hdu2036(多边形面积)

 1 #include<iostream>
 2 #include<cmath>
 3 #include<iomanip>
 4 using namespace std;
 5 
 6 struct point
 7 {
 8           int x;
 9           int y;
10 };
11 
12 point a[100];
13 
14 double area(point m,point n)
15 {
16           return (double)(m.x*n.y - m.y*n.x)/2;
17 }
18 
19 int main()
20 {
21           int n;
22           while(cin>>n)
23           {
24                     if(n==0)
25                               break;
26                     for(int i = 0;i<n;i++)
27                               cin>>a[i].x>>a[i].y;
28                     double sum = area(a[n-1],a[0]);
29                     for(int i = 1;i<n;i++)
30                     {
31                               sum+=area(a[i-1],a[i]);
32                     }
33                     cout<<fixed<<setprecision(1)<<sum<<endl;
34           }
35 
36 }