Control a ball on a line with Left & Right Keys using OpenGL

ball_on_line_gl.cpp

#define GLUT_DISABLE_ATEXIT_HACK
#include <windows.h>
#include <iostream>
#include <gl\glut.h>

// Initial Position of the Ball
float x_position = 0.0;
float y_position = 0.0;

// Radius of ball
float radius = 0.2;

// Speed with which the ball tranlates
float speed = 0.05;

// Limits of line
float line_begin = -1.25 ;
float line_end   =  1.25 ;

void resize_handler(int w,int h)
{
    glViewport(0,0,w,h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45, w/h, 1, 200);
}

void key_handler( int key, int x, int y ) 
{
    if (key == GLUT_KEY_LEFT)
        x_position -= ( x_position > line_begin  ? speed : 0 );
    else if (key == GLUT_KEY_RIGHT)
        x_position += ( x_position < line_end ? speed : 0 );
}

void drawBall(float r, float g, float b, float xpos, float ypos, float rad)
{
    glColor3f(r, g, b);
    glPushMatrix();
        glTranslatef(xpos, ypos, -5.0);
        glutSolidSphere(rad, 100, 100);
    glPopMatrix();    
}

void drawLine( float x1, float y1, float x2, float y2)
{
    glLineWidth(1.0);
    glColor3f(1.0, 1.0, 1.0);
    glBegin(GL_LINES);
        glVertex3f(x1, y1, -5.0);
        glVertex3f(x2, y2, -5.0);
    glEnd();
}

void display()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    drawBall(1.0,1.0,0.0, x_position, y_position, radius);
    drawLine(-1.25, -0.212, 1.25, -0.212);

    glutSwapBuffers();
}

int main(int argc,char **argv)
{
    glutInit(&argc,argv);
    glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB|GLUT_DEPTH);
    glutInitWindowSize(600, 600);

    glutCreateWindow("Collision Window");
    
    glEnable(GL_DEPTH_TEST);
    
    glutDisplayFunc(display);
    glutIdleFunc(display);

    // Resize Handler
    glutReshapeFunc(resize_handler);
    
    // Key Handler
    glutSpecialFunc(key_handler);
    
    glutMainLoop();
    return(0);
}