更改 c++ 中的种子或随机数生成器

Question

I have this program where I use the mersenne twister generator like so:

#include <iostream>
#include <random>
using namespace std;

unsigned seed = 131353;
mt19937 generator(seed);
normal_distribution<real> distribution(0.0, 1.0);

int main() {
    cout << distribution(generator);
}

for me it's convenient to declare this generator outside of main so that it is a global variable, so that I don't have to keep passing it as an argument in the functions I have;

but I want to read the seed from a file, which as far as I am aware can only be done inside the main() function.

Is it possible to declare the generator as a global variable and set the seed inside main()?

Thanks in advance

I tried reading the seed from a file outside main function, without success so far

Answer 1

which as far as I am aware can only be done inside the main() function.

Not really. You can seed it directly when initializing it. Example:

#include <fstream>
#include <iostream>
#include <random>

// call a function/lambda to fetch the seed from the file:
std::mt19937 generator([]{
                           std::mt19937::result_type seed{};
                           if(std::ifstream fs("file_with_seed"); fs)
                               fs >> seed;
                           return seed;
                       }());

std::normal_distribution<double> distribution(0.0, 1.0);

int main() {
    std::cout << distribution(generator);
}

If you want to seed it in main , you can use the seed member function.

#include <fstream>
#include <iostream>
#include <random>

std::mt19937 generator;
std::normal_distribution<double> distribution(0.0, 1.0);

int main() {
    generator.seed([]{
                       std::mt19937::result_type seed{};
                       if(std::ifstream fs("file_with_seed"); fs)
                           fs >> seed;
                       return seed;
                   }());

    std::cout << distribution(generator);
}