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06-30175 The University of Birmingham
Data Structures & Algorithms School of Computer Science
Spring Semester 2010-2020
c Alan P. Sexton 2010-2020
Assignment 04
1 Introduction
This assignment will be marked out of 10 and contributes to your module mark (10% of your final module mark).
Deadline for submission is:
14:00 Friday 13th March
While the first assignment was about linked lists and their implementation, and the second about using standard Java
Collection classes for Lists, Maps, Sets and Arrays, the third about recursion and binary trees, this one is about priority
queues, external merge sorting, CSV files and file handling.
CSV files are data files that represent data as rows of “Comma Separated Values” (https://en.wikipedia.org/
wiki/Comma-separated_values). While there are many options and complexities possible in CSV files, we
restrict ourselves to a very simple core.
2 Marking
• To get full marks, you will have to provide working implementations for all methods where the comment:
“//WRITE YOUR CODE HERE AND REPLACE THE RETURN STATEMENT”
appears so that it matches the specification provided in the corresponding Javadoc comments.
• There will be 1 mark if your submission compiles correctly and can be run and passes the tests provided that already
pass in initial version of the assignment files.
• While a set of tests that cover most of the requirements have been provided for you, more tests, and more and
alternative sample data files, may be used as part of the marking progress. Therefore you are advised to write your
own extra tests.
• If your submission is not structured correctly or does not compile, or crashes or enters an infinite loop before any
tests pass, then no marks will be earned.
3 Plagiarism
Plagiarism will not be tolerated: it is unfair to the other students and prevents you from learning, and would give you a
mark you don’t deserve, and qualifications you don’t deserve, hence being unfair to Society as a whole too. Please behave
well and reward the module lecturer and TA’s by submitting your own, hard work.
All submissions will be checked for copying against other submissions and against sources on the web and elsewhere.
Any student found to have
When the module lecturer decides that there is evidence of plagiarism, the case will be forwarded to the Senior Tutor, who
will look at the evidence and apply penalties and warnings, or, if necessary, forward this to the University.
• University regulations on plagiarism: https://intranet.birmingham.ac.uk/as/studentservices/
conduct/plagiarism/index.aspx
4 Student Welfare
If you miss or are going to miss a deadline, or are getting behind the learning material, for reasons outside of your control,
please contact Student Welfare. Occasionally students represent the University in sports, programming competitions, etc.,
and if this is going to affect your ability to comply with a deadline, Welfare should be informed. It is the Welfare Team, not
the teaching team, who can award extensions and cancellations, and devise other measures to cope with adverse situations.
It is important to contact welfare as soon as possible when such situations arise.
1
5 Details
There are a number of methods to implement for this assignment, all in CsvUtils.java:
• getStudentID() and getStudentName(): the usual ones to capure your information for marking purposes
• splitSortCsv(...): split a CSV file, which is assumed to be in random order, into a collection of ordered
“runs”: individual CSV files which are ordered and which have, between them, all the rows of the original CSV
file.
• mergePairCsv(...): merges two ordered runs into a single (larger) ordered run.
• mergeListCsv(): merges a list of ordered runs into a single ordered run by means of repeated calls to mergePairCsv
(...)
To assist you there is a set of unit tests provided for you in CvsUtilsTest.java and a class that provides methods to
handle the CSV format required in CsvFormatter.java.
A sample method, copyCsv(...) has been provided in CvsUtils.java to show you an appropriate example of
opening files for reading and writing, and reading from and writing to CSV files using the CsvFormatter class. Please
note: this method is provided ONLY as an example of code that correctly reads and writes CSV files. You should NOT
use this method in any of the code that you write.
You should carefully read through all the code before starting on the assignment.
5.1 Paths, Files, Reading and Writing
This assignment requires creating, reading and writing files. Your assignment project directory contains a directory
“data”. This directory contains some CSV files that you should use in development and testing.
You should only read files from this directory and write files to this directory. The only files you write to should
have names that start with “temp_”. The marking software will run your code in a secure sandbox environment and
any attempts to read or write outside this directory or to write to a file whose name does not start with “temp_” will cause
an exception to be thrown and your program to fail.
