COMP 105: Details

You need some Unix experience. You must understand the basics of files, directories, creating and editing files, printing, compiling and loading programs, and using make. You will be much, much happier if you also can write a simple shell script (sh) and use Awk and grep effectively. Kernighan and Pike cover these topics at the appropriate level.

Your programming experience should include work with dynamically allocated data structures and with recursive functions. You should be very comfortable writing recursive functions in the language of your choice, as well as proving that such functions terminate. You should have implemented some of the basic data structures and algorithms used in computer science, like stacks, queues, lists, tables, search trees, standard sorts (quick, insertion, merge, heap), topological sort, and graph algorithms. These topics are well covered in COMP 15 at Tufts. Prior exposure to exhaustive search (backtracking) will also be helpful.

Some students spend many, many hours thrashing out homework assignments. This course uses unusual programming paradigms, and the techniques you are accustomed to may not be much help. Although this material is not a formal prerequisite for this course, you will be happier if you have a nodding acquaintance with formal methods, including the following intellectual tools:

• Loop invariants and termination conditions as they apply to imperative programs.
• Contracts for functions, including preconditions and postconditions.
• Termination conditions for recursive functions. When writing recursive functions, try to develop your understanding of the deep connections between recursion and loops; the ideas of invariants and termination conditions apply in both cases.
• Representation invariants and abstraction functions for abstract data types.

You must have taken Discrete Math (Math 22 or COMP 22). If not, you must produce some other evidence that you can reason precisely about computational objects. You should be able to write an informal mathematical proof. For example, you should be able to prove that a sort routine returns the same set of elements that it was passed. You should be comfortable using basic mathematical formulas with ``forall'' and ``exists'' quantifiers, i.e., the propositional and predicate calculi, although it will not be necessary to write formal proofs using these calculi. You should know basic set theory, e.g., the mathematical definition of functions as sets of ordered pairs. You should be comfortable reading and writing formal mathematical notation, or at least not run screaming from the room.

The most important prerequisites for this course cannot be taught. To do well in this or any other course that involves programming, develop these habits:

• Think carefully about a problem before you begin to write code.
• When having difficulty writing code, stand up, walk away from your computer, and think about the difficulty.
• Never be satisfied with a ``working program,'' but strive for the simplest, clearest, most elegant implementation.

Interactions are Expected

• Part of your job as a student is to get to know some of the faculty. To help you with this part of your education, I count up to five minutes of office-hour visits as part of your course grade. Each minute you spend in conversation with me during my office hours will earn one percent of your overall course grade, up to a possible total of five percent. While you may find it helpful to talk about homework, class, engineering, or Tufts overall, any mutually agreeable topic of conversation is acceptable. (I find the Red Sox disagreeable, but I will talk endlessly about the Patriots.)

• Much of the programming in 105 is about your own individual understanding of new language features and new ideas, and you will tackle this sort of programming on your own. But sometimes you will have the opportunity to build something more substantial. And in the real world, substantial artifacts are seldom built by individuals working alone. COMP 105 will therefore provide you with some opportunity to practice pair programming (more at Wikipedia). You will have the opportunity to practice pair programming on selected problems throughout the term. No single pair may work together on more than three assignments. At need, course staff will help students form pairs.

Spontaneous interactions may be welcome or unwelcome depending on the interaction:

• A particularly useful form of interaction is the question asked in class. Questions are always welcome; if you have a question, chances are other people in class have a similar question. Ask it! One question that is always legitimate is "why are we doing this?"

• A particularly pernicious form of interaction is the telephone call. During class, please put your cell phone on vibrate. If you must take a call, please leave the classroom immediately and do not return until you have finished your phone call.

If you get stuck, seek help early. In a large course, your difficulties could be overlooked until they get out of control. Keep an eye on yourself, and remember that a short conversation during office hours or lab can save hours of aimless frustration.

If you complete and understand all the homework assignments, you are almost certain to do well on the exams and earn a high grade. If you miss assignments or don't really understand the homework, it will be difficult for you to earn a satisfactory grade. Extensive details about grades are available online.

Format of homework

Every assignment should include a README file that describes the work. This description must

• Identify you by name (and on the first assignment, please tell me how to pronounce your name; my name is pronounced NORE-muhn RAM-zee)
• Identify what aspects of the work have been correctly implemented and what have not.
• Identify anyone with whom you have collaborated or discussed the assignment.
• Say approximately how many hours you have spent completing the assignment.

Extra Credit

Extra credit is just that: extra. It is possible to earn an A without doing any extra credit.

Readability of programming assignments

Appropriate explanations vary with the size of the problem.

• For a trivial solution of a few lines of code, it may be best to have no explanation at all. (We're not entering the Obfuscated C Contest!)
• For a solution of a dozen lines of code, a sentence or two is usually sufficient.

• For a solution of a hundred lines or more, I would expect several paragraphs of explanation. Here the explanation needs to cover not so much the code itself, but the organization of the code and the plan that produced it.
• For larger programs, especially programs that are divided into multiple files, a couple of pages on planning, organization, and so on are appropriate.

Good programming style and documentation are essential ingredients in a readable assignment. Good style includes the use of functions, modules, and abstract data types where appropriate. Select algorithms appropriate to the task at hand, and make your code structured and easy to read. Good use and placement of documentation is vital. Lots of comment text is usually a sign of poor documentation. Documentation should focus on high-level issues such as design and organization, data representation, and data invariants. As noted above, even programs that run perfectly will lose points for poor documentation.

