2. Turtle Races

Quick Overview of Day

Use what you know about turtles and looping structures to simulate a basic race between two turtles.

2.1. Random Numbers

Before we begin writing code for this lab, we need to introduce one more Python module. The random module allows us to generate random numbers. It has been used in a few places earlier in the text, so you might already feel comfortable with it. It’s easy to use:

The randrange function, as called in the example above, generates a random number from 1 to 9. Even though we passed in 1,10 the randrange function works just like the range function when it comes to starting and stopping points. In other words, the randrange function will start at 1, and go up to but not including 10. If you run the program over and over again you should see that each time you run it a different number is generated. Random numbers are the basis of all kinds of interesting programs we can write, and the randrange function is just one of many functions available in the random module.

2.2. Turtle Race with a For Loop

In this lab we are going to work step by step through the problem of racing turtles. The idea is that we want to create two or more turtles and have them race across the screen from left to right. For the first version of the race, the turtle that goes the farthest is the winner.

There are several different, and equally plausible, solutions to this problem. Let’s look at what needs to be done, and then look at some of the options for the solution. To start, let’s think about a solution to the simplest form of the problem, a race between two turtles. We’ll look at more complex races later.

When you are faced with a problem like this in computer science it is often a good idea to find a solution to a simple problem first and then figure out how to make the solution more general.

Here is a possible sequence of steps that we will need to accomplish:

  1. Import the modules we need

  2. Create a screen

  3. Create two turtles

  4. Move the turtles to their starting positions

  5. Send them moving across the screen

Here is the Python code for the first 4 steps above

Now, you have several choices for how to fill in code for step 5. Here are some possibilities to try. Try coding each of the following in the box above to see the different kinds of behavior.

  • Use a single call to forward for each turtle, using a random number as the distance to move.

  • Create a for loop, using a random number for the argument passed to the range function. Inside the for loop move one of the turtles forward by some number of units.

  • Create a single for loop using something like range(150) or range(200). Then inside the for loop move each turtle forward using a random number as the argument to forward.

So, which of these programs is better? Which of these programs is most correct? These are excellent questions. Program 1 is certainly the simplest, but it isn’t very satisfying as far as a race is concerned. Each turtle simply moves their distance on their turn. That is not very satisfying as far as a simulated race goes. Program 2 ends up looking a lot like Program 1 when you run it. Program 3 is probably the most ‘realistic’ assuming realism is very important when we’re talking about a simulated race of virtual turtles.

You may be thinking why can’t each turtle just move forward until they cross some artificial finish line? Good question! In order to make that happen, we’ll need to use a while loop and ask about the turtles coordinates.

2.3. Turtle Race with a While Loop

As we’ve seen before, we can ask a turtle about it’s current x or y coordinate using the turtle_name.xcor() and turtle_name.ycor() functions. Consider the following example:

Use the xcor() function to write a version of the turtle race that has the turtles stop after they reach a virtual finish line. You might want to create a third turtle that draws a finish line before the race begins. You will want to recall the logical operators we have studied: specifically, and and or.


After you get the program working, you should try switching your while conditional statement to use and instead of or. Can you predict what the program will do?

2.4. Assignment Work Time

Continue working on your most recent assignment (possibly a turtle graphics assignment focused on loops and conditionals).

Next Section - 3. Functions Introduction