Princeton University
COS 217: Introduction to Programming Systems

Purpose

The purpose of this assignment is to help you learn or review (1) the fundamentals of the C programming language, (2) the details of the "de-commenting" task of the C preprocessor, and (3) how to use the the Linux operating system and GNU software, especially bash, emacs, and gcc217.

Students from past semesters reported taking, on average, 8 hours to complete this assignment.

Rules

Make sure you study the course Policies web page before doing this assignment or any of the COS 217 assignments. In particular, note that you may use a variety of "human" sources of information while doing assignments, including the course staff members, the lab teaching assistants, and other current students via Piazza.

Each assignment has a "extra challenge" part. While doing the "extra challenge" part of an assignment, you are bound to observe the course policies regarding assignment conduct as given in the course Policies web page, plus one additional policy: you may not use any "human" sources of information. That is, you may not consult with the course's staff members, the lab teaching assistants, other current students via Piazza, or any other people while working on the "extra challenge" part of an assignment, except for clarification of requirements.

Design a deterministic finite state automaton (DFA, alias FSA) that expresses the required de-commenting logic. The DFA concept is described in lectures, and in the Wikipedia Deterministic finite automaton page.

Design your DFA so it "accepts" a given sequence of characters if the sequence contains no unterminated comments. That is, when given a sequence of characters that does not contain an unterminated comment, your DFA must end in an "accepting" state. Conversely design your DFA so it "rejects" a given sequence of characters if the sequence contains an unterminated comment. That is, when given a sequence of character that contains an unterminated comment, your DFA must end in a "rejecting" state.

Express your DFA using the traditional "labeled ovals and labeled arrows" notation. Let each oval represent a state. Give each state a descriptive name, and indicate whether it is an "accepting" state or a "rejecting" state. Let each arrow represent a transition from one state to another. Label each arrow with the single character, or class of characters, that causes the transition to occur. We encourage (but do not require) you also to label each arrow with action(s) that must occur (for example, "print the character") when the corresponding transition occurs.

Express as much of the de-commenting logic as you can within your DFA. The more de-commenting logic you express in your DFA, the better your grade on the DFA will be.

To properly report unterminated comments, your program must contain logic to keep track of the current line number of the standard input stream. You need not show that logic in your DFA.

Convert your "labeled ovals and labeled arrows" DFA to a textual representation, placing the result in your project directory in a file named dfa. The document TextualDFAs contains two examples. Make sure you indicate explicitly which state is the DFA's start state, and whether each state is an accepting state or a rejecting state.

The name of the file must be dfa, not dfa.txt, not DFA, not Dfa, etc.

Step 3: Create Source Code

Use emacs to create source code in your project directory in a file named decomment.c. The decomment.c program must implement your DFA.

If your DFA exits while in an "accepting" state, then your program's exit status must be EXIT_SUCCESS or 0. If your DFA exits while in a "rejecting" state, then your program's exit status must be EXIT_FAILURE. In other words, if your program detects no unterminated comments, then its exit status must be EXIT_SUCCESS or 0; if your program detects an unterminated comment, then its exit status must be EXIT_FAILURE.

Your program must not consist of one large main function. Instead your program must consist of multiple small functions, each of which performs a single well-defined task. In this program you must create one function to implement each state of your DFA, as described in lectures.

Generally, a (large) C program must consist of multiple source code files. For this assignment, you need not split your source code into multiple files. Instead, place all source code in a single source code file. Subsequent assignments will ask you to compose programs consisting of multiple source code files.

We suggest that your program use the standard C getchar function to read characters from the standard input stream.

Step 4: Preprocess, Compile, Assemble, and Link

Use the gcc217 command to preprocess, compile, assemble, and link your program. While developing your program, issue the "shortcut" gcc217 command to preprocess, compile, assemble, and link your program all at once. Then one time only, immediately before submitting your work, build your program "the long way" by issuing distinct gcc217 commands to preprocess, compile, assemble, and link your program. Examine the intermediate results to the extent possible.

Step 5: Execute

Execute your program multiple times on various input files that test all statements in your program.

