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Comp 251: Assignment 5

• 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, but this is at your own
risk; helper methods could cause your methods to crash when called from other programs if
not added responsibly. 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 give you an automatic 0.
• The starter code includes a tester class. 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 set of examples. We therefore highly
encourage you to modify that tester class, expand it and share it with other students on the
myCourses 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 one of the files you must submit. 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.
• Do not submit individual files. Include all your files into a .zip file and, when appropriate,
answer the complementary quiz online on MyCourses.
• You will automatically get 0 if the files you submitted on MyCourses do not compile.

• You should compile all your files directly from command line without using a package, by
using the command javac *.java
Assignment specifications: This assignment is different from the previous ones. Instead of
being given a template with specific instructions, you are given three problems with a set of examples for each. Those three problems can be solved by using concepts seen in class, but we will
not be enforcing a particular approach. You must submit three java classes balloon.java ,
mancala.java and islands.java.
Each of these classes must contain a main method that opens an input file with a fixed name
(see each question for that file name), and writes the appropriate output to a text file. You will be
given 6 example inputs and outputs in the appropriate file for each exercise. Do not change the
name of the input file.
Although you are not restricted in the method you want to use to solve the problem, you must
implement this method yourself, and as such you can only use the following import statements:
• import java.io.*;
• import java.util.*;
• import java.util.regex.*;
• import java.math.*;
• import static java.lang.System.out;
COMP 251 – HW5 Page 2 of 4 Fall 2018
Each of the three questions is weighted 50 points. You can get full marks for the assignment by completing two questions, or you can choose to attempt all 3, with a maximum
achievable mark of 120/100. If you choose to not do one of the problems, you must still submit
the java class, but make it a dummy that does not write any file.
1. (50 points) Fun with balloons
After a birthday party held in McIntyre 504, N balloons are left in the room, floating from left
to right. Bored TAs like to play with bows to practice their hunting abilities. During such a
practice, they position themselves at the left end of the room and shoot an arrow towards the
right end of the room from an arbitrary height they choose. Because Computer Science TAs
are not very good at physics, assume the arrows fly in a straight line at the height H chosen by
the shooter. When an arrow comes in contact with a balloon, the balloon disappears and the
arrow continues its way from left to right at a slightly lower height H-1, in a straight line. In
other words, if the arrow travelling at height H comes in contact with a balloon at position i,
then at position i+1, the arrow will be at height H-1, and will maintain that height until it hits
another balloon or the wall. Your task is to write a java class balloon.java that, given an
assignment of balloon positions, will compute the numbers of arrows necessary to destroy all
the balloons.
The input file is named testBalloons.txt. Line 1 contains an integer n that represents
the number of problems included in the file. Line 2 contains n integers mi separated by a
space, each representing the total number of balloons of the ith problem. Starting at line three,
every line describes a new problem. Each line contains mi
integers hj separated by a space,
which represent the respective height of the jth balloon from left to right.
The output file, named testBalloons_solution.txt, contains n lines, one per problem. Each line includes a single integer, representing the number of arrows that need to be
shot to destroy all the balloons of the corresponding input.
To summarize, the command java balloon should generate the output file. Same goes for
the other two exercises.
2. (50 points) Mancala Leapfrog
When TAs have nothing else to do, they like playing games. Because some of them do not have
many friends, they particularly like single-player games, like a specific variant of mancala,
played with small pebbles. In that game, the player is given a board with 12 cavities on a
single line. Some of them contain a pebble, and some don’t. The goal of the game is to remove
as many pebbles as possible from the board. Pebbles are removed through moves. A move
is possible when there is a straight line of three adjacent cavities A, B, C where A is empty
but both B and C contain a pebble. The move consists of moving the pebble from C to A, and
removing the pebble at B from the game. Such a move can be executed in both directions.
Unfortunately, some TAs struggle with the game, and they ask you to write a program to
help them. Your java class mancala.java should, given an assignment of pebbles over a
mancala board of size 12, find a sequence of moves such that as few pebbles as possible are
left on the board.
The input file, named testMancala.txt, has, like exercise 1, the number of included
problems as its first line. Other lines each contain a problem, represented as a sequence of 12
COMP 251 – HW5 Page 3 of 4 Fall 2018
integers. 0 represents an empty slot, and 1 encodes the presence of a pebble at that position.
The output file, named testMancala_solution.txt, contains n lines, one per problem.
Each line includes a single integer, representing the minimum number of remaining pieces at
the end of the optimal move sequence. For example, if the input position can be solved in such
a way that no pebbles are left after 4 moves, the output should be 0.
3. (50 points) Discovering islands in the ocean
Back in the 1830s, one of the first grad students at McGill University had the idea of using
satellites images to search the oceans for new islands to avoid very expensive exploration
voyages. Unfortunately, the resolution on satellite images was not stellar in those times, so the
bitmap images they produce require some post-processing. Your task is to help this visionary
grad student by writing a java class islands.java, which should be given as input an
image, more precisely a 2D array of black or white pixels, and output the number of islands
found in that image. Each pixel in such an image is either black (#) or white (-). Black pixels
are associated with water, and white pixels are parts of an island. All pixels that are adjacent
vertically or horizontally should be considered to be part of the same island.
The input file is named testIslands.txt. Line 1 contains an integer n that represents
the number of problems included in the file. The rest of the lines contain n input arrays of
pixels. Each input begins with a line containing a pair of integers 1 ≤ m, n ≤ 100. This line
is followed by m lines, each containing exactly n pixels.
The output file, named testIslands_solution.txt, contains n lines, one per problem.
Each line includes a single integer, representing the number of unique islands observed in the
corresponding input. Part of an island can still be counted as an observation of an island.
You will submit your three java files in a single zip file.

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