This lab focuses on multi-way decisions and random walks.

Today, we will simulate a random walk using turtles. Random walks are used to model real world scenarios, such as stock prices and market fluctations, as well as robotic search (i.e. search and rescue as well as more prosaic uses such as roomba vacuums).

Our goal is to measure how far a turtle will walk over a 500 turns. Here is the basic outline of our program: 

1.  Set up a turtle, and use it to draw circle boundaries to measure 
    distance from start.
2.  For 500 turns,
3.    Choose a random direction:  north, west, south, or east
         and walk 10 steps in that direction

A sample run of our program looks like this:

Tess, our turtle, first draws our boundary rings (in green, yellow, and red), then changes to purple to begin her random walk. At each turn, she chooses a random direction and walks 10 steps. This is repeated 500 times.

To choose a random direction, we will use the built-in random number generator from the random library. To use a python library, we include an import statement for that library. So, for this program, we will have two import statements: one for the turtle library and one for the python library: 

import turtle
import random
The function random.random() returns a number between 0 and 1. Since we have four different directions, we will divide the possible range into 4 (equal) regions: When our random number is in the first region (between 0 and 1/4), we will turn right 90 degrees. For the next region (between 1/4 and 1/2) we will turn right 180 degrees. For the third region (between (1/2 and 3/4), we will turn left 90 degrees (equivalent to turning right 270 degrees but with a little less whiplash for the poor turtle). Finally, if the random number is in the last region (between 3/4 and 1), we will go straight (i.e. not turn at all).

We can translate all these decisions into python using if and elif constructs: 

def main():
    tess = setUpTurtle()
    
    for steps in range(500):
        direction = random.random()
        if direction < 1/4:
            tess.right(90)
        elif direction < 1/2:
            tess.right(180)
        elif direction < 3/4:
            tess.left(90)
        tess.forward(10)

    turtle.Screen().exitonclick()
Note that the only way to reach the first elif is if direction was already bigger than 1/4, so, there is no need to include it in our test statement on the elif.

We hid all the set up in the function, setUpTurtle(), to make the main() function easier to read. In turn, to make setUpTurtle() easier to read, we placed on the repeated statements for creating rings in a function drawRings(). Here is our complete program: 

#Walking Turtle program for Intro Programming Lab

import turtle
import random

def drawRing(t,radius,color):
    t.up()
    t.goto(0,-radius)
    t.color(color)
    t.down()
    t.circle(radius)

def setUpTurtle():
    #Create turtle
    t = turtle.Turtle()

    #Use the turtle to draw the rings:
    drawRing(t,100,"green")
    drawRing(t,200,"yellow")
    drawRing(t,300,"red")

    #Return turtle to starting position and set shape, speed, and color:
    t.up()
    t.home()
    t.down()
    t.color("purple")
    t.shape("turtle")
    t.speed(10)

    #Return turtle to used elsewhere in the program:
    return t

def main():
    tess = setUpTurtle()
    
    for steps in range(500):
        direction = random.random()
        if direction < 1/4:
            tess.right(90)
        elif direction < 1/2:
            tess.right(180)
        elif direction < 3/4:
            tess.left(90)
        tess.forward(10)

    turtle.Screen().exitonclick()

main()
Try running it several times. The colored circles indicate 100, 200, and 300 steps from the starting position. How often does the turtle leave the outer circle? How often does the turtle finish inside the green circle?