How do I calculate centroid of a polygon if the vertices of the polygon are given?

http://ift.tt/1wbv17e

I visited the above link and tried to implement the formula to formulate the centroid of a non-self-intersecting closed polygon defined by n vertices (x0,y0), (x1,y1), ..., (xn−1,yn−1).

For instance if the co-ordinates are : (0,1),(1,0),(-1,0) and (0,-1), the resulting co-ordinates of centroid should be :

     0.00 0.00



       #include<stdio.h>
       #include<iostream>
       #include<utility>
       using namespace std;
       int main()
       {
    int t,points;       
    scanf("%d",&points);
    pair<float,float>p[points];
    int i;
    for(i=0;i<points;i++)
    {
        scanf("%f %f",&p[i].first,&p[i].second);

    }
    float ar,x,y;
     for(i=0;i<points-1;i++)
     {
         ar+=(p[i].first*p[i+1].second-(p[i+1].first*p[i].second));
         x+=((p[i].first+p[i+1].first)*ar);
         y+=((p[i].second+p[i+1].second)*ar);
     }
 x/=(3*ar);
 y/=(3*ar);
 printf("%.2f %.2f\n",x,y);
 }

However when I run the above code for the given co-ordinates the resulting co-ordinates of centroid are :

  -1.#R -1.#R