CSC115 Assignment 4: Backtracking through a maze

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A maze may be represented as a two-dimensional array of characters, each array item
occupying a cell. We will let the ‘*’ character represent a wall and the space ‘ ’ character
represent an open spot in the maze. One open cell in the first row or the first column will
represent the entry point of the maze and another open cell in the last row or last column
will represent the exit point. Finding a path through the maze means finding a path of
connected cells that contain space characters, cells being connected only horizontally or
vertically.
The path itself is a list of (r, c) pairs where r is the row number from 0 . . .numRows−1
and c is the column number from 0 . . . numCols−1. The path list includes both the starting
cell and the finishing cell.
A maze file is a plain text file containing all the details of the maze itself, minus the actual
path. The following is an example of what a maze file would contain:
9 11
0 1
8 9
* *********
* * *
*** * *** *
* * * *
******* * *
* * *
* ******* *
* *
********* *
The first three lines indicate that the maze described in the file is 9 rows by 11 columns,
the starting cell is (0, 1) and the finishing cell is (8, 9). The rest of the lines represent the
placements of the wall (“*”) squares and the empty (“ ”) cells.
Quick Start
(1) Create a fresh directory to contain this assignment: CSC115/assn4 is a recommended
name. Download this pdf file and the .java files to this directory.
(2) Download and extract the docs.zip, javafiles.zip, and testMazes.zip in the
same directory. A docs directory will be created to store the specifications for the
public classes. All the document links in this document are accessible if they are
stored in the docs directory local to this pdf file.
(3) The following files are complete:
• Cell.java
• CellNode.java
• DequeEmptyException.java
• RunSolver.java
(4) You will create and complete the following two classes:
• CellDeque.java
• Maze.java
Detailed Instructions
The following java classes are completed already:
• Cell defines a cell in a 2-dimensional grid or matrix by its row and column numbers.
We follow the common programming convention that row ∈ {0, . . . , numRows−1} and
col ∈ {0, . . . , numCols−1}.
• CellNode is a Node for a doubly-linked list that holds Cell items.
• DequeEmptyException for throwing when a call to remove an item from an empty
Deque object. This is a standard home-made exception, that should be familiar.
• RunSolver is an external tester that will be described in detail after the Deque and Maze
sections.
You must create and implement the following two classes.
CellDeque
As part of the process of finding a successful path through the maze, we require a list to
contain Cell objects: we add a cell when it is potentially part of the solution or remove it
during the backtracking if it leads to a dead-end. We could use a general List ADT, but
we only need to insert and remove from the beginning or the end of the list as we wander
through the maze. Adding a cell to the list is easy, but when we backtrack, that cell needs
to be removed, a process that a regular Queue will not allow. A Stack ADT allows us to
remove the last try, but when we are finished, the Stack ADT will serve the solution cells
in reverse order.
Hence, our choice is the Double-Ended Queue, or Deque (pronounced “deck”), which is
described briefly in the textbook’s chapter on the Queue ADT. It behaves as if it were a
combination of both a Stack and Queue. You are to implement this class, using the doublylinked CellNode.
See the CellDeque specifications for the required public methods for this class Be sure to
do the necessary internal testing. Add whatever private or public methods help make the
program easy to test and debug. A method that prints out the contents is most helpful.
Maze
You can use whatever data type you wish inside the Maze class to represent the maze
object. We recommend a 2-dimensional array of characters, with specific characters representing a free cell, or a blocked cell, or a cell that has already been visited. A visited cell
lets you know which cells are currently part of the part of the solution, much like leaving
a stone in a maze to let you know that you’ve already tried that direction.
Note that the constructor takes, as input, an array of Strings to represent the maze. Using
what you know about String objects, you can extract whatever you need from this to store
the maze object as the data type of your choice.
Although it is worth thinking about loops or multiple solutions during the algorithm design, the mazes that will be used to test your code will have a single solution or no solution.
You may create whatever data fields and extra methods that you need to make this work.
However there are specific methods you must implement: See the Maze specifications for
the required public methods for this class.
In Java, a public method should not use recursion; we leave that to a private method.
The public method simply initiates the recursion by calling the recursive method with
its initial values. In this assignment, solve must call the following private recursive
method:
• private boolean findPath(Cell src, Cell dest)
This method answers the question: “Is there a path from a given source cell to the destination cell?” The following recursive definition of a path may help: A path from point A to
point B exists
if A is equivalent to B, or
if C is adjacent to A and there is a shorter path from C to B.
In the implementation of this method, the base cases must be clearly identified and dealt
with before the recursive statements. If no path exists in any direction between the source
and destination, then backtracking is necessary. Backtracking may include removing the
current cell from the solution list and/or flagging the current cell as not part of the path
and then returning false.
Since the internal testing involves creating test mazes (lots of typing), we supply a set of
test mazes in files. So for the Maze class, you are not required to create internal test cases.
An external tester RunSolver is supplied, with a single main method that extracts the
information from a maze text file and calls the Maze constructor and solve methods.
RunSolver
This class was written primarily to extract information from tester files to check the correctness of the Maze class. You are expected to understand this class well enough to have
written it yourself. You may add some println statements to help you, but do not change
the statements that interact with the Maze class.
Examples
The following screen shots demonstrate the expected results, using the mazes in files
maze00.txt and maze01.txt
In the example below, we added a statement: System.out.println(this); near the end
of the solver method in the Maze class to test whether the maze looked the way it should.
You can use whatever structure you find meaningful; in our case, we let ‘P’ represent the
current path.
Submission
Submit the following completed file to the Assignment folder on conneX.
• CellDeque.java
• Maze.java
Please make sure you have submitted the required file(s) and conneX has sent you a confirmation email. Do not send [.class] (the byte code) files. Also, make sure you submit your
assignment, not just save a draft. Draft copies are not made available to the instructors, so
they are not collected with the other submissions. We can find your draft submission, but
only if we know that it’s there.
A note about academic integrity
It is OK to talk about your assignment with your classmates, and you are encouraged to
design solutions together, but each student must implement their own solution.
Grading
Marks are allocated for the following:
• Proper programming style is demonstrated, as per the coding conventions on CSC115
conneX Resources.
• The Deque class must be implemented using a doubly-linked list, with no data structures
from the java.util package.
• The Maze findPath method must be
– recursive and
– organized to clearly determine the base case(s) and the recursive statement(s).
• Use of private helper methods where applicable.
• Internal testing in the Deque main method.
You will receive no marks for any Java files that do not compile.