1 Data Structures and Algorithms Queue

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

3 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

4 An Illustration

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

7 Array Implementation of Queue

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

11 Array Implementation of Queue

12 Array Implementation of Queue

13 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

14 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. Array Implementation of Queue

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

16 // 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" Array Implementation of Queue

17 Array Implementation of Queue //Queue.cpp #include 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) }

18 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--;}

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

20 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 3. Linked Queues

21 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

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

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

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

25 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 Linked Queue Class

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

27 Part of Implementation file template void QueueL :: 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++; }

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

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

Implement Queue using ADT of a circular linked List Exercise