C / C++ function - rand

柔情只为你懂 2022-10-29 13:24 141阅读 0赞

# **C / C++ function - rand** #

Defined in header `<stdlib.h>`

int rand (void);

## 0. 产生一定范围内的随机数 ##

产生 `[a, b)` 的随机整数，使用 `(rand() % (b - a)) + a;`。  
`[0, (b - a) - 1] + a = [a, (b - 1)] = [a, b)`  
产生 `[a, b]` 的随机整数，使用 `(rand() % (b - a + 1)) + a;`。  
`[0, (b - a + 1) - 1] + a = [0, (b - a)] + a = [a, b]`  
产生 `(a, b]` 的随机整数，使用 `(rand() % (b - a)) + a + 1;`。  
`[0, (b - a) - 1] + a + 1 = [0, (b - a - 1)] + a + 1 = [a + 1, b] = (a, b]`

产生 0～1 之间的浮点数，可以使用 `rand() / double(RAND_MAX)`。

## 1. Generate random number ##

Returns a pseudo-random integral number in the range between `0` and `RAND_MAX` (`0` and `RAND_MAX` included).  
返回介于 `0` 和 `RAND_MAX` 之间的伪随机整数 (包含 `0` 与 `RAND_MAX`)。  
`RAND_MAX` 是一个常量，它的默认值在不同的实现中会有所不同，但是值至少是 `32767`。

This number is generated by an algorithm that returns a sequence of apparently non-related numbers each time it is called. This algorithm uses a seed to generate the series, which should be initialized to some distinctive value using function `srand`.  
该数字是由一种算法生成的，该算法每次调用时都会返回一个看起来不相关的数字序列。该算法使用种子来生成序列，应使用函数 `srand` 将其初始化为一些独特的值。

`RAND_MAX` is a constant defined in `<cstdlib>`.

`srand()` seeds the pseudo-random number generator used by `rand()`. If `rand()` is used before any calls to `srand()`, `rand()` behaves as if it was seeded with `srand(1)`. Each time `rand()` is seeded with `srand()`, it must produce the same sequence of values.  
`srand()` 播种 `rand()` 所用的伪随机数生成器。若在任何对 `srand()` 的调用前使用 `rand()`，则 `rand()` 表现如同它以 `srand(1)` 播种。每次以 `srand()` 播种 `rand()` 时，它必须产生相同的值数列。

`rand()` is not guaranteed to be thread-safe.  
不保证 `rand()` 为线程安全。

A typical way to generate trivial pseudo-random numbers in a determined range using `rand` is to use the modulo of the returned value by the range span and add the initial value of the range:  
使用 `rand` 生成确定范围内的伪随机数的典型方法是：对返回值取模值并加上初始值。

v1 = rand() % 100;         // v1 in the range 0 to 99 = [0, 99]
    v2 = rand() % 100 + 1;     // v2 in the range 1 to 100 = [0, 99] + 1 = [1, 100]
    v3 = rand() % 30 + 1985;   // v3 in the range 1985 - 2014  = [0, 29] + 1985 = [1985, 2014]
    v4 = rand() % 200 + 1;     // [0, 199] + 1 = [1, 200]

Notice though that this modulo operation does not generate uniformly distributed random numbers in the span (since in most cases this operation makes lower numbers slightly more likely).  
请注意，此模运算不会在跨度范围中生成均匀分布的随机数 (因为在大多数情况下，此运算使较小的数字更有可能出现)。

C++ supports a wide range of powerful tools to generate random and pseudo-random numbers (see `<random>` for more info).  
C++ 支持各种强大的工具来生成随机和伪随机数 (有关更多信息，请参见 `<random>`)。

## 2. Parameters ##

(none)

## 3. Return Value ##

An integer value between `0` and `RAND_MAX`.  
`0` 与 `RAND_MAX` 间包含边界的随机整数值。

## 4. Example ##

### 4.1 Example ###

//============================================================================
    // Name        : Yongqiang Cheng
    // Author      : Yongqiang Cheng
    // Version     : Version 1.0.0
    // Copyright   : Copyright (c) 2019 Yongqiang Cheng
    // Description : Hello World in C++, Ansi-style
    //============================================================================
    
