {

std::random_device rd;
std::mt19937 gen(rd()); 
std::uniform_int_distribution<> d1(1, 20); 
std::uniform_int_distribution<> d2(1, 8);
std::uniform_int_distribution<> d3(1, 6);



int d20 = d1(gen); 
cout << d20 <<  "\n";

int d8 = d2(gen);
cout << d8 << "\n";

int d6 = d3(gen);
cout << d6 << "\n";

}

output:

$ ./darc
12
6
3

$ ./darc
12
6
3

alternate code ive tried:

include

include

include

include

using namespace std;

int main() {

srand(time(0));
std::random_device rd;
std::mt19937 gen(rd()); 
std::uniform_int_distribution<> d1(1, 20); 
std::uniform_int_distribution<> d2(1, 8);
std::uniform_int_distribution<> d3(1, 6);



int d20 = d1(gen); 


int d8 = d2(gen);


int d6 = d3(gen);

if (d20 >= 10) {
cout << "You rolled " << d20 << “: Pass!\n”;
} else {
cout << "You rolled " << d20 << “: Fail!\n”;
}

if (d8 >= 4) {
cout << "You rolled " << d8 << “: Pass!\n”;
} else {
cout << "You rolled " << d8 << “: Fail!\n”;
}

if (d6 >= 3) {
cout << "You rolled " << d6 << “: Pass!\n”;
} else {
cout << "You rolled " << d6 << “: Fail!\n”;
}

}

output:

$ ./darc
You rolled 12: Pass!
You rolled 6: Pass!
You rolled 3: Pass!

$ ./darc
You rolled 12: Pass!
You rolled 6: Pass!
You rolled 3: Pass!

2

Perhaps 12, 6, 3 is the meaning of the universe…?

3

Your code looked correct to me. So i tried compiling and running myself and it works fine. I know this sounds like did you turn it off and on again? but, did you try saving and re-compiling before running it? I know I often forget this lolll.

Screenshot 2023-11-10 at 9.58.55 AM
Screenshot 2023-11-10 at 9.58.58 AM
Screenshot 2023-11-10 at 9.59.01 AM

3 Likes
4

There’s also a small chance your random device seeding could not be working as needed depending on your compiler/architecture?

Try running it on a vm or repl or something to see if it has an issue there. (it runs fine here for example (as well as on my local): https://cpp.sh/

2 Likes
5

Yeah i’ve made sure to save and recompile and everything - I even came close to giving it the old Hollywood smack on the top of the computer but thought better of it…

Im gonna keep looking into it - its definitely working elsewhere so its something to do with my local i guess. Will be back when i figure it out (or give it the Hollywood try)!

6

Figure out what compiler you’re using and your computer’s architecture and OS and you might have better luck in searching around for quirks with random device and those.

1 Like
7

Back with an “answer” and “solution”…

So, turns out the compilers based on the port for windows/microsoft (MingW, MinGW-64) actually have a problem with the std::random_device and bug out from time to time. Best case solution is to use a different compiler (still need to look into that a bit more).

However - a nice short term solution to initialize the code correctly is to seed with:
std::random_device rd;
std::mt19937 gen(std::chrono::high_resolution_clock::now().time_since_epoch().count());

That did the trick nicely - be sure to #inlcude chrono

On reading into it a bit further, theres lots more discussion on the pros and cons of simply including higher resolution seed that is all a little above my head at the time. But we’ll be back to investigate that in a bit. In the meantime the code above works! Now time to try a new friggin compiler… Thanks!!

also for what its worth i found this and hope to look into it as well - an alt lib for rng:
"CSPRNG is a small library that uses your Operating System’s native Cryptographically-Secure Pseudo-Random Number Generator.

It is a superior replacement for the clunky, ill-designed [std::random_device]. It won’t throw. It won’t give you non-CS random numbers. It won’t pretend to tell you anything about entropy. It just works."

1 Like