OPENGL program to draw a circle

#include<GL/glut.h>
#include<math.h>
#define PI 3.1415926535898
GLint circle_points=100;
// if you wanna specify the radius  declare float rd=0.2;

void display(){
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0,1.0,0.0);
        glBegin(GL_LINE_LOOP);
          int i;
          float angle;
          glColor3f(1.0,0.0,0.0);

          for(i=0;i<circle_points;i++)
        {
               angle=2*PI*i/circle_points;
               glVertex2f((cos(angle)),(sin(angle)));
/* if you have specified the radius then use glVertex2f((cos(angle)*rd),(sin(angle)*rd));   instead of above statement */
        }
         glEnd();
glFlush();
}



void initGL()
{
glClearColor(1,1,1,1);
glColor3f(1,0,0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-1,1,-1,1,-1,1);
}


int main(int argc, char** argv) {
   glutInit(&argc, argv);        
   glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
   glutInitWindowSize(500,500); 
   glutInitWindowPosition(50, 50);
   glutCreateWindow("circle");
   glutDisplayFunc(display);           
   initGL();                   
   glutMainLoop();               
   return 0;
}