while (some_control_loop) {
  // calculate the values of x and y using the std eqs.
 
glBegin(GL_LINES);
 
 
// All lines lie in the xy plane.
z = 0.0f;
for(angle = 0.0f; angle <= GL_PI; angle += (GL_PI/20.0f))
  {
  // Top half of the circle
  x = 50.0f*sin(angle);
  y = 50.0f*cos(angle);
  glVertex3f(x, y, z);    // First endpoint of line
 
  // Bottom half of the circle
  x = 50.0f*sin(angle + GL_PI);
  y = 50.0f*cos(angle + GL_PI);
  glVertex3f(x, y, z);    // Second endpoint of line
  }
 
// Done drawing points
glEnd();