As noted previously, we have provided files in CourseLab directory /u/cos217/Assignment1 that you will find helpful:

• sampledecomment is an executable version of a correct assignment solution. Your program must write exactly (character for character) the same data to the standard output stream and the standard error stream as sampledecomment does. You must test your program using commands similar to these:

  ./sampledecomment < somefile.c > output1 2> errors1
  ./decomment < somefile.c > output2 2> errors2
  diff -y output1 output2
  diff -y errors1 errors2
  rm output1 errors1 output2 errors2
  

  The Linux diff program finds differences between two given files. The executions of the diff program shown above generate side-by-side listings of the two given files, and annotate the output to indicate differences. Feel free to send questions to your preceptor or Piazza if the annotations aren't clear. Suggestion: Widen your terminal window before issuing diff -y commands.

  If the output of the command diff -y output1 output2 shows differences between output1 and output2, then sampledecomment and your program have written different characters to the standard output stream. Similarly, if output of the command diff -y errors1 errors2 shows differences between errors1 and errors2, then sampledecomment and your program have written different characters to the standard error stream.

• Many .txt files (that is, files whose names end with .txt) can serve as input files to your program.

• testdecomment and testdecommentdiff are bash scripts that automate the testing process. Comments at the beginning of those files describe how to use them. After copying the scripts to your project directory, you may need to execute the commands chmod 700 testdecomment and chmod 700 testdecommentdiff to give them "executable" permissions.

You also must test your program against its own source code using a command sequence such as this:

./sampledecomment < decomment.c > output1 2> errors1
./decomment < decomment.c > output2 2> errors2
diff -y output1 output2
diff -y errors1 errors2
rm output1 errors1 output2 errors2

Step 6: Create a readme File

Use emacs to edit your copy of the given readme file by answering each question that is expressed therein.

One of the sections of your readme file requires you to list the authorized sources of information that you used to complete the assignment. Another section requires you to list the unauthorized sources of information that you used to complete the assignment. Your grader will not grade your submission unless you have completed those sections. To complete the "authorized sources" section of your readme file, copy the list of authorized sources given in the "Policies" web page to that section, and edit it as appropriate.

Descriptions of your code must not be in the readme file. Instead they must be integrated into your code as comments.

Your readme file must be a plain text file. Don't create your readme file using Microsoft Word or any other word processor. The name of the file must be readme, not readme.txt, not README, not Readme, etc.

Step 7: Provide Feedback

Provide the instructors with your feedback on the assignment. To do that, issue this command:

FeedbackCOS217.py 1

and answer the questions that it asks. That command stores its questions and your answers in a file named feedback in your working directory.

Step 8: Submit

Submit your work. Submit your dfa file, your decomment.c file, the files that gcc217 generated from it, your readme file, and your feedback file electronically by issuing these commands on CourseLab:

submit 1 dfa
submit 1 decomment.c
submit 1 decomment.i decomment.s decomment.o decomment
submit 1 readme feedback

As described in the Assignment 0 specification, you can issue submitandbackup X file1 file2 ... commands instead of submit X file1 file2 ... commands.

We can accept your files only if you submit them by executing submit (or submitandbackup) commands on CourseLab. In particular, we cannot accept your files via e-mail. We cannot accept your DFA in any form other than as a file containing plain text.

Program Style

In part, good program style is defined by the rules given in The Practice of Programming (Kernighan and Pike), as summarized by the Rules of Programming Style document. For this assignment we will pay particular attention to rules 1-24.

These more course-specific rules also apply:

• Indentation: Indent using spaces, not tabs. The emacs editor does that when using the .emacs configuration file that we have provided.

• Line lengths: Limit line lengths in your source code to 72 characters. Doing that allows us to print your code in two columns, and so saves paper. When using the .emacs configuration file that we have provided, emacs indicates lines that exceed 72 characters. Specifically, emacs uses a green background to mark the character in column 72, and a gray background to mark the character in column 73. So it's OK to see a green background, but a gray background indicates that the line is too long.

