CS111 Lab 7 — Fall 2005

CS111, Wellesley College, Fall 2005

Lab 7

Wednesday, October 19, 2005

Today

Recursive JEM
Quick review of turtle world
Using the Java Console
Writing recursive methods (in turtle world)

Some helpful material: Thinking about Recursion

Part 1 -- JEM Diagrams for Recursive methods

Below are the declarations for two classes: a ThrowerWorld class that is a subclass of BuggleWorld and a ThrowerBuggle class that is a subclass of Buggle.

// a BuggleWorld in which Buggles throw Bagels
   public class ThrowerWorld extends BuggleWorld {
     public void run () {
        ThrowerBuggle tara = new ThrowerBuggle();
        tara.throwBagel(3);
      }
   }
    
   class ThrowerBuggle extends Buggle {
    public void throwBagel (int distance) {
       if (distance == 0) {
            dropBagel();
         } else {
            forward();
            throwBagel(distance - 1);
            backward();
         }
     }
 }

Task 1a) Draw the result (in a ThrowerWorld grid) of asking tara to throw bagels at distances of 0, 1, 2, and 3. Use a different grid for each distance. The size of the grid doesn't matter (just make it large enough so the buggle doesn't run into walls). When given recursive code, it is usually a good idea to draw on paper a couple of instances of running the code beginning with the easiest case and working up.

Task 1b) Draw a Java Execution Model diagram that shows all of the execution frames created by invoking the run() method on an instance of the ThrowerWorld class. Your diagram should depict the point in time when the invocation of run() returns. Although Java can discard an execution frame when control returns from it, you should not discard any frames when drawing your diagram.

Part 2 -- Quick review of TurtleWorld

Turtle drawing primitives include the following:

   public void fd (double n)
   Move the turtle forward n steps. 
   
   public void bd (double n); 
   Move the turtle backward n steps.
   
       public void lt (double angle);
   Turn the turtle to the left angle degrees.  
   
   public void rt (double angle);
   Turn the turtle to the right angle degrees. 
   
   public void pu (); 
   Raise the turtle's pen up.
    
       public void pd (); 
   Lower the turtle's pen down.

Additionally, there are also versions of fd, bd, lt, and rt that take int parameters, so you can invoke these methods with either an integer or double floating-point value.

You should not need to use any other Turtle primitives other than those listed above. In fact, many solutions use only a subset of the primitives listed above. (Of course, you may enjoy reviewing the turtle contract after you finish.)

For the tasks below, you will test your work by specifying a number of levels and some size (and perhaps an offset) in a parameter window and then clicking on the Run button in the TurtleWorld window. The Reset button will clear the screen. Good parameter values are in the ranges [0 ... 8] for levels and [100 ... 400] for a size. If your program hangs, you may need to "force quit" it by depressing the option, apple, and escape keys all at the same time. If that doesn't help, restart the computer by pressing the little button with the triangle on the machine itself (under your desk).

Part 3 -- The Console window

You might notice that many of the recursive methods contain System.out.println() statements. These statements can help you understand how your program works, and can be a big help in debugging when your program doesn't work. Each time the method is invoked, a statement is printed to the console window. Here is how you can open the console window to see the printed statements from your program:

Click on Macintosh HD > Applications > Utilities
Your open Utilities window might look something like this:
Double click on the Console icon circled above in red
This will open the Console log that will display the output of any System.out.println() statements from your Java program. Below is the output of a sample of run the BoomerangWorld program:
When viewing the Console window, you can select File>Clear to clear the contents of the window. You can also Print the window from the File menu.

Part 4 -- Exercises

Download the lab7_programs folder from the cs111d account on cs.wellesley.edu.

Most of the programming problems for this lab take place in extensions of TurtleWorld. The problems are arranged in order of difficulty with the easiest problem first. The last problem is a BuggleWorld recursion problem. Note that the exercises are challenging, so don't be disappointed if you don't get to the last ones.

You can load working examples from lab7_programs/test.