Linked Structures - Review Chapter 13. Instructor: Scott Kristjanson CMPT 125/125 SFU Burnaby, Fall 2013

Transcription

1 Linked Structures - Review Chapter 13 Instructor: Scott Kristjanson CMPT 125/125 SFU Burnaby, Fall 2013

2 2 Scope Introduction to Linked Structures: Object references as links Linked vs. array-based structures Managing linked lists Linked implementation of a stack Wk14.1 Slide 2

3 3 Linked Structures An alternative to array-based implementations are linked structures A linked structure uses object references to create links between objects Recall that an object reference variable holds the address of an object Wk14.1 Slide 3

4 4 Linked Lists A Person object could contain a reference to another Person public class Person { private String name; private String addr; private Person next; // Link to Another Person object A series of Person objects could make up a linked list: Wk14.1 Slide 4

5 5 Linked Non-Linear Structures Links could also be used to form more complicated, non-linear structures This is called a graph Wk14.1 Slide 5

6 6 Linked Lists There are no index values built into linked lists To access each node in the list you must follow the references from one node to the next Person current = firstperson; while (current!= null) { System.out.println(current); current = current.next; Wk14.1 Slide 6

7 7 Linked Lists Inserting a node in the Middle 1. Set the next member in obj to refer to the next object in the list 2. Set the next member of the previous object to refer to the new object obj 1 2 prev x next Wk14.1 Slide 7

8 8 Linked Lists Inserting a node at the front Care must be taken to maintain the integrity of the links To insert a node at the front of the list, first point the new node to the front node, then reassign the front reference Wk14.1 Slide 8

9 9 Linked Lists Deleting the First Node To delete the first node, reassign the front reference accordingly If the deleted node is needed elsewhere, a reference to it must be established before reassigning the front pointer Wk14.1 Slide 9

10 10 Put Linked List Details into separate Node Class So far we've assumed that the list contains nodes that are self-referential (Person points to a Person) But often we'll want to make lists of objects that don't contain such references Solution: have a separate Node class that forms the list and holds a reference to the objects being stored Node Node Node Node Node Node Person Person Person Person Person Person Wk14.1 Slide 10

11 Doubly Linked Lists 11 There are many variations on the basic linked list concept For example, we could create a doubly-linked list with next and previous references in each node and a separate pointer to the rear of the list next Node Node Node Node Node Node previous Person Person Person Person Person Person Wk14.1 Slide 11

12 12 Implementing a Stack using Links Let's implement a stack using a linked list to hold the elements Our LinkedStack<T> class stores a generic type T and implements the same StackADT<T> interface used previously A separate LinearNode<T> class forms the list and hold a reference to the element stored An integer count will store how many elements are currently in the stack Wk14.1 Slide 12

13 13 LinkedStack<T> public class LinkedStack<T> implements StackADT<T> { private int count; private LinearNode<T> top; /** * Creates an empty stack. LinkedStack<T> */ public LinkedStack() { count = 0; top = null; LinearNode<T> top int count Linear Node<T> Wk14.1 Slide 13

14 14 LinearNode<T> public class LinearNode<T> { private LinearNode<T> next; private T element; LinearNode<t> top public LinearNode() { next = null; element = null; public LinearNode(T elem) { next = null; element = elem; public LinearNode<T> getnext() { return next; LinearNode<t> next T element Object of Class T Linear Node<T> public void setnext(linearnode<t> node) { next = node; public T getelement() { return element; public void setelement(t elem) { element = elem; Wk14.1 Slide 14

15 15 Implementing a Stack using Links Since all activity on a stack happens on one end, a single reference to the front of the list will represent the top of the stack Linear Node Linear Node Linear Node Linear Node Linear Node Linear Node T T T T T T Wk14.1 Slide 15

16 16 Implementing a Stack using Links The stack after A, B, C, and D are pushed, in that order: Wk14.1 Slide 16

17 17 Implementing a Stack using Links After E is pushed onto the stack: Wk14.1 Slide 17

18 Implementing a Stack using Links 18 package jsjf; /** * Represents a node in a linked list. * Java Foundations 4.0 */ public class LinearNode<T> { private LinearNode<T> next; private T element; /** * Creates an empty node. */ public LinearNode() { next = null; element = null; /** * Creates a node storing the specified element. elem element to be stored */ public LinearNode(T elem) { next = null; element = elem; Wk14.1 Slide 18

19 19 Implementing a Stack using Links /** * Returns the node that follows this one. reference to next node */ public LinearNode<T> getnext() { return next; /** * Sets the node that follows this one. node node to follow this one */ public void setnext(linearnode<t> node) { next = node; /** * Returns the element stored in this node. element stored at the node */ public T getelement() { return element; /** * Sets the element stored in this node. elem element to be stored at this node */ public void setelement(t elem) { element = elem; Wk14.1 Slide 19

