School of Computer Science, McGill University
COMP-206 Introduction to Software Systems, Winter 2021
Mini Assignment C: Intro to C Programming
Due Date March 17th, 18:00 EST
This is an individual assignment. You need to solve these questions on your own. If you have questions, post them on Piazza, but do not post major parts of the assignment code. Though small parts
of code are acceptable, we do not want you sharing your solutions (or large parts of them) on Piazza.
If your question cannot be answered without sharing significant amounts of code, please make a private question on Piazza or utilize TA/Instructors o!ce hours. Late penalty is -15% per day. Even if
you are late only by a few minutes it will be rounded up to a day. Maximum of 2 late days are allowed.
You MUST use mimi.cs.mcgill.ca to create the solution to this assignment. You must not use your
Mac command-line, Windows command-line, nor a Linux distro installed locally on your laptop. You can access
mimi.cs.mcgill.ca from your personal computer using ssh or putty and also transfer files to your computer using
filezilla or scp as seen in class and in Lab A and mini assignment 1.
For this assignment, you will have to turn in one C program source code file and one shell script. Instructors/TAs
upon their discretion may ask you to demonstrate/explain your solution. No points are awarded for commands that
do not compile/execute at all. (Commands that execute, but provide incorrect behavior/output will be given partial
marks.) All questions are graded proportionally. This means that if 40% of the question is correct, you will receive
40% of the grade.
Please read through the entire assignment before you start working on it. You can loose several
points for not following the instructions. There are also some helpful hints given at the end of this document
that can be useful to you.
Lab G provides some background help for this mini assignment.
Total Points: 20
Ex. 1 — Implementing A Simple Text Cipher (14 Points)
In this assignment, we will implement a simple text cipher, that accepts an integer key and a message and encrypts
the message. We will not pursue the decryption aspect in this assignment, but your are free to explore that on your
own.
The cipher’s encrypting logic works in the following way. Let us say that you want to encrypt the message
THISISASECRETMESSAGE using a key 5
The logic involves first distributing the characters in the message in a zig-zag manner across 5 (the key) “levels”.
The dots (.) are merely place holders to help you visualize this better.
T.......E.......S...
.H.....S.C.....S.A..
..I...A...R...E...G.
...S.S.....E.M.....E
....I.......T.......
Then, the encryption basically involves sequentially producing the characters from each level, starting from the top
to the bottom, to the output. For the above example, the encryption produced will be TESHSCSAIAREGSSEMEIT.
Where TES is from the first level, HSCSA is from the second level, and so forth.
1
Clearly, if we change the key, it will change the encryption. For example, with a key of 3, the encryption logic makes
the following arrangement.
T...I...E...T...S...
.H.S.S.S.C.E.M.S.A.E
..I...A...R...E...G.
Resulting in an output of TIETSHSSSCEMSAEIAREG.
1. Use vim to create a C program source file cipher.c . All of your source code logic should be contained in
this file. Further, all of your logic should be in a single function, main. Please read through the entire
assignment before you start working, as the programming constructs that you are allowed to
use are limited.
2. (1 Point) The source code file should include a comment section at the beginning that describes its purpose
(couple of lines), the author (your name), your department, a small “history” section indicating what changes
you did on what date.
/∗
Program to implement a t ex t c ipher
∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗
∗ Author Dept . Date Notes
∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗
∗ Joseph D Comp . S c i e n c e . Mar 10 2021 I n i t i a l v e r s i o n .
∗ Joseph D Comp . S c i e n c e . Mar 11 2021 Added e r r o r h a n dl i n g .
∗/
The code should be properly indented for readability as well as contain any additional comments required to
understand the program logic.
3. The source code should be compilable by (exactly) using the command gcc -o cipher cipher.c. If not,
TAs will not grade it and no points will be awarded for the assignment.
4. (1 Point) Compilation should not produce any warnings!
5. The program accepts 3 arguments, the first is either -e or -d to indicate whether the user wants to encrypt
or decrypt (we are only doing encryption). The second argument is a key, which must be an integer that is
! 2 but 10. The third argument is the message to be encrypted which can be of any length (including 0
which is represented by an empty string argument, ""). You can however, assume that the message argument
will contain only non-space, uppercase alphabets, such as the example given before.
6. (2 Points) If the program is not passed 3 arguments, it should print a usage message and terminate with
code 1.
$ ./cipher
Usage : ./cipher [-e|-d]
$
echo $? 1$
7. (4 Points) The program should print error messages and terminate with error code 1 if any of the first 2
arguments are not valid options, as shown below
$ ./cipher -k 5 HELLOTHERE
Error -k is not a valid option
$$ echo $? 1$
If the first argument is good, the same goes if the user enters a non-valid key as the second argument.
$ ./cipher -e 4.r HELLOTHERE
2
Error 4.r is not a valid key
$ echo $? 1$
8. (6 Points)
For valid arguments, the program should produce the encrypted message in the output (see example below)
and terminate with 0 code.
$ ./cipher -e 3 THISISASECRETMESSAGE
TIETSHSSSCEMSAEIAREG
$ echo $? 0$
Make sure that the program prints a newline character, following the encrypted output, so that your shell
prompt starts in the next line (As seen in the above example. This applies even if the message is empty).
$ ./cipher -e 3 ""
$ echo $? 0$
9. If the program is invoked with -d option, as long as other arguments are valid, just terminate with 0 without
producing anything in the output.
