凸包求面積，周長

#include <stdio.h>#include <stdlib.h>#include <math.h>#include <conio.h>#define N 200#define inf 1e-6typedef struct{    double x;    double y;}point;point points[N]; //點集point chs[N];     //棧int sp; //棧頂指標//計算兩點之間距離double dis(point a, point b){    return sqrt((a.x - b.x) * (a.x - b.x) * 1.0 + (a.y - b.y) * (a.y - b.y));}//通過向量叉積求極角關係（p0p1）(p0p2)//k > 0 ,p0p1在p0p2順時針方向上double multi(point p0, point p1, point p2){    return (p1.x - p0.x) * (p2.y - p0.y) - (p2.x - p0.x) * (p1.y - p0.y);}int cmp(const void *p, const void *q){     point a = *(point *)p;     point b = *(point *)q;    double k = multi(points[0], a, b);    if(k < -inf)        return 1;    else if(fabs(k) < inf && (dis(a, points[0]) - dis(b, points[0])) > inf) //兩點在同一直線上的話，用最近的        return 1;    else return -1;}void convex_hull(int n){    int i, k, d;    double miny = points[0].y;    int index = 0;    for(i=1; i<n; i++) //找最左下頂點    {        if(points[i].y < miny) //找到y座標最小的點        {             miny = points[i].y;             index = i;        }        else if(points[i].y == miny && points[i].x < points[index].x) //相同的話找到x最小的        {             index = i;        }    }    //把最左下頂點放到第一個     point temp;     temp = points[index];     points[index] = points[0];     points[0] = temp;    qsort(points+1, n-1, sizeof(points[0]), cmp); //p[1:n-1]按相對p[0]的極角從小到大排序     chs[0] = points[n-1];     chs[1] = points[0];     sp = 1;     k = 1;    while(k <= n-1)    {        double d = multi(chs[sp], chs[sp-1], points[k]);        if(d <= 0)        {             sp++;             chs[sp] = points[k];             k++;        }        else sp--;    }}int main(){    int i, j, n;    double sum, area;    while(1)    {        scanf("%d",&n);        if(n==0)break;        for(i=0; i<n; i++)            scanf("%lf%lf", &points[i].x, &points[i].y);        convex_hull(n);        sum = 0.;        for(i=1; i<=sp; i++) //求周長            sum += dis(chs[i-1], chs[i]);        sum += dis(chs[0], chs[sp]);        printf("周長：%.4f\n", sum);        area = 0.;        for(i=1; i<sp; i++) {//求面積            double k = multi(chs[0], chs[i], chs[i+1]); //三角形面積是向量叉積的1/2            area += k;        }        area = fabs(area) / 2;        printf("面積：%.4f\n", area);    }    return 0;}