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Assignment 6
CSCI 2132: Software Development
Due April 1, 2019
Assignments are due on the due date before 23:59. All assignments must be submitted electronically via the course SVN
server. Plagiarism in assignment answers will not be tolerated. By submitting your answers to this assignment, you declare
that your answers are your original work and that you did not use any sources for its preparation other than the class notes,
the textbook, and ones explicitly acknowledged in your answers. Any suspected act of plagiarism will be reported to the
Faculty’s Academic Integrity Officer and possibly to the Senate Discipline Committee. The penalty for academic dishonesty
may range from failing the course to expulsion from the university, in accordance with Dalhousie University’s regulations
regarding academic integrity.
General Instructions: How to Submit Your Work
You must submit your assignment answers electronically:
Change into your subversion directory on bluenose: cd ~/csci2132/svn/CSID .
Create a directory a6 for the current assignment.
Change into your assignment directory: cd a6 .
Create files inside the a5 directory as instructed in the questions below and put them under
Subversion control using svn add <filename> . Only add the files you are asked to add!
Once you are done answering all questions in the assignment (or the ones that you are able to
answer—hopefully all), the contents of your a4 directory should look like this:
a6
unique.c
(You will also have executable programs and potentially some data files in this directory, but you
should not add them to SVN.) Submit your work using svn commit -m”Submit Assignment 6″ .
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In this assignment, you are asked to develop a more powerful version of the uniq Unix tool we discussed
earlier in the term. This assignment is broken down into multiple steps to divide the problem into
incremental tasks. This type of incremental development that develops a simple but functional version
of a piece of software first and then incrementally adds features is common in the software industry. It
also helps to break a seemingly overwhelming task into smaller subtasks that suddenly do not seem
that overwhelming at all anymore. In each step, you are expected to modify the solution you developed
in the previous step instead of starting from scratch. For full marks, the code you submit should satisfy
all the requirements stated in the final step. Solutions that address only a subset of the requirements
earn partial marks.1
Recall that the uniq tool reads its input from stdin and sends the lines it reads back to stdout but
drops every line that is identical to the line that immediately precedes it. The final version of the
unique tool you are tasked to develop in this assignment has the following features:
It drops any line that is identical to a line read before, not only the immediately preceding line,
any line that came before it.
It must be able to deal with lines of arbitrary lengths.
It takes an optimal command-line flag -m. If this flag is provided, then it works in“multi-line
mode”. In this mode, any line that ends with a backslash () is to be treated as a single line
together with the line that comes after it. If a whole sequence of lines end with backslashes, then
all of them must be joined to form one logical line. This is the same as escaping line breaks in
Python source code, for example.
We break this into the following incremental steps:
Step 1: Recreate the functionality of uniq. In other words, you should check only adjacent lines
whether they are identical.
Step 2: Extend your implementation of Step 1 so it checks for duplicate lines throughout the document,
not duplicate adjacent lines. In the interest of an efficient implementation, you should use a
sorting algorithm to sort the lines of the input, which guarantees that identical lines are stored
consecutively. Now the solution in Step 1 can be used to eliminate duplicate lines. Note that there
is no requirement in this step that the order of the lines in the document should be preserved in
the output. In this step, you will simply use the qsort function provided as part of the C standard
library to sort the lines.
Step 3: This step adds the last ingredient necessary to meet the first two requirements of the unique
tool listed above. Specifically, you will create the illusion that each input line is considered in
order and output if it is not equal to any line that you have seen before and dropped otherwise.
This can be done by applying the solution from Step 2 but, before producing the final output,
rearranging the lines that were not dropped by the code in Step 2 in the order they appeared in
the input.
1You may be tempted to develop a tool that meets all the requirements in one shot instead of developing the solution
incrementally. If you succeed, this will earn you the same marks as if you had developed the solution incrementally. However,
the incremental approach outlined here is only slightly more work overall and each step is a much more manageable add-on
to the previous step than implementing all the required functionality in one shot. Also, if you get stuck with satisfying one
of the more advanced requirements, completing the earlier steps of this assignment ensures that you have a solution that at
least earns you partial marks. Therefore, you are encouraged to follow the incremental approach outlined here.
