C ++ rand（）在运行进程时给出相同的数字 [英] C++ rand() gives same number when running process

问题描述

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示例：

  #include< iostream& 
 #include< math.h> 
 
 using namespace std; 
 
 void wait（）{
 int e; 
 cin>> e; 
} 
 int random（int low，int high）{
 if（low> high）return high; 
 return low +（rand（）％（high  -  low + 1））; 
} 
 int main（int argc，char * argv []）{
 for（int i = 0; i <5; i ++）cout <随机（2,5）<< endl; 
 wait（）; 
} 
  

输出：

  3 
 5 
 4 
 2 
 3 
  

每次运行程序时，每次都输出相同的数字。

解决方案

未设置随机数生成器的种子

如果你调用srand（time（NULL）），你会得到更多的随机结果：

  #include< iostream> 
 #include< cstdlib> 
 #include< ctime> 
 using namespace std; 
 
 int main（）{
 srand（time（NULL））; 
 cout<< rand（）<< endl; 
 return 0; 
} 
  

原因是从rand（）函数生成的随机数isn' t实际随机。它只是一种变换。维基百科更好地解释了伪随机数发生器的含义：确定性随机位发生器。每次调用rand（）它需要种子和/或最后生成的随机数（C标准没有指定使用的算法，虽然C ++ 11有指定一些流行的算法的功能），运行对这些数字进行数学运算，并返回结果。所以如果种子状态是相同的每一次（因为如果你不调用srand一个真正的随机数字），那么你总是会得到相同的随机数字。

如果您想了解更多，可以阅读以下内容：

http://www.dreamincode.net/forums/topic/24225-random-number-generation-102/

http：// www .dreamincode.net / forums / topic / 29294-making-pseudo-random-number-generators-more-random /

I am fairly new to C++ and have a question about rand()

every time I run a process with rand() it gives me the same results

example :

#include <iostream>
#include <math.h>

using namespace std;

void wait () {
    int e;
    cin >> e;
}
int random (int low, int high) {
    if (low > high) return high;
    return low + (rand() % (high - low + 1));
}
int main (int argc, char* argv []) {
    for (int i = 0; i < 5; i++) cout << random (2, 5) << endl;
    wait ();
}

output :

3
5
4
2
3

Each time I run the program it outputs the same numbers every time. Is there a way around this?

解决方案

The seed for the random number generator is not set.

If you call srand(time(NULL)) then you will get more random results:

#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;

int main() {
    srand(time(NULL));
    cout << rand() << endl;
    return 0;
}

The reason is that a random number generated from the rand() function isn't actually random. It simply is a transformation. Wikipedia gives a better explanation of the meaning of pseudorandom number generator: deterministic random bit generator. Every time you call rand() it takes the seed and/or the last random number(s) generated (the C standard doesn't specify the algorithm used, though C++11 has facilities for specifying some popular algorithms), runs a mathematical operation on those numbers, and returns the result. So if the seed state is the same each time (as it is if you don't call srand with a truly random number), then you will always get the same 'random' numbers out.

If you want to know more, you can read the following:

http://www.dreamincode.net/forums/topic/24225-random-number-generation-102/

http://www.dreamincode.net/forums/topic/29294-making-pseudo-random-number-generators-more-random/

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