Which line of code do i need to change to get the circle shape?

Hello everyone...
Below is a complete sample application in OpenGL C ++.
How can i get the same output but the shape must be circle??
which line of coding do i need to change?

Please help me here.
It is urgent.
Thanks alot

Ram



----------------------------------------------------------------

Code:

#include <windows.h>
#include <gl\gl.h>
#include <gl\glaux.h>
#include<math.h>



//initial square position and its size
GLfloat x=250.0f;
GLfloat y=0.0f;
GLsizei rsize=50;

GLfloat x2=0.0f;
GLfloat y2=0.0f;

GLfloat x3=250.0f;
GLfloat y3=250.0f;

GLfloat x4=0.0f;
GLfloat y4=250.0f;

//step size in x and y directions
//number of pixels to move each time
GLfloat xstep=0.10f;
GLfloat ystep=0.10f;

GLfloat x2step=0.10f;
GLfloat y2step=0.10f;

GLfloat x3step=0.10f;
GLfloat y3step=0.10f;


GLfloat x4step=0.10f;
GLfloat y4step=0.10f;

//keep track of window's changing width and height

GLfloat windowWidth;
GLfloat windowHeight;

//called by AUX library when the windows has changed size

void CALLBACK ChangeSize(GLsizei w, GLsizei h)
{
	//prevent a divide by zero. when window is too short
	// you cannot make a window zero width

	if(h==0)
		h=1;

	//set the viewport to be the entire window
	glViewport(0,0,w,h);

	//Reset the coordinate system before modifying
	glLoadIdentity();

	//keep the square square, this time, save calculated width and height for later use

	if(w<=h)
	{
		windowHeight=250.0f*h/w;
		windowWidth=250.0f;
	}
	else
	{
		windowWidth=250.0f * w/h;
		windowHeight=250.0f;
	}

	//set the clipping volume
	glOrtho(1.0f, windowWidth, 0.01, windowHeight, 1.0f, -1.0f);

}

//called by AUX library to update window
void CALLBACK RenderScene(void)
{
	glClearColor(0.0f,0.0f,1.0f,0.0f);
	glClear(GL_COLOR_BUFFER_BIT);
	//set drawing color to red and draw rectangle at current position
	glColor3f(0.0f,1.0f,0.0f);
	glRectf(x2,y2,x2+rsize,y2+rsize);

	glColor3f(1.0f,0.0f,0.0f);
	glRectf(x3,y3,x3+rsize,y3+rsize);

	glColor3f(1.0f,0.0f,0.0f);
	glRectf(x4,y4,x4+rsize,y4+rsize);


	glColor3f(1.0f, 1.0f, 0.0f);
	glRectf(x,y,x+rsize,y+rsize);
	glFlush();
}

//called by AUX library when idle(window not being resized or moved)

void CALLBACK IdleFunction(void)
{
	//reverse direction when you reach let or right edge

	if(x>windowWidth-rsize || x<0)
		xstep=-xstep;

	//reverse direction when you reach top or bottom edge
	if(y>windowHeight-rsize || y<0)
		ystep=-ystep;

	//check bounds. This is in case the window is made smaller and the retangle is outside the new clipping volume

	if(x>windowWidth-rsize)
		x=windowWidth-rsize-1;

	if(y>windowHeight-rsize)
		y=windowHeight-rsize-1;

	//Actually move the square
	x+=xstep;
	y+=ystep;

	//reverse direction when you reach left or right edge
	if(x2>windowWidth-rsize || x2<0)
		x2step=-x2step;

	//reverse direction when you reach top or bottom edge
	if(y2>windowHeight-rsize || y2<0)
		y2step=-y2step;

	//check bounds. This is in case the window is made smaller and the rectangle is outside the new clipping volume
	if(x2>windowWidth-rsize)
		x2=windowWidth-rsize-1;

	if(y2>windowHeight-rsize)
		y2=windowHeight-rsize-1;

	if(x3>windowWidth-rsize || x3<0)
		x3step=-x3step;

	//reverse direction when you reach top or bottom edge
	if(y3>windowHeight-rsize || y3<0)
		y3step=-y3step;

	//check bounds. This is in case the window is made smaller and the rectangle is outside the new clipping volume
	if(x3>windowWidth-rsize)
		x3=windowWidth-rsize-1;

	if(y3>windowHeight-rsize)
		y3=windowHeight-rsize-1;

	if(x4>windowWidth-rsize || x4<0)
		x4step=-x4step;

	//reverse direction when you reach top or bottom edge
	if(y4>windowHeight-rsize || y4<0)
		y4step=-y4step;

	//check bounds. This is in case the window is made smaller and the rectangle is outside the new clipping volume
	if(x4>windowWidth-rsize)
		x4=windowWidth-rsize-1;

	if(y4>windowHeight-rsize)
		y4=windowHeight-rsize-1;


	//Actually move the square
	x2+=x2step;
	y2+=y2step;

	x3+=x3step;
	y3+=y3step;
	
	x4+=x4step;
	y4+=y4step;
	//redraw the scene with new coordinates
	RenderScene();
}

//main body of the program
void main(void)
{
	//aux window setup and initialization
	auxInitDisplayMode(AUX_DOUBLE || AUX_RGBA);
	auxInitPosition(100,100,250,250);
	auxInitWindow("Assignment 1");

	//set function to call when window is resized
	auxReshapeFunc(ChangeSize);
	auxIdleFunc(IdleFunction);

	//start main loop

	auxMainLoop(RenderScene);
}

//GLAUX.LIB GLU32.LIB glut32.lib glut.lib OPENGL32.LIB

ewwww... lol GLAUX functions... I'd learn glut instead... but even glut is kinda being outdated...

and where in your code are you using any functions from math.h?