C语言标准库函数qsort详解

C语言标准库函数 qsort 详解

来源：http://www.slyar.com/blog/stdlib-qsort.html

qsort包含在<stdlib.h>头文件中，此函数
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 )

{

return *(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;

}