$ ./cipher -d 3 TIETSHSSSCEMSAEIAREG
$ echo $? 0$
10. Your entire C program must be contained in the main. You are not allowed to write any functions of your
own. Further, the only library functions that you are allowed to use are printf and putchar. -3 points
deduction if any of this is not followed.
11. You are not allowed to create/define any new arrays. You may (and have to) use the command line argument
arrays given to main (argv, etc.), and can have additional variables referencing these arrays as needed to make
your program logic simpler. However, you must not modify the contents of any of these arrays either directly
or through other variables referencing them. -3 points deduction if any of this is not followed.
12. If your program crashes for ANY test case case, it will result in a -3 point deduction.
13. If your program is stuck (infinite loop) and the TA has to terminate forcefully, it may not get tested for
remaining test cases in TA’s tester script.
Ex. 2 — A Test Script (6 Points)
1. Write a simple tester script for your cipher program. Name this script, testcipher.sh. Your script need not
be fancy. It just needs to functional. In other words, a person who is executing the script should be able to
observe the input and output of the individual program invocations (including the termination codes of the
process) for each test case and come to the conclusion as to whether the program passes the test case or not.
The test script need not inform the observe whether the test case is passed or not, you can assume that the
observer knows what a valid outcome should look like for each input scenario and make that judgement for
themselves.
The script will be invoked as below:
$ ./testcipher.sh
3
Once the user starts the script, there is no further interaction with the script. (The script just goes and
executes a bunch of test cases you have put inside it). The output thus produced by the script should be send
to standard output and/or standard error as you may chose.
2. (2 Points) The tester script should start by checking if there is a cipher.c in the current directory, if so,
then try to compile it and make an executable (using the gcc command as mentioned previously). After that
it should check if the executable, cipher was built. If any of these steps does not work out, inform the user
of the appropriate situation and exit the test script without attempting to execute any more test cases.
3. (4 Points) At a minimum, your tester script should include the test cases explicitly listed as examples in Ex.
1. You are encouraged to include a few more to ensure that you catch any other potential bugs and make it
easy for you to test your logic.
4. Every other detail is left to you as to how you want to implement the tester script. Remember, you can get
points for your tester script even if your C program does not work correctly for all the test cases.
• You must write a reasonable amount of comments (to understand the logic) in your tester script. -1 point for
not writing su!cient comments.
• As stated in the description, the script output should be understandable to the user (input, output, return
code, etc.) - think how the mini tester in the past assignment was useful for you to judge if your program had
a bug by looking at its output. Lack of this readability in the script output can result in -2 points deduction.
WHAT TO HAND IN
Upload your C program cipher.c, and script, testcipher.sh to MyCourses. If MyCourses is not allowing files with
.sh extension, you may use the tar command as discussed in class to make a tar file or a tar.gz file that contains
your C program and shell script and submit them. Please download your submitted tar files to double check if they
are good. Erroneous invocation of the tar command can sometimes result in a bad file. Such submissions cannot be
unfortunately graded.
DO NOT turn in the executable cipher. TAs will compile your C program on their own as indicated in the
problem descriptions above.
ASSUMPTIONS
• The first and second arguments that might be passed to cipher can be anything, your program should not
crash, but should take appropriate actions.
• You can assume that the third arugment is always made up of ONLY uppercase alphabets (no spaces) and can
be of any length (including 0).
HINTS
This is a high-level outline to get you started with the logic. You are not obliged to follow it.
• Observe the example given at the beginning of Ex.1. We can see that the message has to be visualized into as
many levels as the value of the key is.
• Now look at the space (dots) between consecutive characters in each level. They follow a certain “step-size”
between consecutive characters. This step size is di↵erent depending on whether the zig-zag is going “up” or
“down”. See if you can formulate an equation for these two step-sizes based on the key and the level. (Try to
work it out with a pen and paper first).
• Using the above two concepts, you can iterate through the input message as many times as there are levels and
for each level, print ONLY the characters that fall into the step size(s) for that level.
• Understanding the ASCII table will help you come up with a logic to convert the key argument (which is a
character string/array) into an integer variable. For example, int digit = ’6’-’0’ will store the value 6 into
the integer variable digit. • The concept of computing the length of a character string/array is not very hard, you just count characters
till you encounter a ‘\0’ character.
4
RESTRICTIONS
For your shell script, you may use any bash features, Unix commands, options provided already in mimi.
Please follow the instructions given for C program at the end of Ex.1, on the features and functionality that your
are allowed to use.
TESTING
There is no mini tester associated with this assignment. You are encouraged to use your tester script to test your
program’s behaviour. Start with very short messages so that you can easily look at the output to see if the logic is
working before getting into longer messages.
TAs will test using their own scripts, but it will follow the assumptions that have been setforth in the assignment
description.
QUESTIONS?
Please use piazza. However, before posting your question, use the search functionality to check if it has been already
discussed. You should also look under “Mini 4 general clarifications” pinned post to check if a popular question has
been already included there. They will not get individual responses again.
TAs will not help you with logical errors, you are already expected to have some idea of developing basic programming logic and debugging them from your introductory programming courses like COMP 202. So do not ask
questions of the sort “My code is not printing the correct output” - that is because your logic is not correct. Use
printf statements to print the values of loop indexes, etc., as you are debugging the code to verify if your programming logic is correct. The core of the programming logic is already given to you in the HINT section which you
should be able to develop into the full code.
TA help is limited to clarifying assignment requirements and helping with C programming language syntax and
semantics related questions.