問題描述
有一次，我的老師教我使用 randomize() 和 random() 函數在 C++ Builder 中生成偽隨機數.現在我更喜歡在 VS 2012 中工作，但是當我嘗試在 VS 2012 中使用這些函數時，即使我添加了 #include ，它也會顯示未找到標識符".經過一段時間的谷歌搜索，我發現還有 rand() 和 srand() 函數.它們有什么區別，哪個更好用?
Once, my teacher taught me to use randomize() and random() function for generating pseudorandom numbers in C++ Builder. Now I prefer working in VS 2012, but when I tried to use these functions there it says that "identifier not found", even when I added #include <stdlib.h>. After some time of Googling I found that there are also rand() and srand() functions. What is the difference between them and which is it better to use?
推薦答案
randomize() 和 random() 不是標準庫的一部分.也許您的老師用這些名稱編寫了用于您班級的函數，或者您的意思可能是 random() 和 srandom()，它們是 POSIX 的一部分，在 Windows 上不可用.rand() 和 srand() 是標準庫的一部分，將由任何符合標準的 C++ 實現提供.
randomize() and random() are not part of the standard library. Perhaps your teacher wrote functions with these names for use in your class, or maybe you really mean random() and srandom() which are part of POSIX and not available on Windows. rand() and srand() are part of the standard library and will be provided by any standard conforming implementation of C++.
您應該避免使用 rand() 和 srand() 并使用新的 C++11
You should avoid rand() and srand() and use the new C++11 <random> library. <random> was added as part of the C++11 standard (and VS2012 does provide it).
解釋原因的視頻:rand()認為有害
Video explaining why: rand() Considered Harmful
rand()通常是低質量的 pRNG，不適合需要合理水平不可預測性的應用程序.
rand()is typically a low quality pRNG and not suitable for applications that need a reasonable level of unpredictability.<random>provides a variety of engines with different characteristics suitable for many different use cases.
將rand()的結果轉化為可以直接使用的數字，通常依賴于難讀易出錯的代碼，而使用
Converting the results of rand() into a number you can use directly usually relies on code that is difficult to read and easy to get wrong, whereas using <random> distributions is easy and produces readable code.
使用 rand() 在給定分布中生成值的常用方法進一步降低了生成數據的質量.% 通常會使數據產生偏差，浮點除法仍然會產生非均勻分布.
The common methods of generating values in a given distribution using rand() further decrease the quality of the generated data. % generally biases the data and floating point division still produces non-uniform distributions. <random> distributions are higher quality as well as more readable.
rand() 依賴于一個隱藏的全局資源.除其他問題外，這會導致 rand() 不是線程安全的.一些實現可以保證線程安全，但這不是必需的標準.<random> 提供的引擎將 pRNG 狀態封裝為具有值語義的對象，允許對狀態進行靈活控制.
rand() relies on a hidden global resource. Among other issues this causes rand() to not be thread safe. Some implementations make thread safety guarantees, but this is not required standard. Engines provided by <random> encapsulate pRNG state as objects with value semantics, allowing flexible control over the state.
srand() 只允許有限范圍的種子.seed_seq 還實現了一個常見的 pRNG 預熱.
srand() only permits a limited range of seeds. Engines in <random> can be initialized using seed sequences which permit the maximum possible seed data. seed_seq also implements a common pRNG warm-up.
使用
#include <iostream>
#include <random>
int main() {
// create source of randomness, and initialize it with non-deterministic seed
std::random_device r;
std::seed_seq seed{r(), r(), r(), r(), r(), r(), r(), r()};
std::mt19937 eng{seed};
// a distribution that takes randomness and produces values in specified range
std::uniform_int_distribution<> dist(1,6);
for (int i=0; i<100; ++i) {
std::cout << dist(eng) << '
';
}
}
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