How to bounce the ball forever around the screen

[Peter Otten]

phamtony33 at gmail.com wrote:

> from Tkinter import *
> window = Tk()
> canvas = Canvas(window, width=500, height=500, background="green")
> canvas.pack()
> 
> def move_ball(speed_x, speed_y):
>         box = canvas.bbox("ball")
>         x1 = box[0]
>         y1 = box[1]
>         x2 = box[2]
>         y2 = box[3]
>         
>         if x1 <= 0:
>                 speed_x = 0
>                 speed_y = 0
>                 
>         canvas.move("ball", speed_x, speed_y)
>         canvas.after(30, move_ball, speed_x, speed_y)
>         
> canvas.create_oval(225, 225, 275, 275, fill="blue", tags="ball")
> 
> move_ball(-10, 7)
> 
> mainloop()
> 
>  where in the code should i change to make the ball bounce around forever. 
is it the x and y?

When the ball hits a wall you have to "reflect" it; for a vertical wall 
speed_x has to change:

    if x1 <= 0: # ball hits left wall
        speed_x = -speed_x
    elif ... # ball hits right wall
        speed_x = -speed_x

Can you come up with the correct check for the right wall?
To try if this part works correctly temporarily change

> move_ball(-10, 7)

to

move_ball(-10, 0)

so that the ball only moves in horizontal direction. Once this works handle 
the vertical walls and speed_y the same way.