CS2006 - Data Structures I

Chapter 8

Queue I

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Topics

Introduction Queue Application Implementation

Linked List

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Queues A queue differs from a stack in that it follows the

first-in-first-out (FIFO) principle. Inserting an item to the rear is known as

"enqueueing". Removing an item from the front is known as

"dequeueing".

cashier

Front of line Back of line

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Queues Linear, homogeneous structure Has First-In-First-Out (FIFO) behavior;

Items added at one end and removed from the other end

Middle elements are logically inaccessible

Add/

Enqueue

Remove/Dequeue

Back/Rear Front/Head

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Queue Applications

Real-World Applications Buy a movie ticket Cashier lines in any store

 Computer Science Applications Print lines of a document Convert digit strings to decimal

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ADT Queue

Specification: Elements

Homogeneous Linear

Operations Create an empty queue Check if a queue is empty / Full Enqueue (Enq, Enque, Add, Insert):

Adding new element at the back (rear) Dequeue (Deq, Deque, Remove, Serve):

Deleting an element from the front Retrieve an element from a queue

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Queue Applications Converting Digit Strings to Decimal

Enter characters from the keyboard and retain them in order

Assumption: No typing mistakes Blank spaces may precede or follow the digits

Formula used: Initial value

DigitSum = 0

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Queue Applications Converting Digit Strings to Decimal

Pseudocode// Convert digits in aQueue into decimal integer n// Get first digit, ignoring any leading blanksdo {

ch=queue.dequeue()} while ( ch is blank)// Assertion: ch contains first digit// Compute n from digits in queuen = 0;done = false;do { n = 10 * n + integer that ch represents

if (! queue.isEmpty( ) ) ch=queue.dequeue()

elsedone = true

} while (! done and ch is a digit)// Assertion: n is result

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Queue Applications

Recognizing Palindromes Uses a queue & a stack Idea:

1. Insert the string in both queue & stack2. Remove characters from both stack's Top & queue's Front,

comparing them3. Repeat until:

a) Either the stack or the queue are empty String is a palindrome

b) The character from the stack and the corresponding character from the queue are not similar String isn't a palindrome

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Queue Applications Recognizing Palindromes

PseudocodeIsPal(in str:string) : boolean// Determines whether String is a palindromeaQueue.createQueue ( ) // Create an empty queue aStack.createStack ( ) // Create an empty stack// Insert each character of String into both aQueue and aStacklength = length of strfor ( i = 1 through length){ nextChar = ith character of str

aQueue.enqueue ( nextChar )aStack.push(NextChar)

} // end for// Compare aQueue with aStackcharactersAreEqual = true;while ( ! aQueue.isEmpty() && charactersAreEqual){ queueFront =aQueue.peek ()

stackTop= aStack.top ()if ( queueFront equals stackTop){ aQueue.dequeue ( )

aStack.pop ( )} else charactersAreEqual = false

} // end whilereturn charactersAreEqual

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ADT Queue Implementation

Possible implementations:

Linked List-based Linear Circular

Array-based Linear Circular

ADT List-based

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Linked List-Based Implementation More straightforward than array-based Possible options:

Linear linked list Two external “pointer” (Front & Back)

Circular linked list One “pointer” will be enough (Back)

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Linked List Queue Implementation

Linked List -Based Implementation: Option 1 Insertion to an empty list

front = newNodeback = newNode

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Linked List Queue Implementation

LL -Based Implementation: Option 1 Insertion to a non-empty list

newNode.setNext ( NULL);Back.setNext (newNode);back = newNode;

Deletiontemp = front ;front = front.getNext()temp.setNext ( NULL )

20 15

front

26 84

back

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Linked List Queue Implementation

LL -Based Implementation: option 1 Deletion

form a one-node (one item) queueIf (front = back&&front!=null){

back = null front=null }

We will use option 2 to implement our queue

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Queue Interface

public interface QueueInterface {

public boolean isEmpty(); public void enqueue(Object newItem) throws

QueueException; public Object dequeue() throws QueueException; public void dequeueAll(); public Object peek() throws QueueException;}

