Comp 251: Assignment 2

Comp 251: Assignment 2

• Your solution must be submitted electronically on codePost. Here is a short tutorial to help you
understand how the platform works. You will receive an email inviting you to join the class there
in early October.
• To some extent, collaborations are allowed. These collaborations should not go as far as sharing
code or giving away the answer. You must indicate on your assignments (i.e. as a comment at
the beginning of your java source file) the names of the people with whom you collaborated or
discussed your assignments (including members of the course staff). If you did not collaborate
with anyone, you write “No collaborators”. If asked, you should be able to orally explain your
solution to a member of the course staff.
• This assignment is due on October 16th at 11h59:59 pm. It is your responsibility to guarantee that
your assignment is submitted on time. We do not cover technical issues or unexpected difficulties
you may encounter. Last minute submissions are at your own risk.
• This assignment includes a programming component, which counts for 100% of the grade, and an
optional long answer component designed to prepare you for the exams. This component will not
be graded, but a solution guide will be published.
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• Multiple submissions are allowed before the deadline. We will only grade the last submitted file.
Therefore, we encourage you to submit as early as possible a preliminary version of your solution
to avoid any last minute issue.
• Late submissions can be submitted for 24 hours after the deadline, and will receive a flat penalty
of 20%. We will not accept any submission more than 24 hours after the deadline. The submission
site will be closed, and there will be no exceptions, except medical.
• In exceptional circumstances, we can grant a small extension of the deadline (e.g. 24h) for medical
reasons only. However, such request must be submitted before the deadline, and justified by a
medical note from a doctor, which must also be submitted to the McGill administration.
• Violation of any of the rules above may result in penalties or even absence of grading. If anything is
unclear, it is up to you to clarify it by asking either directly the course staff during office hours, by
email at (cs251@cs.mcgill.ca) or on the discussion board on Piazza (recommended). Please,
note that we reserve the right to make specific/targeted announcements affecting/extending these
rules in class and/or on the website. It is your responsibility to monitor Piazza for announcements.
• The course staff will answer questions about the assignment during office hours or in the online
forum. We urge you to ask your questions as early as possible. We cannot guarantee that questions
asked less than 24h before the submission deadline will be answered in time. In particular, we will
not answer individual emails about the assignment that are sent sent the day of the deadline.
Programming component
• You are provided some starter code that you should fill in as requested. Add your code only where
you are instructed to do so. You can add some helper methods. Do not modify the code in any
other way and in particular, do not change the methods or constructors that are already given to
you, do not import extra code and do not touch the method headers. The format that you see on
the provided code is the only format accepted for programming questions. Any failure to comply
with these rules will result in an automatic 0.
• Public tests cases are available on codePost. You can run them on your code at any time. If your
code fails those tests, it means that there is a mistake somewhere. Even if your code passes those
tests, it may still contain some errors. We will grade your code with a more challenging, private set
of test cases. We therefore highly encourage you to modify that tester class, expand it and share it
with other students on the discussion board. Do not include it in your submission.
• Your code should be properly commented and indented.
• Do not change or alter the name of the files you must submit, or the method headers in these
files. Files with the wrong name will not be graded. Make sure you are not changing file names by
duplicating them. For example, main (2).java will not be graded. Make sure to double-check your
zip file.
• You can submit either a zip file or individual files on codePost. If you get more than 0 on the
public tests, it means codePost accepted your files.
• You will automatically get 0 if the files you submitted on codePost do not compile, since you
can ensure yourself that they do. Note that public test cases do not cover every situation and
your code may crash when tested on a method that is not checked by the public tests. This is why
you need to add your own test cases and compile and run your code from command line on linux.
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Exercise 1 (Disjoint sets (24 points)) We want to implement a disjoint set data structure with union
and find operations. The template for this program is available on the course website and named
DisjointSets.java.
In this question, we model a partition of n elements with distinct integers ranging from 0 to n − 1 (i.e.
each element is represented by an integer in [0, · · · , n − 1], and each integer in [0, · · · , n − 1] represent
one element). We choose to represent the disjoint sets with trees, and to implement the forest of trees
with an array named par. More precisely, the value stored in par[i] is parent of the element i, and
par[i]==i when i is the root of the tree and thus the representative of the disjoint set.
