Polygon formula [for graphics]

[Alex_]

Hello everybody, i need a polygon formula to draw one. I want to make a function that will draw a polygon , depending on how many vertex's it has. I'm not good at math so i can't figure it out how to do it. Though, i do understand that it should look something like: pass=360/vertex , then calculate some end points of lines that are 'pass' degrees from each other; but i don't know how to do that.
Can someone help?Pls!

[nmaillet]

If my math is correct,

x(n) = x + r*cos(a + n*pass)
y(n) = y + r*sin(a + n*pass)

where 0 <= n < vertex , x and y is the center point, a is the angle of the first point to the right of the center point from the horizontal(counter-clockwise) and r is the distance from the center point to each of the vertices(the "radius"). Let me know if this is what you were looking for.

[nucleon]

Also note that you have to use radians (not degrees) for your angles, so pass = 2 * M_PI / vertex (M_PI is in math.h) .

[Alex_]

Sort of ! But not really..
This is what i wrote

void polygon(int x, int y, int n, int r){
int k=0, *poly;
double pass= 360/n, _n=0,pass2=n*pass;
poly=new int[n];
	while(k<n*2){
   	     poly[k++]=x +r * cos(pass2);
	     poly[k++]=y +r * sin(pass2);
	     pass2+=pass;
	}
	drawpoly(n,poly);
}

as i understood , a is a=a+a; at each iteration? If it is so, it is not really what i expected, though it's better than nothing. Thx!
This is the output,at:
[img]http://sites.google.com/site/alexzaim/_/rsrc/1234900573771/Home/temp.jpg[/img]

[Alex_]

tried some modifications

void polygon(int x, int y, int n, int r){
int k=0, *poly;
double pass= 2* M_PI/n,pass2=n*pass, a=360/n;
poly=new int[n];
	while(k<n*2){
	pass2+=a;
	poly[k++]=x +r * cos(pass2);
	poly[k++]=y +r * sin(pass2);
	}
	drawpoly(n,poly);
}

check:
[IMG]http://sites.google.com/site/alexzaim/_/rsrc/1234900573771/Home/temp2.jpg[/IMG]

[Alibeg]

a is constant, but n*pass changes because n changes
i dont know why do you use double pass = 360 / n
and after that pass2 = n * pass, do you mean pass2 = 360?
and why do you use _n?
dont forget about radians, because, i think, sin() and cos() use radians as input parameters
this is not about your previous post but one before it

[Alibeg]

double pass2 = a; //where a is angle between line (first point of polygon joined with centre of polygon) and x axis.....you might like to set pass2 to 0 in the beginning
while(k<n*2)
{
poly[k++]=x +r * cos(pass2);
poly[k++]=y +r * sin(pass2);
pass2 += pass;
}
this is how it goes i think

[Alex_]

Woaaah! That did it! I am very thankfull!! Super, just what i need!

[Alibeg]

no prob, just dont forget to give the credits to a man who gave you the formula in the first place ;) (nmaillet)