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Queue Exception

public class QueueException extends RuntimeException {

public QueueException(String s) {

super(s);

} // end constructor

} // end QueueException

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LListQueue Implementationpublic class LListQueue implements QueueInterface {

private Node lastNode; // we use option 2

public LListQueue() { lastNode = null; } // end default constructor

// queue operations: public boolean isEmpty() { return lastNode == null; } // end isEmpty

public boolean isFull() { return false; } // end isFull

public void dequeueAll() { lastNode = null; } // end dequeueAll

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LListQueue Implementationpublic class LListQueue implements QueueInterface {

private Node lastNode;

public LListQueue() { lastNode = null; } // end default constructor

// queue operations: public boolean isEmpty() { return lastNode == null; } // end isEmpty

public boolean isFull() { return false; } // end isFull

public void dequeueAll() { lastNode = null; } // end dequeueAll

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LListQueue Implementation (2) public void enqueue(Object newItem) { Node newNode = new Node(newItem); if(isFull()) throw new QueueException("QueueException on enqueue:"+ "queue full"); // insert the new node if (isEmpty()) { // insertion into empty queue newNode.setNext(newNode); } else { // insertion into nonempty queue newNode.setNext(lastNode.getNext()); lastNode.setNext(newNode); } // end if lastNode = newNode; // new node is at back } // end enqueue

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LListQueue Implementation (3) public Object dequeue() throws QueueException { if (!isEmpty()) { // queue is not empty; remove front Node firstNode = lastNode.getNext(); if (firstNode == lastNode) { // special case? lastNode = null; // yes, one node in queue } else { lastNode.setNext(firstNode.getNext()); } // end if return firstNode.getItem(); } else { throw new QueueException("QueueException on dequeue:" + "queue empty"); } // end if } // end dequeue

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LListQueue Implementation (4) public Object peek() throws QueueException { if (!isEmpty()) { // queue is not empty; retrieve front Node firstNode = lastNode.getNext(); return firstNode.getItem(); } else { throw new QueueException("QueueException on peek:" + "queue empty"); } // end if } // end peek } // end ListQueue

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LListQueue Implementation (4) public Object peek() throws QueueException { if (!isEmpty()) { // queue is not empty; retrieve front Node firstNode = lastNode.getNext(); return firstNode.getItem(); } else { throw new QueueException("QueueException on peek:" + "queue empty"); } // end if } // end peek } // end ListQueue

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LListQueue Testpublic class LListQueueTest {

public static void main(String[ ] args) { LListQueue aQueue = new LListQueue(); System.out.println("Enqueuing:"); for (int i = 0; i < 9; i++) { System.out.print(" "+i); aQueue.enqueue(new Integer(i)); } // end for System.out.println("\nDequeuing:"); for (int i = 0; i < 9; i++) { System.out.print(" "+aQueue.dequeue()); } // end for System.out.println(); } // end main} // QueueTest

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Review In a queue, items can be added ______.

only at the front of the queue only at the back of the queue either at the front or at the back of the queue at any position in the queue

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Review Operations on a queue can be carried out

at ______. its front only its back only both its front and back any position in the queue

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Review Which of the following is NOT an ADT

queue operation? enqueue isEmpty Pop peek

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Review The ______ operation retrieves and

removes the front of a queue. isEmpty enqueue dequeue peek

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Review The ______ operation retrieves the item

that was added earliest to a queue, but does not remove that item. enqueue dequeue dequeueAll peek

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Review A reference-based implementation of a

queue that uses a linear linked list would need at least ______ external references. one two three four

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Review A reference-based implementation of a

queue that uses a circular linked list would need at least ______ external references. one two three four

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Review Which of the following operations leaves a

queue unchanged? enqueue dequeue dequeueAll peek