2
Step 4: In this step, you need to check the command line arguments of the program, check whether
-m is the only argument and, depending on the outcome, activate multiline mode. Then, when
reading input lines, you need to ensure that you collect all lines separated by escaped newline
characters into a single line record internally. An added complication is that you should consider
the inputs
This is \
a line
and
This is a\
line
to be the same as far as checking for duplicates goes, but in the output of the program, the line
breaks of the line that is kept should be preserved. For example, for the input
This is \
a line
Another line
This is a\
line
the output should be
This is \
a line
Another line
To accomplish this, you need to keep a record of where the line breaks were in the input.
Step 5: The final step is to replace the qsort function with your own implementation of Merge Sort.
In general, it is a better idea to use functions already provided as part of a library unless these
library functions are inadequate for some reason. You are asked to implement your own sorting
algorithm here to make you practice writing recursive functions.
The remainder of this assignment states the requirement for each of these five steps in more detail
and/or gives some hints that should be helpful in completing each step.
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Step 1
Implement a C program unique.c that recreates the functionality of the uniq tool by reading the input
line by line and dropping each line that is identical to the line immediately before it. On the input
abc
abc
def
abc
def
your program should output
abc
def
abc
def
the same output that uniq would produce.
While not strictly necessary for this step, it is important as a basis for subsequent steps that you read
the entire input and store it as a sequence of lines. Specifically, you should represent the input text
as a struct that stores the number of lines in the input and the text as a field of type char **. This
char ** points to a dynamically allocated array of char pointers. Each of these pointers points to a
dynamically allocated char array that stores a single input line.
You need to deal with two potential complications:
- There is no upper limit on the length of each input line. Normally, this would force you to allocate
an array of a certain minimum size for each line and then resize this array if it is not large enough
to hold the current line. Thankfully, the getline function can be used in a fashion that does this
work for you. Read its manpage to find out how you can get it to return a dynamically allocated
character array that stores the read input line. - You do not know the number of input lines in advance. Thus, you have to allow the array of
pointers to the individual lines to grow. To do this, you need to allocate a larger array once your
current array is full, copy the elements of the current array to the new array, and finally free the
current array and start using the new array. In order to avoid a quadratic running time resulting
from excessive copying, the new array you allocate should be twice as big as the current array.
For checking consecutive input lines that are the same, you can use the strcmp standard library function.
Read its manpage to find out how to use it.
An important requirement for your program is that it should not leak any memory. Since all memory
still held by a process is released when the process exits, there is technically no effort required on your
part to meet this requirement. However, it is important for you to practice how to manually manage
allocated memory over the course of a longer-running program. Thus, for full marks you need to
explicitly release all memory allocated to your program before the program exits. Specifically, you need
to call free on the char array holding each line and on the array of char pointers referring to these
char arrays. In later steps, you will add a few more dynamically allocated arrays that you will also
have to release.
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Step 2
Your strategy in this step should be to sort the lines in the input so identical lines in the input are stored
consecutively. Then you can eliminate consecutive duplicate lines as in Step 1. There is no need to
preserve the order of the input lines in the output. For example, on the input
def
abc
abc
def
you are allowed to produce the output
abc
def
To accomplish this step, you need two ingredients: - A function that can sort an array of data items, the array of char pointers representing the input
lines in this case. - A comparator function that is used by the sorting function to determine the correct order of pairs
of input items.
For the latter, you should use the strcmp function again, with a twist. For the former, you should use
the qsort sorting function provided by the C standard library again.
