我们在实现某些应用或者做测试的时候，需要将某些数据乱序。例如1，2，3三个数，随机乱序后，可能是2，1，3。

最简单的方法是：输入一系列数，顺序访问每个位置的数，将当前位置的数与一个随机数相关的位置进行内容交换。

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纯C实现

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

// n : the length of an array
int rand_id(int n){ 
    return rand() % n;
}

// data : 1-D 
// size_ele : size of per element
// len : the length of data
void rand_perm(char* data, int size_ele, int len, int (*rand_id)(int)){
    int i, j, idx;
    char *cptr = data;
    char tmp[16];
    for (i = 0; i < len; i++){
        idx = rand_id(len);
        for (j = 0; j < size_ele; j++){
            tmp[j] = data[idx*size_ele + j];
        }
        for (j = 0; j < size_ele; j++){
            data[idx*size_ele + j] = cptr[j];
        }       
        for (j = 0; j < size_ele; j++){
            cptr[j] = tmp[j];
        }
        cptr += size_ele;
    }
}

#define __N 4
int main(int argc, char** argv){
    // test int
    int a[__N] = {10000000,20000000,30000000,40000000};
    rand_perm((char*)a, sizeof(a[0]), __N, rand_id);
    for (int i = 0; i < __N; i++){
        printf("%d ", a[i]);
    }
    printf("\n");

    // test float
    float b[__N] = {1.E-6f, 2.E-6f, 3.E-6f, 4.E-6f};
    rand_perm((char*)b, sizeof(b[0]), __N, rand_id);
    for (int i = 0; i < __N; i++){
        printf("%f ", b[i]);
    }
    printf("\n");

    // test double
    double c[__N] = { 1.E-6, 2.E-6, 3.E-6, 4.E-6 };
    rand_perm((char*)c, sizeof(c[0]), __N, rand_id);
    for (int i = 0; i < __N; i++){
        printf("%f ", c[i]);
    }
    printf("\n");

    return 0;
}

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输出结果

10000000 40000000 30000000 20000000
0.000001 0.000002 0.000004 0.000003
0.000002 0.000004 0.000003 0.000001 

---

纯C++

C++标准里有随机洗牌函数，我们可以直接调用。

#include <iostream>
#include <algorithm>
using namespace std;

#define __N 4
int main(int argc, char** argv){
    // test int
    int a[__N] = { 10000000, 20000000, 30000000, 40000000 };
    std::random_shuffle(a, a + __N);
    for (int i = 0; i < __N; i++){
        cout << a[i] << " ";
    }
    cout << endl;

    // test float
    float b[__N] = { 1.E-6f, 2.E-6f, 3.E-6f, 4.E-6f };
    std::random_shuffle(b, b + __N);
    for (int i = 0; i < __N; i++){
        cout << std::fixed << b[i] << " ";
    }
    cout << endl;

    // test double
    double c[__N] = { 1.E-6, 2.E-6, 3.E-6, 4.E-6 };
    std::random_shuffle(c, c + __N);
    for (int i = 0; i < __N; i++){
        cout << std::fixed << c[i] << " ";
    }
    cout << endl;
    return EXIT_SUCCESS;
}

---

输出结果

10000000 20000000 40000000 30000000
0.000004 0.000001 0.000003 0.000002
0.000003 0.000001 0.000004 0.000002