You will implement union by rank and the path compression technique seen in class. The rank is an
integer associated with each node. Initially (i.e. when the set contains one single object) its value is 0.
Union operations link the root of the tree with smaller rank to the root of the tree with larger rank. In the
case where the rank of both trees is the same, the rank of the new root increases by 1. You can implement
the rank with a specific array (called rank) that has been added to the template, or use the array par
(this is tricky). Note that path compression does not change the rank of a node.
Download the file DisjointSets.java, and complete the methods find(int i) as well as
union(int i, int j). The constructor takes one argument n (a strictly positive integer) that
indicates the number of elements in the partition, and initializes it by assigning a separate set to each
element.
The method find(int i) will return the representative of the disjoint set that contains i (do not
forget to implement path compression here!). The method union(int i, int j) will merge the
set with smaller rank (for instance i) in the disjoint set with larger rank (in that case j). In that case,
the root of the tree containing i will become a child of the root of the tree containing j), and return the
representative (as an integer) of the new merged set. Do not forget to update the ranks. In the case where
the ranks are identical, you will merge i into j.
Once completed, compile and run the file DisjointSets.java. It should produce the output available in the file unionfind.txt available on the course website.
Note: You will need to complete this question to implement Exercise 2.
Exercise 2 (Minimum Spanning trees (24 points)) We will implement the Kruskal algorithm to calculate the minimum spanning tree (MST) of an undirected weighted graph. Here you will use the file
DisjointSets.java completed in the previous question and two other files: WGraph.java and
Kruskal.java available on the course website. You will need the classes DisjointSets and
WGraph to execute Kruskal.java. Your role will be to complete two methods in the template
Kruskal.java.
The file WGraph.java implements two classes WGraph and Edge. An Edge object stores all informations about edges (i.e. the two vertices and the weight of the edge), which are used to build graphs.
The class WGraph has two constructors WGraph() and WGraph(String file). The first one
creates an empty graph and the second uses a file to initialize a graph. Graphs are encoded using the
following format. The first line of this file is a single integer n that indicates the number of nodes in
the graph. Each vertex is labelled with an number in [0, · · · , n − 1], and each integer in [0, · · · , n − 1]
represents one and only one vertex. The following lines respect the syntax “n1 n2 w”, where n1 and n2
are integers representing the nodes connected by an edge, and w the weight of this edge. n1, n2, and w
must be separated by space(s). An example of such file can be found on the course website with the file
g1.txt. These files will be used as an input in the program Kruskal.java to initialize the graphs.
Thus, an example of a command line is java Kruskal g1.txt.
The class WGraph also provide a method addEdge(Edge e) that adds an edge to a graph (i.e. an
object of this class). Another method listOfEdgesSorted() allows you to access the list of edges
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of a graph in the increasing order of their weight.
You task will be to complete the two static methods isSafe(DisjointSets p, Edge e) and
kruskal(WGraph g) in Kruskal.java. The method isSafe considers a partition of the nodes
p and an edge e, and should return True if it is safe to add the edge e to the MST, and False otherwise.
The method kruskal will take a graph object of the class WGraph as an input, and return another
WGraph object which will be the MST of the input graph.
Once completed, compile all the java files and run the command line java Kruskal g1.txt. An
example of the expected output is available in the file mst1.txt. You are invited to run other examples
of your own to verify that your program is correct. Note that you need to compile it with the two other
files for it to work.
Exercise 3 (Greedy algorithms (52 points)) In this exercise, you will plan your homework with a greedy
algorithm. The input is a list of homeworks defined by two arrays: deadlines and weights (the relative importance of the homework towards your final grade). These arrays have the same size and they
contain integers between 1 and 100. The index of each entry in the arrays represents a single homework, for example, Homework 2 is defined as a homework with deadline deadlines[2] and weight
weights[2]. Each homework takes exactly one hour to complete.
