C++11 random numbers

Question

I need to generate random numbers, but from as wide a range as possible (64 bit at least). I don't care if the distribution is perfect, so std::rand() would work, but it only returns an int. I understand that c++11 has some random number generating capability that can give any size number, but is very complex to use. Can someone post a simple example of how to use it as simply as possible to get the described functionality (64 bit or more random numbers) in as simple a way as possible (like std::rand())?

Answer 1

This is how to use the C++11 random number generation for this purpose (adjusted from http://en.cppreference.com/w/cpp/numeric/random/uniform_int_distribution):

#include <random> #include <iostream> int main() {   /* Initialise. Do this once (not for every      random number). */   std::random_device rd;   std::mt19937_64 gen(rd());    /* This is where you define the number generator for unsigned long long: */   std::uniform_int_distribution<unsigned long long> dis;    /* A few random numbers: */       for (int n=0; n<10; ++n)     std::cout << dis(gen) << ' ';   std::cout << std::endl;   return 0; } 

Instead of unsigned long long, you could use std::uintmax_t from cstdint to get the largest possible integer range (without using an actual big-integer library).

Answer 2

We could easily wrap a random number generator engine into srand/rand-like methods like this:

#include <random> #include <iostream>  struct MT19937 { private:     static std::mt19937_64 rng; public:     // This is equivalent to srand().     static void seed(uint64_t new_seed = std::mt19937_64::default_seed) {         rng.seed(new_seed);     }      // This is equivalent to rand().     static uint64_t get() {         return rng();     } };  std::mt19937_64 MT19937::rng;   int main() {     MT19937::seed(/*put your seed here*/);      for (int i = 0; i < 10; ++ i)         std::cout << MT19937::get() << std::endl; } 

(Like srand and rand, this implementation does not care about thread-safety.)

Well the wrapper functions are so trivial that you could just use the engine directly.

#include <random> #include <iostream>  static std::mt19937_64 rng;  int main() {     rng.seed(/*put your seed here*/);      for (int i = 0; i < 10; ++ i)         std::cout << rng() << std::endl; }