1. Turtle Graphics Eric Roberts Nifty Assignments SIGCSE 2013 March 9, 2013 This is not your typical assignmentCeux ne sont pas vos devoirs typiques

2. The TurtleGraphics Assignment Traditional TurtleGraphics assignments focus on having the students write programs that cause a simulated version of the Project LOGO Turtle to draw figures on the screen. In this assignment, students—typically somewhere in the middle of CS1—build various pieces of the interpreter for the TurtleGraphics microworld. The modules that the students create for this assignment focus on exercising their skills in the following areas: String processing1. Defining and implementing a class2. The beginnings of recursive thinking3.

3. TurtleGraphics Commands The TurtleGraphics interpreter understands programs, which are simply strings composed of the following commands: FnFn LnLn RnRn U D X n { commands } Move the turtle forward n units Turn left n degrees Turn right n degrees Raise the pen to stop line drawing Lower the pen to resume line drawing Repeat the specified commands n times Students can add various other commands (change pen color, change step size, save/restore position) as extensions.

4. TurtleGraphics in Action X36 { X4 { F75 L90 } L10 }

5. Lindenmayer Systems Using strings as the base language allows students to transform programs by making string substitutions. TurtleGraphics programs evolve through repeated application of substitution patterns. Systems that evolve in this way are called L-systems or Lindenmayer systems, after the Hungarian biologist Aristid Lindenmayer, which were originally designed to simulate plant growth.

6. Evolution through Substitution U R150 F47 L150 D X3 { F81 L120 } F81–>F27 R60 F27 L120 F27 R60 F27

7. Evolution through Substitution U R150 F47 L150 D X3 { F27 R60 F27 L120 F27 R60 F27 L120 } F81–>F27 R60 F27 L120 F27 R60 F27

8. Evolution through Substitution U R150 F47 L150 D X3 { F27 R60 F27 L120 F27 R60 F27 L120 } F81–>F27 R60 F27 L120 F27 R60 F27

9. Evolution through Substitution U R150 F47 L150 D X3 { F27 R60 F27 L120 F27 R60 F27 L120 } F27–>F9 R60 F9 L120 F9 R60 F9

10. Evolution through Substitution U R150 F47 L150 D X3 { F9 R60 F9 L120 F9 R60 F9 R60 F9 R60 F9 L120 F9 R60 F9 L120 F9 R60 F9 L120 F9 R60 F9 R60 F9 R60 F9 L120 F9 R60 F9 L120 } F27–>F9 R60 F9 L120 F9 R60 F9F9–>F3 R60 F3 L120 F3 R60 F3

11. Evolution through Substitution U R150 F47 L150 D X3 { F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 } F9–>F3 R60 F3 L120 F3 R60 F3

12. Evolution through Substitution U R150 F47 L150 D X3 { F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 L120 F3 R60 F3 R60 F3 R60 F3 L120 F3 R60 F3 L120 }

13. What the Students Do Students are responsible for three pieces of the application: A TurtleTokenizer class. This class breaks a string into tokens, which consist of either a command letter followed by an optional numeric argument or a string of commands enclosed in braces. 1. The Lindenmayer command substitution model. Lindenmayer systems allow for multiple find-and-replace operations that are carried out simultaneously. For example, the pattern L–>R,R–>L interchanges all L and R commands. 2. Interpreting the command string. For the most part, this phase consists of transforming the command letter and argument returned by the tokenizer into appropriate calls to the GTurtle object. The X command requires an application of recursion that seems natural to students and causes almost no confusion. 3.

14. Why Is This Nifty? Students get to build important pieces of a larger application, which is after all what most people do in industrial software development positions. Students get lots of practice with string manipulation in a context where they can immediately see the results. Students have the chance to define an entire class and then use it in an application. Recursion comes up in a surprisingly natural way as part of the implementation of the X command. Most students seem to love playing with the TurtleGraphics application. There are lots of opportunities for extensions. It is easy to run a contest in parallel with the assignment.

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16. The End