qsort

编译器函数库自带的快速排序函数。

void qsort(void*base,size_t num,size_t width,int(__cdecl*compare)(const void*,const void*));

参数解释：

• void*base-待排序数组首地址
• size_t num-数组中待排序元素数量
• size_t width-各元素的占用空间大小
• int(__cdecl*compare)(const void*,const void*)-指向比较两个元素的函数的指针

qsort函数的用法

#include <stdio.h>      /* printf */
#include <stdlib.h>     /* qsort */

int compare (const void * a, const void * b)
{
  return ( *(int*)a - *(int*)b );
}

int main ()
{
  int n;
  int values[] = { 40, 10, 100, 90, 20, 25 };
  qsort (values, 6, sizeof(int), compare);
  for (n=0; n<6; n++)
     printf ("%d ",values[n]);
  system("pause");
  return 0;
}

例子2：

#include <stdio.h>      /* printf */
#include <stdlib.h>     /* qsort */

int compare(const void * a, const void * b)
{
    return (*(int*)a - *(int*)b);
}

int main()
{
    char s[3][4] = { "z","g","c" };  //字符串数组排序 
    qsort(s, 3, sizeof(s[0]), compare); //sizeof(s[0])=sizeof(char)
    for (int i = 0; i<3; i++)
        printf("%s\n", s[i]);
    system("pause");
    return 0;
}

模拟实现qsort

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

/*
判断n1,n2元素大小，n1比n2大返回正数；小返回负数，相同返回0
*/
int cmp(const void*n1, const void*n2)
{
    return *(char*)n1 - *(char*)n2;                //升序  
}

/*
交换每个字节
sizeof(char) = 1;sizeof(int ) = 4;
sizeof(arr1[0])=1;sizeof(arr[0])=4;
*/
void Swap(char *buf1, char* buf2, int width)
{
    int i = 0;
    for (i = 0; i < width; i++)
    {
        char tmp = *buf1;
        *buf1 = *buf2;
        *buf2 = tmp;
        buf1++;
        buf2++;
    }
}

/*
冒泡排序法
*/
void bubble_sort(void *base, int sz, int width, int(*cmp)(const void* n1, const void*n2))      //模拟实现qsort  
{
    int i = 0;
    for (i = 0; i < sz - 1; i++)
    {
        int j = 0;
        for (j = 0; j < sz - 1 - i; j++)
        {
            int ret = cmp(((char*)base + (j*width)), ((char*)base + (j + 1)*width));
            if (ret>0)
            {
                Swap(((char*)base + (j*width)), ((char*)base + (j + 1)*width), width);
            }
        }
    }
}

int main()
{
    int i = 0;
    int arr[] = { 1, 4, 7, 5, 6, 9 };
    bubble_sort(arr, sizeof(arr) / sizeof(arr[0]), sizeof(arr[0]), cmp);
    for (i = 0; i < sizeof(arr) / sizeof(arr[0]); i++)
    {
        printf("%d ", arr[i]);
    }
    printf("\n");


    /*
    printf输出格式"%s"
    */
    char arr1[] = { 'a','f','y','w','b' };
    bubble_sort(arr1, sizeof(arr1) / sizeof(arr1[0]), sizeof(arr1[0]), cmp);
    for (i = 0; i < sizeof(arr1) / sizeof(arr1[0]); i++)
    {
        printf("%c ", arr1[i]);
    }
    printf("\n");
    system("pause");
    return 0;
}