叉积求面积 hdu2036（模版题）

#include <iostream>
#include <stdio.h>

using namespace std;

typedef struct node
{
    double x;
    double y;
}point;
point p[105];

point make_e(point x,point y)
{
    point a;
    a.x = x.x - y.x;
    a.y = x.y - y.y;
    return a;
}

double cha(point x,point y)
{
    return x.x*y.y - y.x*x.y;
}

int main()
{
    int n,i,j;
    while(scanf("%d",&n)&&n)
    {
        for(i = 0;i < n;i++)
        scanf("%lf %lf",&p[i].x,&p[i].y);

        point e1,e2;
        e1 = make_e(p[0],p[1]);
        e2 = make_e(p[0],p[2]);
        double ans;
        ans = 0;
        ans += cha(e1,e2);
        for(i = 3;i < n;i++)
        {
            e1 = e2;
            e2 = make_e(p[0],p[i]);
            ans+= cha(e1,e2);
        }
        printf("%.1lf\n",ans/2);
    }
    return 0;
}

pc代码

#include<iostream>
#include<cmath>
#include<cstring>
#include<cstdio>
using namespace std;

const int N=102;

struct node
{
    int x,y;
}a,b,c,ss[N];
int n;

void gotp(node *p,int i,int j)
{//向量赋值
    p->x=ss[j].x-ss[i].x;
    p->y=ss[j].y-ss[i].y;
}
double cm(node a,node b)
{//叉积计算面积
    return (a.x*b.y-a.y*b.x);//可能有凹多边形，故未加fabs();
}
double calsum()
{
    int i,j,k;
    double ans=0;
    gotp(&a,0,1);
    gotp(&b,0,2);
    ans+=cm(a,b);//第一个三角形面积的2倍被加入
    for(i=3;i<n;i++)
    {
        a=b;
        gotp(&b,0,i);
        ans+=cm(a,b);
    }
    return ans/2;
}

int main()
{
    while(cin>>n,n)
    {
        memset(ss,0,sizeof(ss));
        for(int i=0;i<n;i++)
        cin>>ss[i].x>>ss[i].y;
        double ans=calsum();
        printf("%.1f\n",ans);
    }
    return 0;
}