    #ifndef _CRT_SECURE_NO_WARNINGS
    #define _CRT_SECURE_NO_WARNINGS
    #endif
    
    /* rand example: guess the number */
    #include <stdio.h>      /* printf, scanf, puts, NULL */
    #include <stdlib.h>     /* srand, rand */
    #include <time.h>       /* time */
    
    int main()
    {
    	int Secret, Guess;
    
    	/* initialize random seed: */
    	srand(time(NULL));
    
    	/* generate secret number between 1 and 10: */
    	Secret = rand() % 10 + 1;
    
    	do {
    		printf("Guess the number (1 to 10): ");
    		scanf("%d", &Guess);
    		if (Secret < Guess) puts("The secret number is lower");
    		else if (Secret > Guess) puts("The secret number is higher");
    	} while (Secret != Guess);
    
    	puts("Congratulations!");
    
    	return 0;
    }

In this example, the random seed is initialized to a value representing the current time (calling `time`) to generate a different value every time the program is run.  
在这个例子中，随机种子被初始化为代表当前时间的值 (称为 `time`)，以在每次程序运行时产生一个不同的值。

Possible output:

Guess the number (1 to 10): 9
    The secret number is lower
    Guess the number (1 to 10): 5
    The secret number is higher
    Guess the number (1 to 10): 6
    The secret number is higher
    Guess the number (1 to 10): 7
    The secret number is higher
    Guess the number (1 to 10): 8
    Congratulations!
    请按任意键继续. . .

### 4.2 Example ###

Random value on [0,32767]: 12901
    请按任意键继续. . .

### 4.3 Example ###

Random value on [0, 32767]: 20719
    28 16 33 2 4 17 29 32 21 17 29 27 45 15 21 2
    请按任意键继续. . .

### 4.4 Example - 生成随机数数组 ###

//============================================================================
    // Name        : Yongqiang Cheng
    // Author      : Yongqiang Cheng
    // Version     : Version 1.0.0
    // Copyright   : Copyright (c) 2019 Yongqiang Cheng
    // Description : Hello World in C++, Ansi-style
    //============================================================================
    
    #ifndef _CRT_SECURE_NO_WARNINGS
    #define _CRT_SECURE_NO_WARNINGS
    #endif
    
    /* rand example: guess the number */
    #include <stdio.h>      /* printf, scanf, puts, NULL */
    #include <stdlib.h>     /* srand, rand */
    #include <time.h>       /* time */
    
    /* 输出 1 到 50 之间的 n 个随机数 */
    int random_number_generator(int *number_array, const int n)
    {
    	for (int i = 0; i < n; ++i) {
    		number_array[i] = rand() % 50 + 1;
    	}
    
    	return 0;
    }
    
    
    int main(void)
    {
    	srand((unsigned int)time(NULL)); // use current time as seed for random generator
    	const int random_variable = rand();
    	printf("Random value on [0, %d]: %d\n", RAND_MAX, random_variable);
    
    	const int n = 16;
    	/* 输出 0 到 49 之间的 16 个随机数 */
    	for (int i = 0; i < n; ++i) {
    		printf("%d ", rand() % 50);
    	}
    
    	printf("\n");
    
    	const int num = 10;
    	int number_array[num] = { 0 };
    	int ret = random_number_generator(number_array, num);
    
    	/* 输出 1 到 50 之间的 10 个随机数 */
    	for (int i = 0; i < num; ++i) {
    		printf("%d ", number_array[i]);
    	}
    
    	printf("\n");
    
    	return(0);
    }

Random value on [0, 32767]: 26738
    19 44 16 23 7 14 13 39 10 19 3 15 34 39 22 19
    31 6 12 48 49 34 41 17 8 10
    请按任意键继续. . .

