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

//============================================================================
// 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(void)
{srand(time(NULL)); //use current time as seed for random generatorint random_variable = rand();printf("Random value on [0,%d]: %d\n", RAND_MAX, random_variable);
}

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

4.3 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(void)
{srand((unsigned int)time(NULL)); //use current time as seed for random generatorconst 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");return(0);
}

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 generatorconst 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 2322^{32}232.
POSIX 要求 rand 所用的伪随机数生成器的周期至少为 2322^{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)

//============================================================================
// 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#include <cstdlib>
#include <iostream>
#include <ctime>int main()
{// use current time as seed for random generator// 以当前时间为随机生成器的种子std::srand(std::time(nullptr));int random_variable = std::rand();std::cout << "Random value on [0 " << RAND_MAX << "]: " << random_variable << '\n';return(0);
}

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

References

http://www.cplusplus.com/reference/cstdlib/rand/
https://en.cppreference.com/w/c/numeric/random/rand
https://en.cppreference.com/w/cpp/numeric/random/rand

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