C语言qsort函数怎么用？

Question

#include
using namespace std;

typedef struct
{
unsigned long id;
int score;
}record;

int main()
{
int id_cmp(const void* s1,const void* s2);
int score_cmp(const void* s1,const void* s2);

record recordset[] = {{3,99},{5,87},{4,56},{2,100},{1,17}};

int recordcount = sizeof(recordset) / sizeof(record);

printf("排序前:\n");
for(int i = 0; i < recordcount;i++)
    printf("%8d %8u %8d\n",i,recordset[i].id,recordset[i].score);

qsort(recordset,recordcount,sizeof(record),id_cmp);
printf("排序后:\n");
for(int i = 0; i < recordcount;i++)
    printf("%8d %8u %8d\n",i,recordset[i].id,recordset[i].score);


qsort(recordset,recordcount,sizeof(record),score_cmp);
printf("排序后:\n");
for(int i = 0; i < recordcount;i++)
    printf("%8d %8u %8d\n",i,recordset[i].id,recordset[i].score);

return 0;

}

int id_cmp(const void* s1,const void* s2)
{
record* p1 = (record* )s1;
record* p2 = (record* )s2;
if(p1->id < p2->id) return -1;
else if(p1->id == p2->id) return 0;
else return 1;
}

int score_cmp(const void* s1,const void* s2) //这个函数指针为什么能改变排序顺序
{
record* p1 = (record* )s1;
record* p2 = (record* )s2;
if(p1->score < p2->score) return -1;
else if(p1->score == p2->score) return 0;
else return 1;
}

Answer 1

因为排序的关键是对要排序的数据两两比较大小，比较大小的不同决定了排序依据的不同，传入不同的函数指针，实现不同的大小比较就能改变排序顺序。