Assignment 6: Set Intersection

Due: 11: a.m., Wednesday, 6 April 2011

Summary: Document, test, and implement a set intersection procedure.

Purposes: To experience test-first program development; to practice writing documentation; to gain further experience with recursion, precondition checking, and locally-bound helper procedures.

Expected Time: One to two hours.

Collaboration: We encourage you to work in groups of size three. You may, however, work alone or work in a group of size two or size four. You may discuss this assignment with anyone, provided you credit such discussions when you submit the assignment.

Submitting: Email your answer to <grader-151-01@cs.grinnell.edu>. The title of your email should have the form CSC151-01 Assignment 6: Set Intersection and should contain your answers to all parts of the assignment. Scheme code should be in the body of the message.

Warning: So that this assignment is a learning experience for everyone, we may spend class time publicly critiquing your work.

Preliminaries

After graduation, you and your colleagues move to Schemeville to found a new company, Sets-R-Us. Your business plan is to develop a library of procedures for set manipulation and sell it to Scheme programmers all over the world (or maybe just computer science professors at Grinnell College). You know that you need thorough documentation and reliable code for your product to sell.

You decide to begin with the set intersection operation. You need to write a procedure, (intersect left right), that takes two lists as parameters. It produces a list containing precisely those values that occur in both left and right. Although left and right might include duplicate values, the value that is produced should not have any duplicates.

Note that the above description of intersect does not specify the order in which the values will occur in the result; your tests should not assume a particular order. Fortunately, the unit test framework lets us test permutations (with test-permutation!).

For example, we might write a test such as the following.

(test-permutation! (intersection (list 1 2) (list 3)) (list 1 2 3))

This indicates that we want the values 1, 2, and 3 to appear in the resulting list, but we don't care what order those values appear in. The list (1 2 3) would be a valid result, and so would (3 2 1), and even (2 3 1).

Assignment

(a) The above description of the intersect procedure is clearly casual. Write formal documentation for intersect, using the six P's.

(b) Write a test suite, using test-permutation! and the rest of the MediaScheme unit testing tools, that will help you and your colleagues test for errors in an implementation of intersect. You should be confident that anything that passes your test suite is correct. (You might also want to be able to use this test suite to convince potential customers that your implementation is correct!)

(c) Finally, write your own implementation of the intersect procedure. If the user calls your procedure in ways that do not satisify the preconditions, your procedure should give a helpful error message. If you use the husk-and-kernel style for checking preconditions (and I encourage you to do so), then hide the kernel using letrec or named let.

Important Evaluation Criteria

We will evaluate the carefulness of your documentation, the completeness of your tests, and the correctness and elegance of your implementation.