E - 改革春风吹满地

 1 #include <iostream>
 2 #include <cstring>
 3 #include <string>
 4 #include <map>
 5 #include <set>
 6 #include <algorithm>
 7 #include <fstream>
 8 #include <cstdio>
 9 #include <cmath>
10 #include <stack>
11 #include <queue>
12 using namespace std;
13 const double Pi=3.14159265358979323846;
14 typedef long long ll;
15 const int MAXN=5000+5;
16 const int dx[5]={0,0,0,1,-1};
17 const int dy[5]={1,-1,0,0,0};
18 const int INF = 0x3f3f3f3f;
19 const int NINF = 0xc0c0c0c0;
20 const ll mod=1e9+7;
21 struct point{
22     double x;double y;
23     point (double xx,double yy):x(xx),y(yy) {}
24 };
25 
26 double ComAre(const vector <point> &V)
27 {
28     int len=V.size();
29     if(len<3) return 0.0;
30     double s=0;
31     for(int i=0;i<len;i++)
32     {
33         s+=V[i].x*V[(i+1)%len].y-V[i].y*V[(i+1)%len].x;
34     }
35     return fabs(s/2);
36 }
37 
38 int main()
39 {
40     int n;
41     while(cin>>n&&n)
42     {
43         vector <point> V;
44         for(int i=1;i<=n;i++)    
45         {
46             double a,b;cin>>a>>b;
47             V.push_back(point(a,b));
48         }
49         double ans=ComAre(V);
50         printf("%.1lf\n",ans);
51     }
52     return 0;
53  }