这是一个看似常见的问题,所以我希望我听起来不是多余的。但是从 rand() 返回的范围应该在 0 和 RAND_MAX 之间,但是,当我执行一个非常简单的 rand 语句时,我总是得到一个非常小的范围内的返回值。
该范围类似于 1,4XX,XXX,XXX。我认为这可能是一个时钟问题,所以我等了三十分钟,但仍然得到相同范围内的数字。
以下是二十分钟前的一些示例输出:
Matthews-Macbook-Pro:Data_Structures matthewwright$ ./main
1439810968
80
Matthews-Macbook-Pro:Data_Structures matthewwright$ ./main
1439827775
29
Matthews-Macbook-Pro:Data_Structures matthewwright$ ./main
1439827775
29
Matthews-Macbook-Pro:Data_Structures matthewwright$ ./main
1439844582
78
Matthews-Macbook-Pro:Data_Structures matthewwright$ ./main
1439878196
29
Matthews-Macbook-Pro:Data_Structures matthewwright$ ./main
1439895003
78
这是刚才的输出示例:
Matthews-Macbook-Pro:Data_Structures matthewwright$ ./main
1456483512
78
Matthews-Macbook-Pro:Data_Structures matthewwright$ ./main
1456500319
80
Matthews-Macbook-Pro:Data_Structures matthewwright$ ./main
1456500319
80
Matthews-Macbook-Pro:Data_Structures matthewwright$ ./main
1456517126
29
Matthews-Macbook-Pro:Data_Structures matthewwright$ ./main
1456533933
78
我知道 rand() 并不完美,但这看起来太相似而不是正确的。如果范围是 0 - RAND_MAX,则返回的每个数字都在同一范围内似乎很奇怪。
这是我正在测试的代码:
#include <iostream>
#include <stdio.h> /* printf, scanf, puts, NULL */
#include <stdlib.h> /* srand, rand */
#include <time.h> /* time */
int main(int argc, char const *argv[])
{
/* declarations */
srand(time(NULL));
std::cout << std::rand() << std::endl;
std::cout << std::rand()%100 << std::endl;
return 0;
}
我认为我不需要所有这些 #include 语句,但我看到其他人使用它们,所以我将它们包含在内,以防它会影响我的输出,但事实并非如此。
Edit
所以@Mgetz 和@Curious 提供的链接非常有帮助。以巩固,
信息页面:http://en.cppreference.com/w/cpp/numeric/random/uniform_int_distribution http://en.cppreference.com/w/cpp/numeric/random/uniform_int_distribution
超级有用的讲座(真正的,观看这个):https://channel9.msdn.com/Events/GoingNative/2013/rand-Considered-Harmful https://channel9.msdn.com/Events/GoingNative/2013/rand-Considered-Harmful
我把我在讲座中听到的内容总结在自己的笔记上,这样如果我忘记了,就不用再重新研究了。我没有在这里编写代码,大部分代码都在上面链接的“信息页面”中。大多数评论包含讲座中的信息,尽管不是讲座中的逐字内容。再次,我真的建议观看。它内容丰富,信息丰富。
#include <iostream>
#include <random>
int main(int argc, char const *argv[])
{
/* https://channel9.msdn.com/Events/GoingNative/2013/rand-Considered-Harmful */
/* Randomness Verson 1 : Deterministic */
std::mt19937 mt(1234);
std::uniform_int_distribution<int> dist(0,127);
/* Default is int, but we could specify others.
* The range is [inclusive, inclusive]
*
* Above is Mersenne Twister RNG. It is deterministic, meaning we can get the same result
* if we use "std::mt19937 mt(1234)"; or something like that. This could be useful for some
* people (He mentions games, some experiments, et cetera). It is stupid fast.
*
* However, it isn't cryptographically secure, but it pretty random as random goes. If you
* track the output though, you could guess the next numbers, so don't use it for anything
* secure.
*/
/* Randomness Verson 2 */
std::random_device rd; //Will be used to obtain a seed for the random number engine
std::mt19937 gen(rd()); //Standard mersenne_twister_engine seeded with rd()
std::uniform_int_distribution<int> dis(0, 127); // Inclusive
/* This is not reproducible. This is not deterministic.
* "Possibly Crypto-secure." Seems like using Random Device makes this near perfect random,
* assuming some conditions. I'm not a man who's written security software, and if you are
* writing security software, I assume you're not looking at StackOverflow to figure out how
* to do random numbers. The way he talked about it in the lecture made this seem much more
* secure, but I'm not sure what I'm talking about when it comes to these things
*/
for (int i = 0; i < 3; ++i)
{
/* Below would output the pure Mersenne Twister output, deterministic. This seems to
* be pretty random, but it isn't totally random. */
std::cout << dist(mt) << " ";
/* And below would output the random device output. This should be slower, but
* more truly random. */
//Use dis to transform the random unsigned int generated by gen into an int in [1, 6]
std::cout << dis(gen) << " ";
std::cout<< std::endl;
}
}