Computer Science 211
Data Structures

First, carefully read the lab description in the text and this handout. You are certain to have questions, so be sure to leave plenty of time to discuss this assignment with me and the TAs.

Before you begin, copy the starter files from the the shared area under labs/postscript. Familiarize yourself with these three classes and make sure you understand how they work.

Next, develop a sketch of the Interpreter class. This is where you will process the tokens being delivered to your interpreter by the provided Iterator class. No later than Monday, October 26, please either see me to go over your sketch of this class or email it to me for comment and discussion (for 3 points).

• Please name the file with your interpreter and main method Interpreter.java.
• Make use of the functionality of the classes you are given to start. Be careful not to spend time developing code that replicates functionality that is already there!
• Make your main method very short. All it should do is create an Interpreter object and tell it to start parsing the PostScript program presented at the command line. Create a method interpret that takes a single parameter of type java.util.Iterator<Token> (which will be an instance of the provided Reader class) and processes the PostScript tokens returned by that Iterator<Token>.
• A "debugging mode" for your program may prove useful to facilitate debugging and testing. In my version, if I specify the word "debug" as a command-line parameter, the program enters a debugging mode and prints out informative messages about what it is doing as it processes the PostScript tokens.
• Develop your interpret method incrementally. Get the simple opertations like pushing a token encountered on the input, pop, and pstack working, then move on to the arithmetic operators, and finally the definition and usage of symbols.
• Your program should throw exceptions when it encounters invalid input, but these should contain meaningful error messages. You can use Assert.condition() and Assert.fail() for this.

For a bonus point, you can implement procedures as described in thought question 3. If you design everything else properly, this should be almost a trivial extension.

For another bonus point, you can implement the if operator as described in thought question 4.

When you're finished, please submit the following by email to jteresco AT mtholyoke.edu:

• Your well-documented source code for Interpreter.java. Indicate clearly in the comment at the top of your program if you have implemented the extensions in thought questions 3 and 4.
• A README file that contains your answers to thought questions 1, 2, and 5.

You should not include the three files from the textbook's web page in your submission.

This lab assignment is graded out of 30 points. As in all labs, you will be graded on design, documentation, style, and correctness. Be sure to document your program with appropriate comments (use Javadoc!), including your name and a general description at the top of the file, a description of each method with pre and postconditions where appropriate. Also use comments and descriptive variable names to clarify sections of the code which may not be clear to someone trying to understand it.

The program design grade will be based on the design choices you make in the implementation of the Interperter class. The program style grade will be based on code formatting and approriate use of Java naming conventions. The program documentation grade is, of course, based on the comments you provide (including Javadoc and pre and postconditions as appropriate). The program correctness grade is based on how well your program meets the functionality requirements.