CS 5010 F '09
Pair Programming
The Recipes
The Style
Set 1
Set 2
Set 3
Set 4
Set 5
Set 6
Set 7
Set 8
Set 9
Set 10
Set 11
Set 12

Problem Set 5


Due date: 10/21


The goal of this problem set is to practice the creation of abstractions and the use of existing abstractions. The table on page 313 is your friend.


HtDP: 21.2.1, 21.2.2, 21.2.3

The TAs have requested that you design a small world program for drill club. They believe that few, if any students, have the ability to present and justify the design of 'complete' programs.

"Snake" (aka "worm") is another popular computer game from the middle ages. A worm always has a head and may have a tail---a sequence of segments. Think of all these things as circle of a fixed size; if you wish, think of a marker to distinguish the head.

Design a world program that consumes the number of tail segments and that moves the worm in reaction to key events. Specifically, only the four arrow keys should move the worm in the specified direction; the "tail" of the worm--if any--just moves 'along' one segment at a time. One way to think about movements is to say that the worm's head moves and the tail's last segment is cut off. Draw images to understand the problem.

You should proceed in two steps. First design the program using a fixed number of tail segments; specify the fixed number as a constant and ensure that you can change the constant and the program's test cases still pass. Second, re-organize your program so that the main function consumes the number of tail segments.

Required Problems:

Note: You must use DrScheme's HtDP Intermediate Student Language plus Lambda for this problem set.

  1. Problem 4.1 required the design of the functions: xexpr-find and xexpr-depth. Because the description of the input requires three data definitions, those of you who designed the functions according to the design recipe came up with three mutually interconnected functions (plus perhaps some auxiliary functions).

    Task 1: Fix all the problems that the code review panels criticized about these functions.

    Task 2: The first step to editing such clusters of functions after they have been properly designed and corrected is to re-organize them into a single function, just like the problem statement (or your boss's request) states for both of the two functions in this problem. Do so.

    The reason for organizing code in this manner is to create an organizational structure for future readers. If xexpr-find or xexpr-depth occur in the context of a large module or program that some reader wishes to understand, he may just read the purpose statement and ignore the internal nest of local functions during a first pass. This provides a first overview of the module and puts everything in context. After that, the reader can "drill" down wherever needed.

    Task 3: XML experts refer to (the interpretation of) an Xexpr as an element. Design the function xexpr-elements, which consumes an X-expression x and symbol tand extracts a list of all elements whose tag is t. Note that an element with tag t may occur nested inside of some other element with the same tag.

  2. Use existing ISL abstractions to "edit" your solution of problem 4.3.
  3. Use existing ISL abstractions to "edit" your solution of problem 3.4
If the word "edit" looks vague in the above statements, you have either not read part IV of HtDP or you haven't read it properly. While we will spend some time on the ideas in this chapter on Monday (10/19) and Tuesday (10/20), it will not be the focus of the evening.

For each of the above problems, you are free to choose either partner's code base as the starting point. Your solution must attribute the code to the original code base. Failure to do so will result in point deductions. You may not use any other person's code, i.e., collaboration outside of the current partnership, especially with your former partner, is considered theft.

last updated on Wed Dec 2 17:58:10 EST 2009generated with PLT Scheme