OpenGl code to make a moving Circle

This source code is for making a circle which can move from one side to another side.

OpenGL program:-

#include<iostream>
#include<stdlib.h>

#ifdef __APPLE__
#include<openGL/openGL.h>
#include<GLUT/glut.h>
#else 
#include<GL/glut.h>
#endif

using namespace std;

float ballX = -0.3f;
float ballY = 0.0f;
float ballZ = -1.0f;
/*
float ballX2 = 0.3f;
float ballY2 = 0.0f;
float ballZ2 = -1.0f;
*/
static int flag=1;

void drawBall(void) {

        glColor3f(0.0, 1.0, 0.0); //set ball colour
        glTranslatef(ballX,ballY,ballZ); //moving it toward the screen a bit on creation
        glutSolidSphere (0.1, 10, 10); //create ball.

}

void keyPress(int key, int x, int y)
{
      if(key==GLUT_KEY_RIGHT)
        ballX -= 0.05f;
    if(key==GLUT_KEY_LEFT)
        ballX  += 0.05f;

    glutPostRedisplay();
}

void initRendering()
{
    glEnable(GL_DEPTH_TEST);
}

//Called when the window is resized
void handleResize(int w, int h) {
    //Tell OpenGL how to convert from coordinates to pixel values
    glViewport(0, 0, w, h);
    
    glMatrixMode(GL_PROJECTION); //Switch to setting the camera perspective
    
    //Set the camera perspective
    glLoadIdentity(); //Reset the camera
    gluPerspective(45.0,                  //The camera angle
                   (double)w / (double)h, //The width-to-height ratio
                   1.0,                   //The near z clipping coordinate
                   200.0);                //The far z clipping coordinate
}

void drawScene()
{
    glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
   
    glMatrixMode(GL_MODELVIEW);
   
    glLoadIdentity();
    drawBall();
    //drawBall2();

    glutSwapBuffers();
}

//float _angle = 30.0f;
void update(int value) {
    if(flag)
    {
    ballX += 0.001f;
       if(ballX>0.3)
       {
           flag=0;

       }
    }
    if (!flag)
    {
        ballX -= 0.001f;
        if(ballX<-0.3)
       {
           flag=1;

       }
    }
   
    glutPostRedisplay(); //Tell GLUT that the display has changed
   
    //Tell GLUT to call update again in 25 milliseconds
    glutTimerFunc(25, update, 0);
}

int main(int argc,char** argv)
{
    glutInit(&argc,argv);
   
    glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB|GLUT_DEPTH);
   
    glutInitWindowSize(400,400);
   
    glutCreateWindow("Moving Circle");
   
    initRendering();
   
    glutDisplayFunc(drawScene);

    glutSpecialFunc(keyPress);
    glutReshapeFunc(handleResize);
    //autorot();

    glutTimerFunc(25, update, 0);

    glutMainLoop();
   
    return(0);
}

//Output of the above Program:-

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