Documentation is a deep topic in its own right, which I am not prepared to address here, but here are a few suggestions. Good large-scale documentation should answer such questions as:

• What algorithms are used, if any?
• Why is the implementation correct?
• Why does it terminate?
• What are the important invariants, preconditions, and postconditions?
• What are the major data structures and their representations?

Small-scale documentation (comments in the code) should say

• What are the arguments of the most significant procedures? For example, is a list of edges the list of all edges in the graph or the list of edges not yet visited?
• If any code is tricky (e.g., you had trouble getting it right the first time), what are the local invariants? What are the pre- and post-conditions of the procedures? What is the `recursion invariant' (analogous to a loop invariant)?

; (example of poor commenting style)
;
;  Function: caar
;  Description: Returns the car of the car of the given element.
;
(define caar (x)
  (car (car x))
)

; (example of better commenting style)
;
; Visit vertex and every successor of vertex that appears in edges,
; in depth-first order.  Executed for side effects on the global 
; variables output and visited.
; Graph is the complete list of edges in the graph; 
; we use structural recursion on edges.  

(define dfsvisit (vertex edges graph) 
  (if (null? edges) '()
      ( ...  (dfsvisit mumble (cdr edges) graph) ...)))

Here are two examples of very poor commenting style:

   ++i;   /* Use the prefix autoincrement operator to increase i by one. */

   for (;;) {   /* Infinite loop. */

Guidelines for coding style

Your code must not wrap when displayed in 80 columns.

Your ML code must not have redundant parentheses, as described in the relevant section of the ML supplement.

We emphasize readability and clarity, but we do not enforce any particular coding style. Work that we find obscure or difficult to read will earn lower grades.

Collaboration

When you reach the coding stage, therefore, group discussions are no longer appropriate. Each program, unless explicitly assigned as a pair problem, must be entirely your own work.

Do not, under any circumstances, permit any other student to see any part of your program, and do not permit yourself to see any part of another student's program. In particular, you may not test or debug another student's code, nor may you have another student test or debug your code. (If you can't get code to work, consult your section leader or the instructor.) Using another's code in any form or writing code for use by another violates the University's academic regulations.

Suspected violations will be reported to the University's Judicial Officer for investigation and adjudication. Be careful! As described in the handbook on academic integrity, the penalties for violation can be severe. A single bad decision made in a moment of weakness could lead to a permanent blot on your academic record.

The same standards apply to all homework assignments; work you submit under your name must be entirely your own work. Always acknowledge those with whom you discuss problems!

Use of the library

Some students rely heavily on the library. Although this is permitted within the letter of the rules, I discourage it. I assign homework problems not because I want to know the answers, but because doing homework is the best way for you to learn. While library skills are important in our profession, the homework in this course is designed to develop other skills that are even more important. Remember, you will not have the library with you when you write your exams or go on job interviews!

Wikipedia considered harmful

• For ordinary difficulties, each student is automatically issued five ``extension tokens.'' By expending an extension token, you can get an automatic 24-hour extension on all deadlines associated with a single assignment.

  Expenditure of extension tokens is governed by these rules:

  When expending an extension token, you must notify the course staff. We must receive this notification before the assignment is due. (Notification ensures that we don't post solutions before all homework is in.)
  At most two extension tokens may be expended on any single assignment. This rule enable us to post solutions in a timely manner.
  When you are out of tokens, late homework will no longer be accepted. It will be returned ungraded, and you will receive No Credit for the work.

• If a serious illness affects your ability to complete homework on time, your first step is to report the illness using the "Illness Notification Form" that is available in WebCenter for Students. We will make suitable arrangements.

• For extraordinary difficulties, such as bereavement, family emergencies, or other extraordinary unpleasant events, your first step should be to make contact with your associate dean for undergraduate education. You must take this step before the assignment is due. Ask your dean to drop me an email or give me a call, and we will make special arrangements that are suited to your circumstances.

• Let us hope there is not another outbreak of swine flu.

Regrading

Submission and automatic validation

use comp105

use -q comp105

A submission script is named submit105-name, so for example the submission script for the first assignment is called submit105-intro. (Actually there are two submission scripts: for your individual work, there is submit105-intro-solo, and for the part of the work you do with a partner, there is submit105-intro-pair.) Normally you should change to the directory in which you have placed your solutions and run the script. Most submission scripts do some sort of sanity checking. If the submission script complains, fix the problems and resubmit.

We encourage you to submit work early and often, even if it is incomplete, so that you have may have an independent check that what you plan to submit is what the course staff are expecting.

Questions and discussion

It is a great idea to post some kinds of questions to Stackoverflow; please follow our guidelines.

It is OK to send instant messages to the course instructor via Skype; his id is norman-ramsey.

We will make every effort to answer questions once a day, even on weekends. In the last two days before an assignment is due, we strive for even quicker turnaround. Answers to questions and other useful information will be sent to the class. Perhaps at some future point we will be able to have a web forum. Meanwhile, you can view the mailing-list archives at https://www.eecs.tufts.edu/mailman/private/comp105/. You will need your mailing-list password, which should have been emailed to you.

Sections are mandatory

Handouts

  git clone linux.cs.tufts.edu:/comp/105/book-code

If you need an account for CS machines, please send email to staff@cs.tufts.edu. Ask for bash as your login shell.

Stupid software tricks