Your task is to output a homeworkPlan: an array of length equal to the last deadline. Each entry in
the array represents a one-hour timeslot, starting at 0 and ending at ’last deadline - 1’. For each time slot,
homeworkPlan indicates the homework which you plan to do during that slot. You can only complete
a single homework in one 1-hour slot. The homeworks are due at the beginning of a time slot, in other
words if an assignment’s deadline is x, then the last time slot when you can do it is x - 1. For example,
if the homework is due at t=14, then you can complete it before or during the slot t=13. If your solution
plans to do Homework 2 first, then you should have homeworkPlan[0]=2 in the output. Note that
sometimes you will be given too much homework to complete in time, and that is okay.
Your homework plan should maximize the sum of the weights of completed assignments.
To organize your schedule, we give you a class HW_Sched.java, which defines an Assignment
object, with a number (its index in the input array), a weight and a deadline.
The input arrays are unsorted. As part of the greedy algorithm, the template we provide sorts
the homeworks using Java’s Collections.sort(). This sort function uses Java’s compare()
method, which takes two objects as input, compares them, and outputs the order they should appear
in. The template will ask you to override this compare() method, which will alter the way in which
Assignments will be ordered. You have to determine what comparison criterion you want to use. Given
two assignments A1 and A2, the method should output:
• 0, if the two items are equivalent
• 1, if a1 should appear after a2 in the sorted list
• -1, if a2 should appear after a1 in the sorted list
The compare method (called by Collections.sort()) should be the only tool you use to reorganize lists and arrays in this problem. You will then implement the rest of the SelectAssignments()
method.
Submit all the Java files you modified on codePost.
Exercise 4 (Change-Making Problem (0 points) In this problem, we give a formal look at the problem
of returning a given amount of money using the minimum number of coins (including bills), for a given
coin system.
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For example, the best way to give someone 7 dollars is to add up a 5 dollars bill and a 2 dollars coin.
For simplicity, we will consider all denominations to be coins.
First, let us define the problem: a coin system is a m-tuple c = (c1, c2, . . . , cm) such that c1 c2
· · · cm = 1. For a given coin system c and a positive integer x representing the amount of money we
want to gather, we want to find a solution (a m-tuple of non-negative integers) k = (k1, k2, . . . , km) such
that x =
Pm
i=1 kici so as to minimize Pm
i=1 ki
.
There exists a greedy algorithm to find the optimal solution for certain coin systems: for a given x,
we select the largest coin ci ≤ x. Then, we repeat this step for x − ci
, and continue repeating it until
x becomes 0. For instance, with the coin system (10, 5, 2, 1), the algorithm decomposes x = 27 into
10, 10, 5, and 2.
We describe a coin system as canonical if and only if the solution given by the greedy algorithm
described above is optimal for any positive integer x. For example, all systems (a, 1) with a 1 are
canonical. For any positive integer x, we can express such a change system in the form of Euclidean
division: x = aq + r with r < a. The greedy solution for x is then the tuple g = (q, r). To prove that the
solution is optimal, we can proceed as follows:
Let’s consider g
0 = (q
0
, r0
) different than g such that x = aq0 + r
0
. Because q is already at its
highest value under x and cannot be above x, we have q
0 < q, otherwise q
0 = q causes g
0 = g. Since
(q
0 + r
0
) − (q + r) = (q
0 + x − aq0
) − (q + x − aq) = (a− 1)(q − q
0
) 0, g
0 would sum up to more coins
than the initial solution g, for any g
0
satisfying the problem definition. Thus, the solution g is optimal,
and the system (a, 1) is canonical.
4.1 (0 points) Design a non-canonical system of 3-tuple c = (c1, c2, c3) and prove your system is noncanonical.
4.2 (0 points) Let q and n be two integers ≥ 2. Prove that the system c = (q
n
, qn−1
, . . . , q, 1) is canonical.
4.3 (0 bonus points) Prove that the Euro system c = (200, 100, 50, 20, 10, 5, 2, 1) is canonical. There
will be no partial credit for this question.
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