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Software Development Methods Assignment 3

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Software Engineering 265
Software Development Methods

Assignment 3
Programming environment
For this assignment please ensure your work executes correctly on the Linux
machines in ELW B238. You are welcome to use your own laptops and desktops for
much of your programming; if you do this, give yourself a few days before the due
date to iron out any bugs in the Python program you have uploaded to the lab
machines. (Bugs in this kind of programming tend to be platform specific, and
something that works perfectly at home may end up crashing on a different
hardware configuration.)
Individual work
This assignment is to be completed by each individual student (i.e., no group work).
Naturally you will want to discuss aspects of the problem with fellow students, and
such discussion is encouraged. However, sharing of code fragments is strictly
forbidden without the express written permission of the course instructor
(Zastre). If you are still unsure regarding what is permitted or have other questions
about what constitutes appropriate collaboration, please contact me as soon as
possible. (Code-similarity analysis tools will be used to examine submitted
programs.)
Objectives of this assignment
• Understand a problem description, along with the role used by sample input
and output for providing such a description.
• Use the Python programming language (specifically Python 3) and its module
mechanism to write an improved version of the MTF encoder and decoder.
• Use git to manage changes in your source code and annotate the evolution of
your solution with “messages” with committed changes.
• Test your code against the twenty provided test cases.
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This assignment: “coding2.py”
We will continue with our MTF coding exercise. The main differences this time
around are that:
• The module mechanism in Python will be used to organize your application.
• You will allow for a much larger dictionary.
Module mechanism
Two files have been provided to you named encode.py and decode.py. They refer to
the functionality within some coding2.py module, and it is this latter module which
you must write. You should assume these two provided files are located in the same
directory as your coding2.py file. Each of the provided files calls one of the functions
in the coding2 module (i.e., passes the name of the file to be processed).
Note that we will no longer use symbolic links to your Python script as we did in
previous assignment.
Larger dictionary
Our original implementations of MTF coding were seriously hampered by the
limitation of a dictionary to a maximum of 120 words. In order to apply such coding
to realistic texts we need a larger dictionary.
To increase the range of codes we will use the following scheme:
• A code from 1 to 120 will be encoded as (code + 128); that is, the code will fit
in one char.
• Codes from 121 to 375 will be represented using two chars: the first char is
(121 + 128), and the second code is (code – 121). For example, if the code
would be 300, then the first output char is 0xF9, and the second would be
0xB3. (The code for 121 would be F9 00.)
• Codes from 376 to 65912 will be represented using three chars: the first char
is (122 + 128), the second is ((code – 376) // 256), and third is ((code – 376)
% 256). In Python integer division is indicated by the “//” symbol. For
example, if the code is 30000, then the first output char would be 0xFA, the
second 0x73, and the third 0xB8. (The code for 376 would be FA 00 00.)
Since the resulting MTF files will produce codes that cannot be used by earlier
versions of the encoder and decoder, we need to change the magic number for our
MTF files. It will now be BA5EBA12 (i.e., last hex digit is changed). Note, however,
that this third program must be able to process MTF files created in the previous
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two assignments. (That is, your new program should check that one of the two
magic numbers – BA5EBA11 or BA5EBA12 – starts the MTF file.)
You are, of course, free to use any part of your assignment #2 solution as will help
with completion of assignment #3.
Exercises for this assignment
1. Within your git repo ensure there is an a3 subdirectory. All directories and
program files you create are to be source-controlled by git. You need not add
the directory containing test files to your project unless you wish to do so.
New test files are available at /home/zastre/seng265/a3/tests) (and the older
tests from A#1 and A#2 are still in /home/zastre/seng265/a1/tests).
2. The scripts encode.py and decode.py are in the /home/zastre/seng265/a3
directory. The only modification you are permitted to make to these two
files is changing the path of the Python interpreter that appears on the
first line.
3. Write your program.
4. You are welcome to use classes and regular expressions if you wish.
5. Use the test files to guide your implementation effort. Start with the simple
example in test 01 and move onto 02, 03, etc. in order. (Note that test19 is
particularly punishing.) Refrain from writing the program all at once, and
budget time to anticipate when things go wrong! Use the Unix command
cmp to compare your MTF files and diff to compare text files.
6. For this assignment you can assume all test inputs will be well-formed (i.e.,
our marking team will not test your submission for handling of input or for
arguments containing errors). The next assignment might specify errorhandling as part of its requirements.
7. Write two test scripts, i.e., Unix scripts that go through all of the twenty
provided test cases (one script for encoding, one script for decoding) and
reports which test cases fail.
What you must submit
• A single Python script named coding2.py within your git repository
containing a solution to Assignment #3.
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• Two Unix scripts (i.e., one for testing encoding functionality, one for testing
decoding functionality).
Evaluation
Our grading scheme is relatively simple.
• “A” grade: An exceptional submission demonstrating creativity and initiative.
The code within coding2.py runs without any problems. Test scripts are
provided. The program is clearly written and is structured in a way that also
uses functions appropriate.
• “B” grade: A submission completing the requirements of the assignment. The
code within coding2.py runs without any problems. Test scripts are
provided. The program is clearly written.
• “C” grade: A submission completing most of the requirements of the
assignment. The code within coding2.py runs with some problems. Test
scripts might be missing.
• “D” grade: A serious attempt at completing requirements for the assignment.
The code within coding2.py runs with quite a few problems. At least a
handful of tests pass. Test scripts might be missing.
• “F” grade: Submission either represents very little work or cannot be
executed for testing.
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Appendix
Python runs on many different platforms and operating systems, and the language
compensates for the different ways newlines are represented. Unfortunately this
can cause us some grief. For example, here are different ways of indicating a
newline:
• “\n”, or ASCII code 10
• “\r”, or ASCII code 13
• “\r\n” or ASCII codes 10 plus 13 (on Windows)
The problem for us is not so much these differences as with the way Python
attempts to isolate the programmer from the differences. Specifically the problem
has to do reading newline character(s). Even if a file is opened with “latin-1”
encoding:
• “\n” (or 0x0a) in the file is returned as 0x0a
• “\r” (or 0x0d) in the file is returned as 0x0a
That’s no typo! If you expect to read ASCII 13, Python will instead give you ASCII 10.
There is one solution to this that involves one more parameter to open(). To ensure
ASCII 13 is input as ASCII 13, use the following:
file = open(<filename>, encoding=”latin-1″, mode=”r”, newline=””)
This will prevent Python from performing newline-character substitutions when
reading newlines from this file object.
(Those of you with solutions reading the file as raw bytes might not encounter this
situation.)

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