Random double C++11 | 易学教程

问题

The code below show how to random double in C++11. In this solution each time, when I run this program the result are the same - I don't know why? How to change it to get different solution in every time when I run the program?

#include <random>
int main(int argc, char **argv)
{
  double lower_bound = 0.;
  double upper_bound = 1.;
  std::uniform_real_distribution<double> unif(lower_bound, upper_bound);
  std::default_random_engine re;      
  double a_random_double = unif(re);
  cout << a_random_double << endl;
  return 0;
}
//compilation: "g++ -std=c++0x program_name.cpp -o program_name"

回答1:

You need to seed the random number generator before you generate samples. You do this when you construct the default_random_engine instance. For example:

std::random_device rd;
std::default_random_engine re(rd());

If you wish to be more prescriptive about the generator you use then you should specify one rather than relying on the library implementor's choice for default_random_engine. The available choices are listed here: http://en.cppreference.com/w/cpp/numeric/random#Predefined_random_number_generators

Also beware that some implementations do not use a non-deterministic source for random_device. If you are faced with such an implementation, you'll need to find an alternative source for your seed.

回答2:

You have to initialize the random engine with some initial seed value.
An option is to use std::random_device.

#include <iostream>
#include <random>

int main() {
    const double lower_bound = 0;
    const double upper_bound = 1;
    std::uniform_real_distribution<double> unif(lower_bound, upper_bound);
    
    std::random_device rand_dev;          // Use random_device to get a random seed.

    std::mt19937 rand_engine(rand_dev()); // mt19937 is a good pseudo-random number 
                                          // generator.

    double x = unif(rand_engine);
    std::cout << x << std::endl;
}

You may want to watch a great presentation freely available online: "rand() Considered Harmful" (from GoingNative 2013 C++ event).

回答3:

The engine was not initialized with a seed.

#include <chrono>
...
std::default_random_engine re(std::chrono::system_clock::now().time_since_epoch().count());

回答4:

As others have noted, you haven't given the generator an explicit seed.

Let's look at what's happening. GCC implements std::default_random_engine as a std::linear_congruential_engine<uint_fast32_t, 16807UL, 0UL, 2147483647UL>. When you write std::default_random_engine re;, you call the constructor for this class with no arguments, which defaults to a seed value of linear_congruential_engine::default_seed = 1u. So if you want a seed other than 1, you have to provide it yourself.

Reference: http://gcc.gnu.org/onlinedocs/gcc-4.8.2/libstdc++/api/a00860.html

来源：https://stackoverflow.com/questions/21102105/random-double-c11

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