In this lesson we will add code to randomly set the direction of the ball when the game starts.  We will also add code so that the ball bounces off the walls.  We will use concepts learned in previous lessons.

When the game begins, the ball starts moving from a random location. To make the game more interesting we should make the ball move in a random direction as well. To keep our code simple, we will have the ball start moving downward and either left or right so that the ball moves diagonally down across the screen.  

If we want the ball to move to the right, we add to the position. For example, if we want the ball to move by 10 pixels our statement is:

    x = x + 10

If we want the ball to move to the left 10 pixels, then our statement is:

    x = x - 10

Notice that the equations are identical with the exception of the + or - operator.  We can write one equation to handle both situations by introducing another variable to keep track of the direction:

    x = x + dirX * 10

In this equation, the variable dirX will be set to 1 to move the ball to the right and to -1 to move the ball to the left. 

This concept is illustrated in the example below.

Now we will discuss the code needed to make the ball bounce off the walls.  Assume an object is moving to the left. We keep moving the object to the left as long as its position is to the right of the wall.  When the object's position equals 0, the location of the left wall, we reverse its direction by changing the sign of the direction variable:

    if(px <=0) dirX = -dirX

We can apply the same concept to the other walls.  The example below illustrates these concepts.

1.  Modify your game page so that the ball starts moving diagonally down towards the left or right when the user begins the game.

2. Modify your game so that the ball bounces off the walls. In the next lesson we will learn to make the ball bounce off the paddle.