Chapter 8 Queue I CS2006 - Data Structures I COSC2006 April 24, 2017 Chapter 7: Queues

Topics Introduction Queue Application Implementation Linked List

COSC2006 April 24, 2017 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 Chapter 7: Queues

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

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

ADT Queue Specification: Elements Operations Homogeneous Linear 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

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

Queue Applications Converting Digit Strings to Decimal Pseudocode // Convert digits in aQueue into decimal integer n // Get first digit, ignoring any leading blanks do { ch=queue.dequeue() } while ( ch is blank) // Assertion: ch contains first digit // Compute n from digits in queue n = 0; done = false; do { n = 10 * n + integer that ch represents if (! queue.isEmpty( ) ) else done = true } while (! done and ch is a digit) // Assertion: n is result

Queue Applications Recognizing Palindromes Uses a queue & a stack Idea: Insert the string in both queue & stack Remove characters from both stack's Top & queue's Front, comparing them 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

Queue Applications Recognizing Palindromes Pseudocode IsPal(in str:string) : boolean // Determines whether String is a palindrome aQueue.createQueue ( ) // Create an empty queue aStack.createStack ( ) // Create an empty stack // Insert each character of String into both aQueue and aStack length = length of str for ( i = 1 through length) { nextChar = ith character of str aQueue.enqueue ( nextChar ) aStack.push(NextChar) } // end for // Compare aQueue with aStack charactersAreEqual = true; while ( ! aQueue.isEmpty() && charactersAreEqual) { queueFront =aQueue.peek () stackTop= aStack.top () if ( queueFront equals stackTop) { aQueue.dequeue ( ) aStack.pop ( ) } else charactersAreEqual = false } // end while return charactersAreEqual

ADT Queue Implementation Possible implementations: Linked List-based Linear Circular Array-based ADT List-based

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)

Linked List Queue Implementation Linked List -Based Implementation: Option 1 Insertion to an empty list front = newNode back = newNode

Linked List Queue Implementation LL -Based Implementation: Option 1 Insertion to a non-empty list newNode.setNext ( NULL); Back.setNext (newNode); back = newNode; Deletion temp = front ; front = front.getNext() temp.setNext ( NULL ) 20 15 front 26 84 back

Linked List Queue Implementation LL -Based Implementation: option 1 Deletion form a one-node (one item) queue If (front = back&&front!=null){ back = null front=null } We will use option 2 to implement our queue

Queue Interface public interface QueueInterface { COSC2006 April 24, 2017 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; } Chapter 7: Queues

Queue Exception public class QueueException extends RuntimeException { COSC2006 April 24, 2017 Queue Exception public class QueueException extends RuntimeException { public QueueException(String s) { super(s); } // end constructor } // end QueueException Chapter 7: Queues

LListQueue Implementation COSC2006 April 24, 2017 LListQueue Implementation public 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() { } // end dequeueAll Chapter 7: Queues

LListQueue Implementation COSC2006 April 24, 2017 LListQueue Implementation public 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() { } // end dequeueAll Chapter 7: Queues

LListQueue Implementation (2) COSC2006 April 24, 2017 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 Chapter 7: Queues

LListQueue Implementation (3) COSC2006 April 24, 2017 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(); throw new QueueException("QueueException on dequeue:" + "queue empty"); } // end dequeue Chapter 7: Queues

LListQueue Implementation (4) COSC2006 April 24, 2017 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 Chapter 7: Queues

LListQueue Implementation (4) COSC2006 April 24, 2017 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 Chapter 7: Queues

LListQueue Test public class LListQueueTest { COSC2006 April 24, 2017 LListQueue Test public 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:"); System.out.print(" "+aQueue.dequeue()); System.out.println(); } // end main } // QueueTest Chapter 7: Queues

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

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

Review Which of the following is NOT an ADT queue operation? enqueue isEmpty Pop peek

Review The ______ operation retrieves and removes the front of a queue. isEmpty enqueue dequeue peek

Review The ______ operation retrieves the item that was added earliest to a queue, but does not remove that item. enqueue dequeue dequeueAll peek

Review A reference-based implementation of a queue that uses a linear linked list would need at least ______ external references. one two three four

Review A reference-based implementation of a queue that uses a circular linked list would need at least ______ external references. one two three four

Review Which of the following operations leaves a queue unchanged? enqueue dequeue dequeueAll peek