C++ rand() 每次程序运行时返回相同的数字
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/13896450/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
rand() returns the same number each time the program is run
提问by codedude
In this rather basic C++ code snippet involving random number generation:
在这个涉及随机数生成的相当基本的 C++ 代码片段中：
include <iostream>
using namespace std;
int main() {
cout << (rand() % 100);
return 0;
}
Why am I always getting an output of 41? I'm trying to get it to output some random number between 0 and 100. Maybe I'm not understanding something about how the rand function works?
为什么我总是得到 41 的输出？我试图让它输出 0 到 100 之间的一些随机数。也许我不了解 rand 函数的工作原理？
采纳答案by Aaron
You need to "seed" the generator. Check out this short video, it will clear things up.
您需要“播种”发电机。看看这个短视频，它会清除一切。
回答by 0x90
You need to change the seed.
您需要更改种子。
int main() {
srand(time(NULL));
cout << (rand() % 101);
return 0;
}
the srandseeding thing is true also for a clanguage code.
srand对于c语言代码，播种也是如此。
See also: http://xkcd.com/221/
另见：http: //xkcd.com/221/
回答by DuncanACoulter
For what its worth you are also only generating numbers between 0 and 99 (inclusive). If you wanted to generate values between 0 and 100 you would need.
对于它的价值，您也只生成 0 到 99（含）之间的数字。如果您想生成 0 到 100 之间的值，则需要。
rand() % 101
in addition to calling srand() as mentioned by others.
除了像其他人提到的那样调用 srand() 。
回答by DuncanACoulter
You are not seeding the number.
你不是在播种号码。
Use This:
用这个：
#include <iostream>
#include <ctime>
using namespace std;
int main()
{
srand(static_cast<unsigned int>(time(0)));
cout << (rand() % 100) << endl;
return 0;
}
You only need to seed it once though. Basically don't seed it every random number.
不过你只需要播种一次。基本上不要为每个随机数播种。
回答by DuncanACoulter
srand()seeds the random number generator. Without a seed, the generator is unable to generate the numbers you are looking for. As long as one's need for random numbers is not security-critical (e.g. any sort of cryptography), common practice is to use the system time as a seed by using the time()function from the <ctime>library as such: srand(time(0)). This will seed the random number generator with the system time expressed as a Unix timestamp (i.e. the number of seconds since the date 1/1/1970). You can then use rand()to generate a pseudo-random number.
srand()随机数生成器的种子。如果没有种子，生成器将无法生成您要查找的数字。只要对随机数的需求不是安全关键的（例如任何类型的密码学），通常的做法是使用系统时间作为种子，使用库中的time()函数，<ctime>如下所示：srand(time(0)). 这将为随机数生成器提供以 Unix 时间戳表示的系统时间（即自 1/1/1970 以来的秒数）。然后您可以使用它rand()来生成一个伪随机数。
Here is a quote from a duplicate question:
这是一个重复问题的引述：
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/
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 具有指定一些流行算法的功能），运行一个对这些数字进行数学运算，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/
回答by Alexey Ryabinin
random functions like borland complier
像 borland 编译器这样的随机函数
using namespace std;
int sys_random(int min, int max) {
return (rand() % (max - min+1) + min);
}
void sys_randomize() {
srand(time(0));
}
