C 中的随机数组 [英] Shuffle array in C

问题描述

我正在 ANSI C 中寻找一个函数，它可以像 PHP 的 shuffle() 那样随机化一个数组.有没有这样的功能还是必须自己写?如果我必须自己编写它，最好/最高效的方法是什么?

I'm looking for a function in ANSI C that would randomize an array just like PHP's shuffle() does. Is there such a function or do I have to write it on my own? And if I have to write it on my own, what's the best/most performant way to do it?

我目前的想法:

• 遍历数组，例如 100 次，然后将一个随机索引与另一个随机索引交换
• 创建一个新数组并用第一个数组中的随机索引填充它，每次检查索引是否已经被占用(性能 = 0 复杂性 = 严重)

推荐答案

粘贴自 Asmodiel 的 链接到Ben Pfaff 的著作，为了坚持:

Pasted from Asmodiel's link to Ben Pfaff's Writings, for persistence:

#include <stdlib.h>

/* Arrange the N elements of ARRAY in random order.
   Only effective if N is much smaller than RAND_MAX;
   if this may not be the case, use a better random
   number generator. */
void shuffle(int *array, size_t n)
{
    if (n > 1) 
    {
        size_t i;
        for (i = 0; i < n - 1; i++) 
        {
          size_t j = i + rand() / (RAND_MAX / (n - i) + 1);
          int t = array[j];
          array[j] = array[i];
          array[i] = t;
        }
    }
}

EDIT:这是一个通用版本，适用于通过 memcpy 的任何类型(int、struct、...).要运行示例程序，它需要 VLA，并非每个编译器都支持此功能，因此您可能希望将其更改为 malloc(这将导致性能不佳)或足够大的静态缓冲区以容纳您抛出的任何类型在它:

EDIT: And here's a generic version that works for any type (int, struct, ...) through memcpy. With an example program to run, it requires VLAs, not every compiler supports this so you might want to change that to malloc (which will perform badly) or a static buffer large enough to accommodate any type you throw at it:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

/* compile and run with
 * cc shuffle.c -o shuffle && ./shuffle */

#define NELEMS(x)  (sizeof(x) / sizeof(x[0]))

/* arrange the N elements of ARRAY in random order.
 * Only effective if N is much smaller than RAND_MAX;
 * if this may not be the case, use a better random
 * number generator. */
static void shuffle(void *array, size_t n, size_t size) {
    char tmp[size];
    char *arr = array;
    size_t stride = size * sizeof(char);

    if (n > 1) {
        size_t i;
        for (i = 0; i < n - 1; ++i) {
            size_t rnd = (size_t) rand();
            size_t j = i + rnd / (RAND_MAX / (n - i) + 1);

            memcpy(tmp, arr + j * stride, size);
            memcpy(arr + j * stride, arr + i * stride, size);
            memcpy(arr + i * stride, tmp, size);
        }
    }
}

#define print_type(count, stmt) \
    do { \
    printf("["); \
    for (size_t i = 0; i < (count); ++i) { \
        stmt; \
    } \
    printf("]\n"); \
    } while (0)

struct cmplex {
    int foo;
    double bar;
};

int main() {
    srand(time(NULL));

    int intarr[] = { 1, -5, 7, 3, 20, 2 };

    print_type(NELEMS(intarr), printf("%d,", intarr[i]));
    shuffle(intarr, NELEMS(intarr), sizeof(intarr[0]));
    print_type(NELEMS(intarr), printf("%d,", intarr[i]));

    struct cmplex cmparr[] = {
        { 1, 3.14 },
        { 5, 7.12 },
        { 9, 8.94 },
        { 20, 1.84 }
    };

    print_type(NELEMS(intarr), printf("{%d %f},", cmparr[i].foo, cmparr[i].bar));
    shuffle(cmparr, NELEMS(cmparr), sizeof(cmparr[0]));
    print_type(NELEMS(intarr), printf("{%d %f},", cmparr[i].foo, cmparr[i].bar));

    return 0;
}

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