## 5. Compatibility ##

In C, the generation algorithm used by `rand` is guaranteed to only be advanced by calls to this function. In C++, this constraint is relaxed, and a library implementation is allowed to advance the generator on other circumstances (such as calls to elements of `<random>`).  
在 C 语言中，只能通过调用此函数来保证 `rand` 使用的生成算法。在 C++ 中，此约束得到了放松，a library implementation is allowed to advance the generator on other circumstances (例如，对 `<random>` 元素的调用)。

## 6. Data races ##

The function accesses and modifies internal state objects, which may cause data races with concurrent calls to `rand` or `srand`.  
该函数访问和修改内部状态对象，这可能导致并发调用 `rand` 或 `srand` 的数据竞争。

Some libraries provide an alternative function that explicitly avoids this kind of data race: `rand_r` (non-portable).  
一些库提供了一个替代函数，它明确避免了这种数据竞争：`rand_r` (不可移植)。

C++ library implementations are allowed to guarantee no `data races` for calling this function.  
允许 C++ 库实现保证不存在调用该函数的数据竞争。

There are no guarantees as to the quality of the random sequence produced. In the past, some implementations of `rand()` have had serious shortcomings in the randomness, distribution and period of the sequence produced (in one well-known example, the low-order bit simply alternated between `1` and `0` between calls). `rand()` is not recommended for serious random-number generation needs, like cryptography.  
无对产生的随机数质量的保证。过去，某些 `rand()` 的实现在随机性、分布和产生的数列周期中有严重缺陷 (在一个广为人知的例子中，最低位在调用间简单地于 1 和 0 间改变)。不推荐将 `rand()` 用于严肃的随机数生成需求，如加密。

POSIX requires that the period of the pseudo-random number generator used by `rand` is at least  2 32 2^\{32\} 232.  
POSIX 要求 `rand` 所用的伪随机数生成器的周期至少为  2 32 2^\{32\} 232。

POSIX offered a thread-safe version of rand called `rand_r`, which is obsolete in favor of the `drand48` family of functions.  
POSIX 提供 `rand` 的线程安全版本，名为 `rand_r`，它由于 `drand48` 函数族而过时。

## 7. Exceptions (C++) ##

**No-throw guarantee:** this function never throws exceptions (该函数从不抛出异常。).

## 8. `std::rand` ##

Defined in header `<cstdlib>`

Returns a pseudo-random integral value between `0` and `RAND_MAX` (`0` and `RAND_MAX` included).  
返回 `0` 与 `RAND_MAX` (包含 `0` 与 `RAND_MAX`) 的随机数。

`std::srand()` seeds the pseudo-random number generator used by `rand()`. If `rand()` is used before any calls to `srand()`, `rand()` behaves as if it was seeded with `srand(1)`.  
`std::srand()` 播种 `rand()` 所用的伪随机数生成器。若在任何到 `srand()` 的调用前使用 `rand()`，则 `rand()` 表现如同它以 `srand(1)` 播种。

Each time `rand()` is seeded with `srand()`, it must produce the same sequence of values on successive calls.  
每次以 `srand()` 播种 `rand()` ，它必须在后续调用上产生相同的值数列。

Other functions in the standard library may call `rand`. It is implementation-defined which functions do so.  
标准库中的其他函数可调用 `rand`。

It is implementation-defined whether `rand()` is thread-safe.  
`rand()` 是否线程安全是实现定义的。

`rand()` is not recommended for serious random-number generation needs. It is recommended to use C++11’s random number generation facilities to replace `rand()`. (since C++11)  
对于严肃的随机数生成需求不推荐 `rand()`。推荐用 C++11 的随机数生成函数替换 `rand()`。(C++11 起)

Random value on [0 32767]: 2085
    请按任意键继续. . .

## References ##

[http://www.cplusplus.com/reference/cstdlib/rand/][http_www.cplusplus.com_reference_cstdlib_rand]  
[https://en.cppreference.com/w/c/numeric/random/rand][https_en.cppreference.com_w_c_numeric_random_rand]  
[https://en.cppreference.com/w/cpp/numeric/random/rand][https_en.cppreference.com_w_cpp_numeric_random_rand]

[http_www.cplusplus.com_reference_cstdlib_rand]: http://www.cplusplus.com/reference/cstdlib/rand/
[https_en.cppreference.com_w_c_numeric_random_rand]: https://en.cppreference.com/w/c/numeric/random/rand
[https_en.cppreference.com_w_cpp_numeric_random_rand]: https://en.cppreference.com/w/cpp/numeric/random/rand