CS 171 Assignment 3 Summer 2022
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CS 171 (Summer 2022)
Assignment 3: Maze Traversal
Goals
● Practice the use of Queues and Stacks
● Practice the use of links for storing and constructing paths
● Implement the path finding algorithm to solve the maze search problem
Problem and Requirement
You are to implement two versions of a path finding algorithm for finding a path through a square maze; one version will use a stack, and the other will use a queue to store a list of positions to explore as the search algo- rithm proceeds. A maze is a square space with an entrance at the upper left-hand corner, an exit at the lower right hand corner, and some internal walls. Your algorithm will find a path through a given maze, starting from the entrance and finishing at the exits that does not go through any walls (a path must go around walls).
Your program will take a command line argument, the filename (e.g. maze1.txt) for the maze description. (To input a command line argument in Eclipse, you can click “Run” – “Run Configurations” – “Arguments” and then type in the argument in the “Program Arguments” box.) Your program next prints the maze to stdout, then tries to find a path through the maze. If a path is found, the positions in the path are printed to stdout with a success message. Otherwise, an error message is printed to the screen. Your program will print the paths that result from executing both implementations of the path finding algorithm.
The maze will be represented as an NxN array of 1s and 0s; if maze[i][j] == 1 then there is an internal wall in that position of the maze. Otherwise, there is no wall. The search algorithm will start at maze[0][0] and find a path to maze[N-1][N-1]. A path is represented by a sequence of [i][j] position coordinates, starting with [0][0] and ending with [N-1][N-1]. From position [i][j] in the path, the next position, the next position in the path can only be the position to the left, right, up or down from position [i][j]; a path cannot move diagonally from one position to another. For example, the following is the array representation
of a 10x10 maze:
ENTER --> 0 0
0
0
0
1
0
0
0
0 |
1
0
1
1
1
1
0
0
1
0 |
The following path is
1
0
0
0
0
1
1
1
1
0
possible
1
1
1
0
0
0
0
0
0
0 |
0
0
1
1
1
1
0
0
1
1 |
0
0
0
0
0
0
0
0
0
0 |
0
0
0
1
1
1
1
1
1
1 |
through this maze:
0
1
1
1
1
0
0
0
0
1
0
0
0
0
0
0
1
1
0
0
0
0
0
0
0
0
1
1
0
Path: ([0][0], [1][0], [1][1], [1][2], [2][2], [3][2], [3][3], [4][3], [5][3], [6][3], [6][4], [6][5], [5][5], [4][5], [3][5], [2][5], [2][6], [1][6], [0][6], [0][7], [0][8], [1][8], [2][8], [3][8], [4][8], [5][8], [5][7], [6][7], [7][7], [8][7], [8][8], [8][9], [9][9])
ENTER --> X 1 1 1 0 0 X----X----X 0
| | |
X----X----X 1 0 0 X 1 X 0
| | |
0 1 X 1 1 X----X 1 X 0
| | |
0 1 X----X 1 X 1 1 X 0
| | |
0 1 0 X 1 X 1 1 X 0
| | |
1 1 1 X 1 X 1 X----X 0
| | |
0 0 1 X----X----X 1 X 1 1
|
0 0 1 0 0 0 1 X 1 1
|
0 1 1 0 1 0 1 X----X----X
|
0 0 0 0 1 0 1 1 0 X --> EXIT
Program Operation
Your program will perform the following actions:
1. Take the name of the maze file from command line.
2. Use the provided method readMaze to read the maze into the array representation and print the maze to stdout.
3. Next, your program will search for a path from the maze entrance point to the exit point using both versions of the path searching algorithm: stackSearch and queueSearch.
4. For both algorithms, your program will either print an error message (if there is no path through the maze) or will print:
(a) The path as a list of [i][j] positions, starting at the entrance point and ending at the maze exit point.
(b) The maze with the path coordinates indicated by ’X’ characters.
The Path Finding Algorithm
You should implement the following algorithm for finding a path (some of the details are left for you to fill in):
Create a search list for positions yet to explore, add the entrance position, (0,0) to the search list while the list is not empty do
Remove the next position from the search list
if it is the exit position [n-1][n-1] then
a path is found, construct the path and return the path
else
mark the position as visited, add all valid up, down, left, or right neighbor positions to the search list
end if
end while
if the list is empty and the method has not returned then
there is no path
end if
In one version of the algorithm, you will use a queue of Position objects to enqueue neighbor elements and to dequeue the next element at each step. The other version will use a stack to push neighbor elements and to pop the next element at each step. Once you implement one version of the algorithm, you can implement the second version by simply copying the code to a new method and replacing the data structure use and the calls to enqueue/dequeue with push/pop.
To construct the path, you need to keep track of the previous position (parent) that leads to each position on the explored paths. The Position class has been defined for you, which contains the position information and a reference to its previous (parent) Position on a path. This is similar to the NOde class in the linked list examples we have learned which contains a data item and a reference to its next Node. When adding a neighbor position of the current position to the search list, you need to create a new Position object or the neighbor position to be added with a reference to its parent – note that the current position is in fact the parent of the neighbor position. When the exit position is reached or a path is found, you can follow the previous (parent) links from the exit position to construct the path.
Think carefully about when a neighbor is “valid” and how you can ensure that your implementation termi- nates.
Data Structures
You can use java .util .ArrayDeque class for storing the search list. See the lecture notes and the Java API for examples of usage and documentation.
The maze is stored as a 2-dimensional array of char values defined as char[][]. You can see how to declare initialize, and use 2-dimensional arrays in the provided method readMaze.
Getting Started
1. Get the starter code PathFinder .java from ~cs171001/share/hw3 directory and understand it. It has the following methods that are already implemented:
– char[][] readMaze(String filename) : reads maze from file, returns 2-dimensional array with each entry containing a 0 or a 1(wall)
– printMaze(char[][] maze) : prints a visual representation of the 2D maze array to stdout
– main(): reads and prints a maze, finds a path using stackSearch (to be implemented) and queueSearch (to be implemented), prints the path using printPath method (to be implemented) and prints the maze for each version of the algorithm. You do not need to change the main method.
2. Fill in your implementation for the stackSearch, queueSearch and printPath methods.
3. Test your program with the provided mazes (maze1 .txt, maze2 .txt, maze3 .txt, maze4 .txt)
4. Think about the following question (you do not need to submit the answers)
– Why do both versions of the algorithm work?
– Why do they sometimes output different paths for some of the mazes?
Honor Code
The assignment is governed by the College Honor Code and Departmental Policy. Please remember to have the following comment included at the top of the file.
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UgIr cPDE W4r Jr PWa WPsK , IU W4r rPJscEr PJUrIDE PF UgPrE 4RRsPvED
·/
WsIUUEa WIUgPJU cParJLUIaG 4ar
Br UgE IarUsJcUPs . rm r o a - e a g a o a
Submission:
Place your completed PathFinder.java file directly under your ~/cs171/hw3 directory. Then use the turnin command:
~cs171001/turnin PathFinder .java hw3
Grading:
If your program does not compile, you will get 0 points
Correct usage of ArrayDeque to implement the stackSearch algorithm: 25 points
Correct usage of ArrayDeque to implement the queueSearch algorithm: 25 points stackSearch algorithm correctly finds the path or returns null if no path exists: 15 points queueSearch algorithm correctly finds the path or returns null if no path exists: 15 points The path to exit (if any) is correctly marked in maze and printed: 15 points
Coding style: 5 points
2022-07-21