for example:

define n=30; R=100.; and declear xp[n],yp[n] at Variable Panel
Then I can add the following code at Initialize Panel to draw a circle.
[code]
double dc=Math.PI/n,c;
for(int i=0;i<n;i++){
c=i*dc;
xp[i]=R*Math.cos(c);
yp[i]=R*Math.sin(c);
}
[/code]
Assign xp,yp as x,y property for polygon, and set the number of point to n