20 20 Implementing a Stack using Links package jsjf; import jsjf.exceptions.*; import java.util.iterator; /** * Represents a linked implementation of a stack. * Java Foundations 4.0 */ public class LinkedStack<T> implements StackADT<T> { private int count; private LinearNode<T> top; /** * Creates an empty stack. */ public LinkedStack() { count = 0; top = null; Wk14.1 Slide 20

21 21 Implementing a Stack using Links /** * Adds the specified element to the top of this stack. element element to be pushed on stack */ public void push(t element) { LinearNode<T> temp = new LinearNode<T>(element); temp.setnext(top); top = temp; count++; /** * Removes the element at the top of this stack and returns a * reference to it. element from top of stack EmptyCollectionException if the stack is empty */ public T pop() throws EmptyCollectionException { if (isempty()) throw new EmptyCollectionException("stack"); T result = top.getelement(); top = top.getnext(); count--; return result; Wk14.1 Slide 21

22 22 Implementing a Stack using Links Wk14.1 Slide 22

23 23 Key Things to take away: Linked Objects: Object Reference variables can be used to create linked structures A Linked List is composed on objects that each point to the next in the list Objects stored in a collection should not contain any implementation details of the underlying data structure that The order in which references are changed are very important Dealing with the first node in the list often requires special handling A Linked List implementation of a Stack adds elements to, or removes elements from, one end of the linked list. Queues, Trees, and other structures can be created with Linked Objects Wk14.1 Slide 23

24 24 Practice Array Questions Problem 1: Write a program that shuffles a deck of cards, then deals out 5 cards each to two players. Problem 2: Create a method that accepts two 2-dimensional arrays A and B as formal parameters and returns the matrix product A*B. Problem 3: What is wrong with Bob s Array code? Problem 4: Write a method called sumarray that accepts an array of floating point values and returns the sum of the values stored in the array Problem 5: Write a method called sum2darray that accepts an int[][] array and sums all the numbers in the array and returns the sum of the values stored in the array. Wk14.1 Slide 24

25 25 Problem 1: Write a program to shuffle and deal cards Step 1: Create the Shuffled Deck using a Stack for(int i=0; i<card.numsuits*card.numvalues; i++) { boolean foundnextcard = false; int tries = 0; while (!foundnextcard) { int cardvalue = valuegen.nextint(card.numvalues); int cardsuit = suitgen.nextint(card.numsuits ); tries++; if (carddealt[cardsuit][cardvalue] == false) { Card nextcard = new Card(cardValue+1, cardsuit); deckofcards.push(nextcard); carddealt[cardsuit][cardvalue] = true; foundnextcard = true; System.out.println("Card "+(i+1)+" : "+ nextcard+" found after "+tries+" tries"); Wk14.1 Slide 25

26 26 Problem 1: Write a program to shuffle and deal cards Step 2: Deal five cards to each player for(int p=0; p<numplayers; p++) for(int c=0; c<numcardsperhand; c++) hand[p][c] = deckofcards.pop(); Wk14.1 Slide 26

27 27 Problem 1: Write a program to shuffle and deal cards Step 3: Display both player s hands for(int p=0; p<numplayers; p++) { System.out.println("\nPlayer "+p+" hand:"); for(int c=0; c<numcardsperhand; c++) { System.out.println( "Card " + c + ": "+hand[p][c]); Wk14.1 Slide 27

28 28 Problem 1: Write a program to shuffle and deal cards Discussion: How does a Card get converted to a String? System.out.println("Card "+c+": "+hand[p][c]); Using the tostring method from the Card class! public String tostring() { if (value == 0) return "Joker"; else return cardnames[value]+" of "+suit; enum Suit {Hearts, Diamonds, Clubs, Spades; private final static String[] cardnames = {"Joker","Ace", "Two", "Three","Four","Five", "Six", "Seven","Eight","Nine","Ten", "Jack","Queen","King"; Wk14.1 Slide 28

29 29 Problem 2: Method to return the matrix product A*B. public static int[][] matrixmultiply(int[][] A, int [][] B) { int arows = A.length; int acols = A[0].length; int brows = B.length; int bcols = B[0].length; int N = A.length; // Assume A and B are both NxN if (acols!= brows) throw new IllegalArgumentException( "A Columns: " + acols + " did not match number of B Rows " + brows); int[][] C = new int[arows][bcols]; for (int i = 0; i < arows; i++) // A Row for (int j = 0; j < bcols; j++) // B Column for (int k = 0; k < acols; k++) // A Column C[i][j] += A[i][k] * B[k][j]; return C; Wk14.1 Slide 29

30 30 And the Winner for best TicTacToe GUI is Curtis Babnik Human vs Smarter: Human vs Learns : Wk14.1 Slide 30

31 31 TicTacToe UML A UML Diagram using a Eclipse UML Plugin: Wk14.1 Slide 31

32 32 References: 1. J. Lewis, P. DePasquale, and J. Chase., Java Foundations: Introduction to Program Design & Data Structures. Addison-Wesley, Boston, Massachusetts, 3rd edition, 2014, ISBN Wk14.1 Slide 32