C语⾔qsort函数详解
原⽂链接：
qsort包含在<stdlib.h>头⽂件中，此函数根据你给的⽐较条件进⾏快速排序，通过指针移动实现排序。排序之后的结果仍然放在原数组中。使⽤qsort函数必须⾃⼰写⼀个⽐较函数。
函数原型：
void qsort ( void * base, size_t num, size_t size, int ( * comparator ) ( const void *, const void * ) );
⽤法以及参数说明：
Sorts the num elements of the array pointed by base, each element size bytes long, using the comparator function to determine the order. The sorting algorithm used by this function compares pairs of values by calling the specified comparator function with two pointers to elements of the array.
The function does not return any value, but modifies the content of the array pointed by base reordering its elements to the newly sorted order.
base Pointer to the first element of the array to be sorted.(数组起始地址)
num Number of elements in the array pointed by base.(数组元素个数)
size Size in bytes of each element in the array.(每⼀个元素的⼤⼩)
comparator Function that compares two elements.(函数指针，指向⽐较函数)
1、The function must accept two parameters that are pointers to elements, type-casted as void*. These parameters should be cast back to some data type and be compared.
2、The return value of this function should represent whether elem1 is considered less than, equal to, or greater than elem2 by returning, respectively, a negative value, zero or a positive value.
Return Value none (⽆返回值)
⼀、对int类型数组排序
int num[100];
int cmp ( const void *a , const void *b )
{
结构体sizeofreturn *(int *)a - *(int *)b;
}
qsort(num,100,sizeof(num[0]),cmp);
⼆、对char类型数组排序（同int类型）
char word[100];
int cmp( const void *a , const void *b )
{
return *(char *)a - *(int *)b;
}
qsort(word,100,sizeof(word[0]),cmp);
三、对double类型数组排序
double in[100];
int cmp( const void *a , const void *b )
{
return *(double *)a > *(double *)b ? 1 : -1;
}
qsort(in,100,sizeof(in[0]),cmp);
四、对结构体⼀级排序
struct Sample
{
double data;
int other;
}s[100]
//按照data的值从⼩到⼤将结构体排序
int cmp( const void *a ,const void *b)
{
return (*(Sample *)a).data > (*(Sample *)b).data ? 1 : -1; }
qsort(s,100,sizeof(s[0]),cmp);
五、对结构体⼆级排序
struct Sample
{
int x;
int y;
}s[100];
//按照x从⼩到⼤排序，当x相等时按照y从⼤到⼩排序int cmp( const void *a , const void *b )
{
struct Sample *c = (Sample *)a;
struct Sample *d = (Sample *)b;
if(c->x != d->x) return c->x - d->x;
else return d->y - c->y;
}
qsort(s,100,sizeof(s[0]),cmp);
六、对字符串进⾏排序
struct Sample
{
int data;
char str[100];
}s[100];
//按照结构体中字符串str的字典顺序排序
int cmp ( const void *a , const void *b )
{
return strcmp( (*(Sample *)a).str , (*(Sample *)b).str ); }
qsort(s,100,sizeof(s[0]),cmp);
附加⼀个完整点的代码，对字符串⼆维数组排序：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char s[2001][1001];
int cmp(const void *a, const void *b){
return strcmp((char *)a,(char *)b);
}
int main(){
int i,n;
scanf("%d",&n);
getchar();
for(i=0;i<n;i++) gets(s[i]);
qsort(s,n,1001*sizeof(char),cmp);
for(i=0;i<n;i++) puts(s[i]);
return 0;
}

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