TurtleGraphics Python: Bouncing turtle off the walls?

Question

So, I am trying to make a realistic bouncing function, where the turtle hits a wall and bounces off at the corresponding angle. My code looks like this:

def bounce(num_steps, step_size, initial_heading):
   turtle.reset()
   top = turtle.window_height()/2
   bottom = -top
   right = turtle.window_width()/2
   left = -right

   turtle.left(initial_heading)
   for step in range(num_steps):
      turtle.forward(step_size)
      x, y = turtle.position()
      if left <= x <= right and bottom <= y <= top:
         pass
      else:
         turtle.left(180-2 * (turtle.heading()))

So, this works for the side walls, but I don't get how to make it bounce correctly off the top/bottom. Any suggestions?

Answer 1

Try something like this:

if not (left <= x <= right):
    turtle.left(180 - 2 * turtle.heading())
elif not (bottom <= y <= top):
    turtle.left(-2 * turtle.heading())
else:
    pass

My python syntax is a little rusty, sorry :P. But the math is a little different for a horizontal vs. a vertical flip.

EDIT:

I suspect that what is happening is your turtle is getting into a situation where it is pointing upwards and stuck above the top wall. That would lead it to just flip indefinitely. You could try adding the following conditions:

if (x <= left and 90 <= turtle.heading() <= 270) or (right <= x and not 90 <= turtle.heading() <= 270):
    turtle.left(180 - 2 * turtle.heading())
elif (y <= bottom and turtle.heading() >= 180) or (top <= y and turtle.heading <= 180):
    turtle.left(-2 * turtle.heading())
else:
    pass

If that works, there is probably a bug elsewhere in your code. Edge handling is tricky to get right. I assume that turtle.heading() will always return something between 0 and 360 - if not then it will be even more tricky to get right.

Answer 2

Gday,

Your problem seems to be that you are using the same trigonometry to calculate the right and left walls, as you are the top and bottom. A piece of paper and a pencil should suffice to calculate the required deflections.

def inbounds(limit, value):
    'returns boolean answer to question "is turtle position within my axis limits"'
    return -limit < value * 2 < limit

def bounce(num_steps, step_size, initial_heading):
    '''given the number of steps, the size of the steps 
        and an initial heading in degrees, plot the resultant course
        on a turtle window, taking into account elastic collisions 
        with window borders.
    '''

    turtle.reset()
    height = turtle.window_height()
    width = turtle.window_width()
    turtle.left(initial_heading)

    for step in xrange(num_steps):
        turtle.forward(step_size)
        x, y = turtle.position()

        if not inbounds(height, y):
            turtle.setheading(-turtle.heading())

        if not inbounds(width, x):
            turtle.setheading(180 - turtle.heading())

I've used the setheading function and a helper function (inbounds) to further declare the intent of the code here. Providing some kind of doc-string is also good practice in any code that you write (provided the message it states is accurate!!)

Your mileage may vary on the use of xrange, Python 3.0+ renames it to simply range.