为什么 rand() 每次运行都会产生相同的数字序列?

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

问题描述

每次我用 rand() 运行程序时，它都会给我相同的结果.

示例:

#include #include 使用命名空间标准；int随机(int低，int高){如果(低>高)回报高；返回低 + (rand() % (高 - 低 + 1));}int main (int argc, char* argv []) {for (int i = 0; i <5; i++)cout<<随机 (2, 5) <<结束;}

输出:

35423

每次我运行程序时，它每次都会输出相同的数字.有没有办法解决这个问题?

未设置随机数生成器的种子.

如果你调用 srand((unsigned int)time(NULL)) 那么你会得到更多的随机结果:

#include #include #include <ctime>使用命名空间标准；int main() {srand((unsigned int)time(NULL));cout<<兰特()<<结束;返回0；}

原因是从 rand() 函数生成的随机数实际上并不是随机的.这简直就是一种转变.维基百科对伪随机数生成器的含义给出了更好的解释: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 标准没有指定使用的算法，尽管 C++11 具有指定一些流行的算法)，对这些数字运行数学运算，并返回结果.因此，如果种子状态每次都相同(就像您不使用真正的随机数调用 srand 一样)，那么您将始终得到相同的随机"数.

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/

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

Example:

#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;
}

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?

The seed for the random number generator is not set.

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;
}

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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