Hdu-2036-reform spring breeze blowing all over the ground (polygon area template)

The spring breeze of reform is sweeping the ground

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 30103    Accepted Submission(s): 15517

Problem Description

“ The spring breeze of reform is sweeping the ground ,
Can't AC No problem ;
I can't go back to my hometown ,
And a third of an acre .
thank you !（ The band plays ）”

In other words, some students have excellent mentality , I know the game every day , This is such a simple test , It's also in the clouds , and , Even so many Doggerels .
Okay , The teacher's responsibility is to help you solve problems , Since you want to farm , Then share it with you .
This field is located in linjiapuzi village, Lingxi Town, Cangnan County, Wenzhou City, Zhejiang Province , A polygon shaped piece of land , It was linle Of , I'm ready to give it to you now . however , Nothing is that simple , You must first tell me the area of this land , If you answer correctly, you can really get this land .
Worry ？ Just to let you know , Farming is also needed AC Knowledgeable ！ You'd better practice hard in the future ...

 

Input

Input data contains multiple test instances , One line per test instance , The beginning of each line is an integer n(3<=n<=100), It represents the number of sides of a polygon （ It's also the number of vertices ）, Then it is given in counterclockwise order n The coordinates of the vertices （x1, y1, x2, y2... xn, yn）, To simplify the problem , All coordinates here are expressed as integers .
All integers in the input data are in 32 Bit integer range ,n=0 Indicates the end of the data , Don't deal with it .

 

Output

For each test case , Please output the corresponding polygon area , The result is accurate to one decimal place after the decimal point .
The output of each instance takes up one line .

 

Sample Input

  
   

    
   3 0 0 1 0 0 1
4 1 0 0 1 -1 0 0 -1
0
  
   

 

Sample Output

  
   

    
   0.5
2.0
  
   

 

Author

lcy

 

Source

ACM Programming final exam （2006/06/07）

#include <cstdio>

struct Lpoint{
    double x;
    double y;
}plg[110];// Counterclockwise polygon vertex set 
double areaofp(int vcount,Lpoint plg[]){
    int i;
    double s;
    if(vcount<3) return 0;
    s=plg[0].y*(plg[vcount-1].x-plg[1].x);
    for(i=1;i<vcount;i++){
        s+=plg[i].y*(plg[i-1].x-plg[(i+1)%vcount].x);
    }
    return s/2;
}
int main(){
    int n,i;
    while(scanf("%d",&n),n){
        for(i=0;i<n;i++){
            scanf("%lf%lf",&plg[i].x,&plg[i].y);
        }
        printf("%.1f\n",areaofp(n,plg));
    }
    return 0;
}