为什么是< random>库每次使用std :: uniform_int_distribution时产生相同的结果 [英] Why is <random> library producing the same results every time when using std::uniform_int_distribution

问题描述

注意：

1. 我的编译器是g ++ 5.1.0-2，使用c ++ 14语言标准


2. 我的IDE是Code :: Blocks（我也尝试过Dev-C ++的代码）


3. 我测试的代码是在线IDE， http://cpp.sh/ （C + + shell）和 https://www.codechef.com/ide （CodeChef.com的IDE）都运行c ++ 14


4. 使用在线IDE时代码运行正常，




这是代码：

  #include< iostream> 
 #include< random> 
 #include< cstdlib> 
 #include< ctime> 
 #include< chrono> 
 
 int main（）{
 srand（time（0））; 
 long long seed = rand（）; 
 std :: default_random_engine rand_num（seed）; 
 std :: uniform_int_distribution< long long>范围（0，10）; 
 long long a = range（rand_num）; 
 long long b = rand_num（）; 
 std :: cout<< seed<<\\\
; //种子每次不同（测试）
 std :: cout<< a<<\\\
; 
 std :: cout<< b<<\\\
; 
 system（pause）; 
 return 0; 
} 
  

运行std :: uniform_int_distribution（a）





没有std :: uniform_int_distribution（b）的代码在两个在线IDE上都可以正常工作和我自己的电脑。





有什么问题？

更新：该代码适用于mt19937引擎（std :: mt19937和std :: mt19937_64）

解决方案

这似乎是Windows上的g ++实现的一个已知问题。查看接受的类似问题的答案：为什么我得到相同的序列为styr :: random_device与mingw gcc4.8.1





为了避免这个问题，你可以使用< chrono> 种子随机数生成器的设施：

  #include < iostream> 
 #include< random> 
 #include< cstdlib> 
 #include< ctime> 
 #include< chrono> 
 
 int main（）{
 srand（time（0））; 
 long long seed = rand（）; 
 
 std :: default_random_engine rand_num {static_cast< long unsigned int>（std :: chrono :: high_resolution_clock :: now（）。time_since_epoch（）。count（））}; 
 // ^ ^ ^ ^ ^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 
 std :: uniform_int_distribution< long long>范围{1,10}; 
 
 long long a = range（rand_num）; 
 long long b = rand_num（）; 
 std :: cout<< seed<<\\\
; //种子每次不同（测试）
 std :: cout<< a<<\\\
; 
 std :: cout<< b<<\\\
; 
 system（pause）; 
 return 0; 
} 
  



这在每次运行时都有不同的结果（在Windows上使用g ++） / p>


Notes:

1. My compiler is g++ 5.1.0-2 with c++14 language standards
2. My IDE is Code::Blocks (I tried the code on Dev-C++ too)
3. The online IDEs I tested the code on were http://cpp.sh/ (C++ shell) and https://www.codechef.com/ide (CodeChef.com's IDE) both running c++14
4. The code runs ok when using an online IDE, which puzzles me even more.

Here is the code:

#include <iostream>
#include <random>
#include <cstdlib>
#include <ctime>
#include <chrono>

int main() {
    srand(time(0));
    long long seed = rand();
    std::default_random_engine rand_num(seed);
    std::uniform_int_distribution<long long> range(0, 10);
    long long a = range(rand_num);
    long long b = rand_num();
    std::cout<<seed<<"\n"; // the seed is different every time (tested)
    std::cout<<a<<"\n";
    std::cout<<b<<"\n";
    system("pause");
    return 0;
}

The one with std::uniform_int_distribution (a) isn't random when running the code on my own computer but works ok and creates a random number on the online IDEs.

The one without std::uniform_int_distribution (b) works ok with both online IDEs and my own computer.

What's the problem? How can I fix it?

UPDATE: The code works ok with mt19937 engine (std::mt19937 and std::mt19937_64)

解决方案

It seems to be a known problem of g++ implementation on Windows. See the accepted answer to a similar question: why do I get same sequence for everyrun with std::random_device with mingw gcc4.8.1

To avoid the problem you can use <chrono> facilities to seed the random number generator:

#include <iostream>
#include <random>
#include <cstdlib>
#include <ctime>
#include <chrono>

int main() {
    srand(time(0));
    long long seed = rand();

    std::default_random_engine rand_num{static_cast<long unsigned int>(std::chrono::high_resolution_clock::now().time_since_epoch().count())};
    //                                  ^ ^ ^ ^ ^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
    std::uniform_int_distribution<long long> range{1,10};

    long long a = range(rand_num);
    long long b = rand_num();
    std::cout<<seed<<"\n"; // the seed is different every time (tested)
    std::cout<<a<<"\n";
    std::cout<<b<<"\n";
    system("pause");
    return 0;
}

This gave me different results at every run ( with g++ on Windows).

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