The java.nio.file.Path class represents full and partial file path strings to be handled. By default, when your
program starts, it does so with your project directory as its current working directory. Therefore if you give a relative path
name for a file you wish to read, e.g. “data/state-abbrevs.csv”, then it will look for that file path starting at your
project directory. You will frequently want to be able to create paths to new files in the data folder:
• Given a path variable dataDir to a directory, you can construct a path variable to a file given by a String
filename, "temp_copy.csv", with: dataDir.resolve("temp_copy.csv")
• You can also create a Path from Strings using calls like Paths.get("data") or Paths.get("data",
"temp_copy.csv")
• You can get the String name of a file (the last component of a path) with path.getFileName(). This actually
returns a Path object with a single component, but the toString() method of Path will turn it into a string.
• If you have the path of a file, and want to create the path of a new file in the same directory, you can use:
path.resolveSibling("temp_new_file.java")
• You will need to generate names for your output files that do not clash with your input file names or any other
output file names. To do this, make an output file name from the input filename in the following form: if the input
filename is "abc.csv", make the output file name be "temp_00000_abc.csv", where the number part can
be incremented for each output that you need. If your input file Path is in the variable fromPath, and your
temporary file number counter is in the integer variable tempNum, this can be done with:
Path tmpPath = fromPath.resolveSibling(
String.format("temp_%05d_%s", tempNum, fromPath.getFileName()) );
5.1.1 Scanner and PrintWriter
For this assignment, you should use the Scanner class for reading CSV files and the PrintWriter class for writing
them. This ensures that all unicode characters are handled appropriately.
• You can open a Scanner on a file specified by a Path object with:
Scanner from = new Scanner(fromPath);
2
• The PrintWriter class does not currently support opening a file from a Path object, so you first have to turn
the Path object into a File object:
PrintWriter to = new PrintWriter(toPath.toFile()))
Once you have opened a file, it is important to ensure that you close it:
• Not closing it can mean that your program can crash if you open many files: there is a limit on how many files you
can have open at the same time. While this limit is large, it is possible to hit the limit.
• Not closing open files is a memory leak and wastes memory
• Under some circumstances, not closing a file may cause some of the last writes to that file to not be saved to the file
on disk
Therefore you should get in the habit of always closing your files. While you can close them directly, you have to ensure
that they get closed even if an exception occurs before you make the call to close them. You can handle this with the try
... catch ... finally construct, but there is a much easier and more convenient way: the try-with-resources construct:
try ( Scanner from = new Scanner(fromPath);
PrintWriter to = new PrintWriter(toPath.toFile()))
{
// do your reading and writing here - do not close
}
This form of try ensures that every resource that is constructed in the “try” part is open within the following curly braces
and is automatically closed when the program leaves the braces, whether that leave is caused by an exception, a return,
or simply completing the code in the braces. Note that the code in the try part is a series of assignments separated by
semi-colons. You can have as many as you like. You can also nest them, if you wish:
try ( Scanner from = new Scanner(fromPath) )
{
// read from "from" here
try ( PrintWriter to = new PrintWriter(toPath.toFile()) )
{
// Read from "from" and write to "to" here
}
// read from "from" here
}
5.2 Priority Queues and Comparators
A PriorityQueue is Java’s implementation of the Priority Queue data structure. To use it you need to use a Comparator
object which tells the PriorityQueue how to compare the objects that you put into it. The CsvFormatter class
contains a nested CsvComparator class that is suitable for this purpose. Since it is nested inside CsvFormatter,
you can only create a CsvComparator object from a CsvFormatter object as follows:
CsvFormatter formatter = ... // get a formatter object
String columnName = "abbreviation"; // the header of the column used for
comparing CSV rows
CsvFormatter.RowComparator comparator = formatter.new RowComparator(columnName)
For this assignment, you will need to put CSV rows into your priority queues. These CSV rows are implemented as arrays
of Strings, so, given the comparator created above, you can create a suitable new PriorityQueue as follows:
PriorityQueue

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