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Computer Science 187

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public Maze(String fileName, boolean IsDebugOn) throws IOException ... public void closeFile() throws IOException. closes the maze file and frees I/O resources ... – PowerPoint PPT presentation

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Title: Computer Science 187

1
Computer Science 187
Introduction to Programming with Data Structures
Lecture 11 Stacks and Mazes
Announcements

Midterm exam October 29 in class
A new OWL assignment is up, due 10/20.
Help for studying for the midterm is available
on the WIKI.

2
Implementing Stacks

Book shows four possibilities
As an extension to Vector
With a list component
Using an array
As a linked structure
Included in these notes but not covered in class
Book defines a stack interface on page 259 Java
has no stack interface

3
Implementing Stack as Extension of Vector

Collection hierarchy includes java.util.Stack
The Vector class offers a growable array of
objects
Elements of a Vector accessed by index
Size can grow or shrink as needed
That is Vector is just like ArrayList

4
Implementing Stack as Extension of Vector (2)
Top element of the Stack is at the highest index
5
Stack Code

public class StackltEgt extends VectorltEgt
public E push (E e)
add(e)
return e
public E pop () throws EmptyStackException
try
return remove(size()-1)
catch (ArrayIndexOutOfBoundsException ex)
throw new EmptyStackException()
...

6
Stack Code (2)

public class StackltEgt extends VectorltEgt
...
public E peek () throws EmptyStackException
try
return get(size()-1)
catch (ArrayIndexOutOfBoundsException ex)
throw new EmptyStackException()
public boolean empty ()
return size() 0

7
Implementing Stack with a List

Can use ArrayList, Vector, or LinkedList
All implement the List interface
Name of class illustrated in text is ListStack
ListStack is an adapter class
Adapts methods of another class to ...
Gives different names to essentially the same
operations

8
ListStack Code

public class ListStackltEgt
implements StackIntltEgt
private ListltEgt list
public ListStack ()
list new ArrayListltEgt()
// or new VectorltEgt or new LinkedListltEgt
public E push (E e)
list.add(e)
return e
...

9
ListStack Code (2)

public E peek ()
if (empty())
throw new EmptyStackException()
return list.get(list.size()-1)
public E pop ()
if (empty())
throw new EmptyStackException()
return list.remove(list.size()-1)
public boolean empty ()
return list.size() 0

10
Implementing Stack Using an Array

Must allocate array with some default capacity
Need to keep track of the top of the stack
Have no size method, so must track size
Similar to growable PhoneDirectory
Why not just use vector?

11
Implementing Stack Using an Array (2)
12
ArrayStack Code

public class ArrayStackltEgt
implements StackIntltEgt
private static final int INITIAL_CAPACITY 10
private E data
(E) new ObjectINITIAL_CAPACITY
private int top -1
public ArrayStack ()
public boolean empty () return top lt 0
...

13
ArrayStack Code (2)

public E push (E e)
if (top data.length) reallocate()
return datatop e
public E pop ()
if (empty()) throw new EmptyStackException()
return datatop--
public E peek ()
if (empty()) throw new EmptyStackException()
return datatop

14
ArrayStack Code (3)

private void reallocate ()
E newData (E) new Objectdata.length2
System.arraycopy(data, 0, newData, 0,
data.length)
data newData

15
Implementing Stack as a Linked Structure

We can implement Stack using a linked list

16
LinkedStack Code

public class LinkedStackltEgt
implements StackIntltEgt
private NodeltEgt top null
public LinkedStack ()
public boolean empty () return top null
public E push (E e)
top new NodeltEgt(e, top)
return e
...

17
LinkedStack Code (2)

public E pop ()
if (empty()) throw new EmptyStackException()
E result top.data
top top.next
return result
public E peek ()
if (empty()) throw new EmptyStackException()
return top.data

18
LinkedStack Code (3)

private static class NodeltEgt
private E data
private NodeltEgt next
Node (E data, NodeltEgt next)
this.data data
this.next next

19
Comparison of Stack Implementations

Vector is a poor choice exposes Vector methods
Likewise, extending other List classes exposes
their methods
Using a List as a component avoids exposing
LinkedStack operations are O(1)
But Node objects add space overhead
ArrayList component is perhaps easiest
Still some space overhead (StackArrayList)
Array as a component best in space and time
But somewhat harder to implement

20
(No Transcript)
21
M. C. Escher Relativity
22
Isabelle de Beaufort's environmental maze in the
village of reignac sur Indre in
France. http//cccw.adh.bton.ac.uk/schoolofdesign/
MA.COURSE/LMaze.html
23
An Application of Stacks Solving a Maze

Mazes and Labyrinths whats the difference?

Enter
Exit
http//www.astrolog.org/labyrnth
24
The Palace at Knossos
Greek myth of Thesesus and the Minotaur.
http//www.unmuseum.org/maze.htm
25
Whats a Plausible Approach to Solving a Maze?
Exit
Enter
26
Solving a Maze

Represent as an array with black and white spaces

Path
Important decision / choice points which path
to follow?
Wall
Dead-ends
No-brainers
Special
27
The General Cases
North

What direction can we move in?
not walls
not back where we came from

West
East
South
Choice
N E S W 1 0 1 1
U
(choice)
0
U
Dead-End
No-Brainer
N E S W 0 0 0 1
(deadend)
(no choice)
allowed directions
0
0
28
What to do in each of the cases?

