为什么rand()每次运行都会产生相同的数字序列? [英] Why does rand() yield the same sequence of numbers on every run?

问题描述

每次我使用rand()运行程序时，都会得到相同的结果.

Every time I run a program with rand() it gives me the same results.

示例:

#include <iostream>
#include <cstdlib>

using namespace std;

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;
}

输出:

3
5
4
2
3

每次运行程序时，每次输出相同的数字.有办法解决吗?

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

推荐答案

未设置随机数生成器的种子.

The seed for the random number generator is not set.

如果您致电srand((unsigned int)time(NULL))，您将获得更多随机结果:

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

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

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

原因是从rand()函数生成的随机数实际上不是随机数.这只是一个转变.维基百科更好地解释了伪随机数生成器的含义:确定性随机位生成器.每次调用rand()时，它都会获取生成的种子和/或最后一个随机数(C标准未指定所使用的算法，尽管C ++ 11具有指定某些流行算法的功能)，对这些数字进行数学运算，并返回结果.因此，如果每次的种子状态都是相同的(例如，如果您不使用真正的随机数调用srand)，那么您总是会得到相同的随机"数.

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.

如果您想了解更多信息，可以阅读以下内容:

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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