Listing 6. Beginning of the moveBall function.
void moveBall(){
  //Save current location of ball.
  tempX = x;
  tempY = y;

  //The code in the switch statement is identical to the
  // code in the earlier version of the program.
  switch(dir){
    case 0:
      //Direction is northwest.
      if((x <= radius) || (y <= radius)){
        //Ball has collided with either the left wall or
        // the top wall.
        //Get a new direction. Note that if the new
        // direction is the same as the old one, control
        // will come back to here to get still another
        // direction the next time the function is called.
        dir = rand() % 4;
      }else{
        //No collision, set new location for the ball
        --x;
        --y;
      }//end else
    break;
    case 1:
      //Direction is southwest.
      if(((x <= radius) || (y >= (height - radius)))){
        //get a new direction
        dir = rand() % 4;
      }else{
        //set new location for the ball
        --x;
        ++y;
      }//end else
    break;
    case 2:
      //Direction is northeast.
      if(((x >= (width - radius)) || (y <= radius))){
        //get a new direction
        dir = rand() % 4;
      }else{
        //set new location for the ball
        ++x;
        --y;
      }//end else
    break;
    case 3:
      //Direction is southeast
      if((((x >= (width - radius)) ||
                              (y >= (height - radius))))){
        //get a new direction
        dir = rand() % 4;
      }else{
        //set new location for the ball
        ++x;
        ++y;
      }//end else

  }//end switch