No-Brainer Case
only one direction that we can move in.
suppose I know my current location and the
direction to move?
I want to take a step .whats my next
coordinates?

U
29
What to do in each of the cases?

The choice or decision case
need to remember all possibilities push each
onto a stack
need to choose one pop top one off the stack
need to take a step in that direction

Helper Method
location direction
location direction
location direction
location direction
location direction
N E S W 1 0 1 0
U
location direction
POP
possible next steps
30
What to do in each of the cases?

Dead-end case
need to go back to previous choice point and
explore an alternative path BACKTRACKING
if there are NO previous choice points, then
there is no path through the maze!

Wheres the next alternative(s) from the previous
choice point?? ON THE TOP OF THE STACK.
location direction
location direction
POP
Becomes new location and direction
31
Putting it all together
Helper Methods

Given current location (start) and goal
location
Move one step into maze
get current direction given starting location
step in that direction
Repeat forever

v Is current location the goal? if yes,
report it found and stop.
v Get the allowed directions from here.
v Handle the cases
Only one choice.
get direction to go in
step in that direction
More than one choice
push a decision point onto the stack for EACH
possible direction
pop the top location and set current location and
direction accordingly
take a step in that direction
No choices (dead-end)
If the stack is not empty, get top element (pop),
set current location and direction, and step in
that direction
If the stack IS empty, report failure and stop.

32
The Maze Program Classes Used

All the stack classes
LinkedStack.java,
StackfullException.java, StackEmptyException.java,
Node.java, Stack.java
The interface defining compass directions
CardinalDirections.java
int NORTH 0 int EAST 1 int SOUTH 2 int
WEST 3
The class defining a location
Location.java protected int row protected int
col
The class defining the structure of the
information pushed onto the stack
decBlock.java myLocation currPos
myDirmyDirection
The class defining the maze itself
Maze.java
A test program
TestMaze.java reads in maze, prints it, and
solves it,

33
A Maze in a File
The size of the maze (rows, columns)

5 5
2 0
3 4
.
..
...
....

The starting square (row, col)
The goal square
The symbol used for wall and path
The maze itself
34
Maze.java

public class Maze implements CardinalDirections
protected char maze //the maze upper
left corner of maze is (0,0)
protected int nrows, ncols //size of maze
protected location start //(row,col)
coordinates of starting point
protected location goal //(row,col) coordinates
of goal (i.e. the exit from the maze)
protected char wallSym //symbol used in file to
designate a wall
protected char pathSym //symbol used in file to
designate a path
protected boolean debug //debug flag...if true,
debugging statements are on
private int currDir //my current direction
North0, East1, etc.
private location currLoc //my current location
in the maze (location is a row and column)
private int allowed //array of four possible
directions I can move in from my
//current location with 0 indicating I can't go
that way, and 1 indicating
//I can in order NESW
private LinkedStack choices //the stack to be
used in the method mazeSolve()
private BufferedReader B //reader for maze file
private String fname //file that the maze
is on

35
The public methods of Java.maze

public Maze(String fileName, boolean IsDebugOn)
throws IOException
sets up a buffered reader for the maze file
reads the array size, start location, goal
location, and symbols from file
creates the maze array
creates the choices stack
public char getMaze()
get method which returns the two dimensional
maze array
public void readMaze() throws IOException
reads the maze array itself from the file and
fills maze array
public void writeMaze ()
prints the maze
public boolean mazeSolve()
returns true if the maze has a path from start to
goal and false otherwise
public void closeFile() throws IOException
closes the maze file and frees I/O resources

36
The private methods of Maze.Java

private char toChar(String s)
returns the character equivalent of the first
element of s
private int getDirection()
returns the direction corresponding to location
of first allowed direction in the allowed
direction vector
private void step ()
given a direction, moves us to the next maze
square in that direction by modifying the current
location
private int getStartDir()
given the starting location, determine the
direction to step into the maze
private boolean isGoal()
compares current location and goal location
returns true if they match
private void getAllowed()
returns the four element vector indicating
whether or not (1 or 0) we can move in the
corresponding direction from the current location
private void updateStack()
for every allowed direction from the current
location, pushes a location block onto the stack.
Each block contains the current location and a
direction to move in.

37
The mazeSolve method

public boolean mazeSolve()
currDir getStartDir() //get direction
to go in to step into the maze
step()
//step into it
while (true)
if(isGoal()) return true //weve
found a solution, so were done.
getAllowed() //where can
I go from here?
int sumsumAllowed() //add up the
elements of the allowed vector
switch(sum)
case 0
//We have nowhere to go from this square
(deadend)
if( !choices.isEmpty()) //Go back
to previous decision point- search from there.
decBlock old (decBlock)
choices.pop()
currLoc old.decLocation
//update our location and direction
currDir old.myDir
step()