Data Structures and Algorithms

Data Structures and Algorithms Queue 1

The Queue ADT • Introduction to the Queue data structure • Designing a Queue class using dynamic arrays • Linked Queues • An Application of Queues 2

1. introduction to the Queue Data Structure • A simple data container consisting of a linear list of elements • Access is by position (order of insertion) • Insertions at one end (rear)(back) , deletions at another end (front). • You cannot add/extract entry in the middle of the queue. • A queue is open at two ends. • First In First Out (FIFO) structure • Two basic operations: enqueue: add to rear dequeue: remove from front 3

An Illustration 4

An Illustration 5

Some Queue Applications • Simulation of waiting lines • Simulation of serviceable events • Job scheduling • Input/Output Buffering 6

Array Implementation of Queue 7

Array Implementation of Queue • When last array element is reached, we move back to start • The queue is viewed as a circular array • To enqueue: rear = (rear + 1) % size • To dequeue: front = (front + 1) % size • Both rear and front advance clockwise 10

Array Implementation of Queue • construct: construct an empty queue • queueIsEmpty  bool : return True if queue is empty • queueIsFull  bool : return True if queue is full • enqueue(el) : add element (el) at the rear • dequeue(el): retrieve and remove the front element • queueFront(el): retrieve front without removing it • queueLength  int:return the current queue length 13

Array Implementation of Queue • The queue may be implemented as a dynamic array. • The capacity (MaxSize) will be input as a parameter to the constructor (default is 128) • The queue ADT will be implemented as a template class to allow for different element types. 14

Array Implementation of Queue // File: Queuet.h // Queue template class definition // Dynamic array implementation #ifndef QUEUET_H #define QUEUET_H template <class Type> class Queuet { public: Queuet (int s = 128); // Constructor ~Queuet (); // Destructor 15

Array Implementation of Queue // Member Functions void enqueue(Type ); // Add to rear void dequeue(); // Remove from front Type queueFront() const; // Retrieve front bool queueIsEmpty() const; // Test for Empty queue bool queueIsFull() const; // Test for Full queue int queueLength() const; // Queue Length private: Type *queue; // pointer to dynamic array int front, rear, count, MaxSize; }; #endif// QUEUET_H #include "Queuet.cpp" 16

Array Implementation of Queue //Queue.cpp #include <iostream> Using names pace std; #include " QUEUET_H.h“ Queuet: : Queuet (int s) : Front(0) , rear(0), count(0) { MaxSize=s ; queue=new int[s] ; } ~ Queuet() {delete [] queue ;} //queueIsEmpty() bool Queue: :queueIsEmpty() const { return (count==0);// if(front == rear) } 17

Array Implementation of Queue // enqueue() void Queuet::enqueue( Type alue) { Type newBack = (rear+ 1) % MaxSize;// circular if (newBack != front) // queue Isn't full { queue[rear] = value; rear = newBack; count++;} } // Type queueFront() Type Queuet:: queueFront() const { if ( ! queueIsEmpty() ) return (queue[front]); } //dequeue void Queuet::dequeue() { i f ( ! queueIsEmpty() ) front = (front + 1) % MaxSize; count--;} 18

Array Implementation of Queue // queueIsfull() bool Queuet:: queueIsfull ( ) const { return (count==MaxSize);} //queueLength() const; Int Queuet::queueLength() const { return count; } 19

3. Linked Queues • A Queue can be implemented as a linked structure. • Requires more space than array implementations, but more flexible in size. • Two pointers are needed: front for dequeue and rear for enqueue 20

Node Specification // The linked structure for a node can be // specified as a Class in the private part of // the main queue class. class node // Hidden from user { public: Type e; // stack element node *next; // pointer to next node }; // end of class node declaration typedef node * NodePointer; NodePointer front , rear; // pointers 21

2 New 1 pnew Enqueue Operation rear front enqueue(v): NodePointer pnew = new node; pnew->e = v; pnew->next = NULL; rear->next = pnew; rear = pnew; 22

Dequeue Operation 1 rear cursor front 3 2 dequeue(v): v = front->e; cursor = front; front = front->next; delete cursor; 23

Linked Queue Class // File: QueueL.h // Linked List Queue class definition #ifndef QUEUEL_H #define QUEUEL_H template <class Type> class QueueL { public: QueueL(); // Constructor ~QueueL(); // Destructor void enqueue(Type ); // Add to rear 24

Linked Queue Class void dequeue(); // Remove from front void queueFront() const; // retrieve front bool queueIsEmpty() const; // Test for Empty queue int queueLength() const; // Queue Length private: // Node Class class node { public: Type e; // queue element node *next; // pointer to next node }; // end of class node declaration 25

Linked Queue Class typedef node * NodePointer; NodePointer front , rear; // Pointers int count; // length }; #endif // QUEUEL_H #include "QueueL.cpp" 26

Part of Implementation file template <class Type> void QueueL<Type>::enqueue(Type v) { NodePointer pnew = new node; pnew->e = v; pnew->next = NULL; if(queueIsEmpty()) { front = pnew; rear = pnew;} else {rear->next = pnew; rear = pnew;} count++; } 27

Part of Implementation file template <class Type> void QueueL<Type>::dequeue() { NodePointer cursor; if(queueIsEmpty()) cout<<” Queue is empty “<<endl; • else { • cursor = front; • front =front->next; • delete cursor; • count--; • }} 28

Part of Implementation file template <class Type> void QueueL<Type>::queueFront() const { NodePointer cursor; if(queueIsEmpty()) cout << "Queue is Empty" << endl; else {return front->e; } } 29

Exercise • Implement Queue using ADT of a circular linked List

Data Structures and Algorithms