• Names: Use a clear and consistent style for variable and function names. One example of such a style is to prefix each variable name with characters that indicate its type. For example, the prefix c might indicate that the variable is of type char, i might indicate int, pc might mean char*, ui might mean unsigned int, etc. But it is fine to use another style — a style that does not include the type of a variable in its name — as long as the result is a clear and readable program.

• File Comments: Begin each source code file with a comment that includes the name of the file and your name.

• Variable Comments: Compose a comment for each global variable. The comment must appear immediately before the definition/declaration of the global variable. Compose a comment for each local variable definition/declaration whose purpose is unclear. The comment must appear immediately before the definition/declaration of the local variable, or at the end of the line containing the definition/declaration.

• Function Comments: Compose a comment for each function. A function definition/declaration's comment must immediately precede the function's definition/declaration. A function's comment must describe what the function does from the point of view of the function's callers. A function's comment must refer explicitly to the function's parameters (by name) and the function's return value. A function's comment must state what, if anything, the function reads from standard input or any other stream, and what, if anything, the function writes to standard output, standard error, or any other stream. In short, a function's comment must describe the flow of data into and out of the function. A function's comment must not describe how the function works. To describe how the function works, add local comments (that is, comments within the function definition) as necessary.

Grading

Minimal requirements to receive credit for decomment.c:

• decomment.c must build.

• decomment.c must handle "normal" files (that is, files that contain no comments) properly.

To receive credit for the "extra challenge" part of the assignment, your program must write proper line numbers within its "unterminated comment" error messages, and must do so without using global variables. The next section of this assignment specification elaborates.

We will grade your work on two kinds of quality:

• Quality from the user's point of view. From the user's point of view, your code has quality if it behaves as it must. The correct behavior of your program is defined by the previous sections of this assignment specification, and by the behavior of the given sampledecomment program.

• Quality from the programmer's point of view. From the programmer's point of view, your code has quality if it is well styled and thereby easy to maintain. Good program style is defined by the previous section of this assignment specification.

To encourage good coding practices, we will deduct points if gcc217 generates warning messages.

Avoiding Global Variables

As noted above, when the standard input stream contains an unterminated comment, your decomment program must write an error message. The error message must contain the number of the line at which the unterminated comment begins. The "extra challenge" part of the assignment is to implement that logic without using global variables. Here's an elaboration...

Suppose your program's main function calls some state handling function, and that main wishes to pass some values to the state handling function. The main function can do that using ordinary parameters.

But suppose the state handling function wishes to pass some values back to main. The state handling function can use its return value to pass the first value (for example, the next DFA state) back to main. But how can it pass additional values (for example, a line number) back to main?

One approach is to use global variables, where a global variable is one which is defined outside of all functions. The state handling function could assign the additional values (for example, a line number) to global variables; main then could fetch those values by accessing the global variables.

However, as prescribed by Kernighan and Pike style rule 25, and for reasons that we will discuss later in the course, generally you should avoid using global variables. Instead all communication of values into and out of a function should occur via the function's parameters and its return value.

Indeed in your decomment program you must avoid using global variables. You can do that using either of these two approaches:

• Express the program's logic such that no state handling function must pass more than one value back to main. You can do that by enhancing the logic in main.

• Use pointer parameters in your state handling functions, as described in Section 11.4 of our King book.

Some notes:

• The COS 217 course has not covered pointers yet. So you'll need to work ahead of the pace of the course if you use the second approach.

• Although you must avoid defining variables at the global level, it's fine to define data types at the global level. For example, code of this form:

  enum SomeEnum {SOMEVALUE1, SOMEVALUE2, ...};
  

  at the global level is fine.

• As noted in the Grading section of this document, to be awarded points for the "extra challenge" part of the assignment your program must write proper line numbers within its "unterminated comments" error messages, and must do so without using global variables. However, if your program writes proper line numbers within its "unterminated comment" error messages but does so using global variables, then your grade will be penalized only 2 percent. We will not think less of you if you decide to accept the small deduction instead of doing that part of the assignment.

This assignment was written by Robert M. Dondero, Jr.
with input from many other faculty members