As you can see in the documentation of the qsort function, it calls the comparator function with
pointers to the two data items to be compared as arguments. This creates a small wrinkle here: The
items we want to sort are of type char *, since this is how we represent each input line. Thus, the
arguments passed to the comparator function by qsort are of type char **. strcmp, however, expects
to arguments of type char * (because it is meant to compare two strings, not two pointers to strings).
Therefore, you need to write a wrapper for strcmp suitable to be used by qsort. All this wrapper needs
to do is dereference its char * arguments to obtain to char values that can be passed to strcmp,
call strcmp on these two char pointers, and then return the result.
To complete this step, all you need to do is to combine the discussed ingredients: - Sort the lines of text using qsort and your strcmp wrapper.
- Eliminate duplicate lines using your solution developed in Step 1.
- Print the lines that remain as in Step 1.
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Step 3
Next you need to modify your code so the lines that are included in the output are output in the same
order as they appear in the input. For the input
def
abc
ghi
abc
def
this means that the output should be
def
abc
ghi
not
abc
def
ghi
(Recall that, of all identical lines in the input, the first one should be kept.)
The strategy is fairly simple: You sort the input lines and eliminate duplicates as in the previous step.
Then you rearrange the remaining lines in their original input order and output the resulting list.
To support this, you need to - Number the lines that you read in the order you read them and change your representation of
each input line from a simple char to a struct that stores the line number and this char . - Change your sorting step from Step 2 so it sorts the lines by their content and identical lines by
their line numbers. This is a simple extension of the comparison function for pairs of lines yur
developed in Step 2 and ensures that the elimination strategy from Step 1 indeed keeps the first
line in each set of identical lines. - Restore lines to their original order after eliminating duplicates. You do this by sorting the lines
that were not eliminated a second time, this time only by their line numbers, ignoring their
contents. To do so, you need to implement a second comparison function that compares lines by
line number.
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Step 4
Here, you add the ability to invoke unique with an optional command line flag -m. You should parse
the command line as practiced in Lab 7. unique can be invoked without command line arguments
or with the -m argument. Any other set of command line arguments should be rejected with an error
message.
The -m flag turns on“multi-line mode”. This means that lines that end with a backslash () should
be treated as if they were one line with the one that comes after it. A sequence of multiple lines that
all end with backslashes should be treated as a single“logical line”. The backslashes should not be
included in this logical line. Thus, the input
A little \
dwarf built\
a house
on a hill
A little dwarf \
built a house
should be treated as if it were the input
A little dwarf built a house
on a hill
A little dwarf built a house
Duplicate elimination transforms this into
A little dwarf built a house
on a hill
The output of your program should preserve the line breaks from the input, that is, this sequence of
lines produced by eliminating duplicates should be printed as
A little \
dwarf built\
a house
on a hill
because this is the way the first A little dwarf … line was broken in the input.
The interesting part in this step is how to treat lines separated by escaped newline characters as single
lines when eliminating duplicates but preserve line breaks in the output. There are two ways you can
do this.
The first is to eliminate the escaped newline characters from your internal representation of the line.
This makes sorting and checking for duplicates straightforward using exactly the same strategy as
in Step 3. However, the string does not store any information about where the line breaks were in
the input. Thus, you need to extend your line representation so it includes an array of the line break
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positions. When outputting the lines that are not dropped, you can then use the array of these line
break positions to recreate the line breaks in the output.
The second option is to preserve all escaped line breaks in your internal representation of each line.
This means that you can simply print each line that is part of the final output without the need for any
special handling. Sorting and checking of duplicates, however, requires you to implement your own
string comparison function, one that ignores escaped line breaks, that is, treats them as if they weren’t
present at all.
Step 5
We will discuss Merge Sort in class. Implement this algorithm and use it instead of qsort as your
sorting function. Your mergesort function should be a drop-in replacement for qsort, that is, it should
have the prototype:
void mergesort(void *array, size_t n_elems, size_t elem_size,
int (cmp)(const void , const void *));
(The description of this step is deceptively short. It is by far the most complicated step of this
assignment.)
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