5. Randomly Walking Turtles

Suppose we want to entertain ourselves by watching a turtle wander around randomly inside the screen. When we run the program we want it to behave in the following way:

  1. The turtle begins in the center of the screen.
  2. Flip a coin. If it’s heads then the turtle turns left 90 degrees. If it’s tails then the turtle turns right 90 degrees.
  3. The turtle takes 50 steps forward.
  4. If the turtle has moved outside the screen then stop, otherwise go back to step 2 and repeat.

Notice that we cannot predict how many times the turtle will need to flip the coin before it wanders out of the screen, so we can’t use a for loop in this case. In fact, although very unlikely, this program might never end, that is why we call this type of iteration indefinite.

Based on the problem description above, we can outline a program as follows:

create a window and a turtle

while the turtle is still in the window:
    generate a random number between 0 and 1
    if the number == 0 (heads):
        turn left
        turn right
    move the turtle forward 50 steps

The only thing that might seem a bit confusing is the part about whether or not the turtle is still in the screen. But this is the nice thing about programming: we can delay the tough stuff and get something in our program working right away.

The way we are going to do this is to delegate the work of deciding whether the turtle is still in the screen or not to a boolean function. Let’s call this boolean function is_in_screen We can write a very simple version of this boolean function by having it always return True, or by having it decide randomly, the point is to have it do something simple so that we can focus on the parts we already know how to do well and get them working. Since having it always return True would not be a good idea, we will write our version to decide randomly. Let’s say that there is a 90% chance the turtle is still in the window and a 10% chance that the turtle has escaped.

Now we have a working program that draws a random walk of our turtle that has a 90% chance of staying on the screen. We are in a good position because a large part of our program is working and we can focus on the next bit of work: deciding whether the turtle is inside the screen boundaries or not.

We can find out the width and the height of the screen using the window_width and window_height methods of the screen object. However, remember that the turtle starts at position (0,0) in the middle of the screen. So we never want the turtle to go farther right than width/2 or farther left than negative width/2. We also never want the turtle to go further up than height/2 or further down than negative height/2. Once we know what the boundaries are we can use some conditionals to check the turtle position against the boundaries and return False if the turtle is outside or True if the turtle is inside.

Once we have computed our boundaries we can get the current position of the turtle and then use conditionals to decide if it is still in the window. Here is one implementation:

def is_in_screen(screen, t):
    left_bound = -(screen.window_width() / 2)
    right_bound = screen.window_width() / 2
    top_bound = screen.window_height() / 2
    bottom_bound = -(screen.window_height() / 2)

    turtle_x = t.xcor()
    turtle_y = t.ycor()

    still_in = True
    if turtle_x > right_bound or turtle_x < left_bound:
        still_in = False
    if turtle_y > top_bound or turtle_y < bottom_bound:
        still_in = False

    return still_in

There are lots of ways that the conditional could be written. In this case, we have given still_in the default value of True and we use two if statements to set the value to False if the conditions determine the turtle has left the screen. Another way you could write this is to use nested conditionals or elif statements and set still_in to True in an else clause.

Here is the full version of our random walk program.

We could have written this program without using a boolean function. As an exercise, you could to try to rewrite it using a complex condition in the while statement. However, using a boolean function makes the program much more readable and easier to understand. It also gives us a reusable function for use if this program were larger and we needed to have a check in another part of the program for whether the turtle was still in the screen. Breaking up this program into a couple of parts is another example of functional decomposition.

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