Thread: Half of the circle

So i ran into a problem with circle. I could draw a whole circle easily, but when i tried half of the circle, it doesn't go in the way i wanted.

in my draw circle function, i set the vectors' coordinate base on the information of the class. Then use "gl triangle fan" to draw the circle:

Code:

vect2* BuildBar()
	{
		//Pi() is the function return Pi's value
		Vertices = new vect2[numVertices+2];
		for(int j=0; j<numVertices+2; j++)
		{
			double a =  2*Pi() * (j/double(numVertices));
			Vertices[j] = vect2(cos(a), sin(a)) * (R); //R is the radius
		}
		return Vertices;
	}
void Render()
	{

		glLoadIdentity();		
		glTranslatef(Pos.x, Pos.y, 0);
		glBegin(GL_TRIANGLE_FAN);
			for(int j=0; j<numVertices+2; j++)
			{					
				glVertex2d(Vertices[j].x, Vertices[j].y);
			}			
		glEnd();

	}

and this is how it looks like:
http://www.albumtown.com/data/953ecc...66_p935793.gif

But my intention is just only part of the circle. I was trying to get something like this:
http://www.albumtown.com/data/953ecc...66_p935796.gif


so i change my build function and tried to get that half circle:

Code:

vect2* BuildBar()
	{
		/**/
		Vertices = new vect2[numVertices+2];
		for(int j=0; j<numVertices+2; j++)
		{
			double a =  Pi()/1.5 * (j/double(numVertices));	
		                Vertices[j] = vect2(cos(a), sin(a)) * (R);
		}
		return Vertices;
	}

and i get this:
http://www.albumtown.com/data/953ecc...66_p935794.gif

No matter what way i rotate, the whole thing just won't stay straight like the second picture. So, what should i set for the sin() and cos() in order to straight up the circle like the second picture?