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1 Queues – Chapter 3 A queue is a data structure in which all additions are made at one end called the rear of the queue and all deletions are made from the other end called the front of the queue Alternatively, in a queue the element deleted is the one that stayed in the queue the longest. This is also called first-in-first-out (FIFO) Classical example of queues is a queue of people waiting to pay bills.

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2 Queue Concept and Example A queue has a Front and a Rear The insert operation is often called Enqueue The delete operation is often called Dequeue Front of the queue Next person to be served Rear of the queue Next person will join the queue from the rear Queue of people waiting to pay bills

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3 Queue ADT A queue is a data structure in which all additions are made at one end called the rear of the queue and all deletions are made from the other end called the front of the queue Common queue operations: –Enqueue(item) – Add the item to the end of the Q –Dequeue() – Remove & return the item at the front –isEmpty() – Return true if the Q is empty –isFull() – Return true if the Q is full

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4 How do we implement queue ADT? 2 ways to implement a queue –Using an array –Using a linked list

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5 Array Implementation of Queues: Operations Use an array int Q[N] front points to the front element of the queue, i.e., holds the index of the array Q that contains the front of the queue rear points to next slot after the last element in the queue noItems contains the current number of items in the queue. An empty queue is one where noItems = 0 A full queue is one where noItems = N

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6 Array Implementation of Queues We can implement a queue of at most “N” elements with an array int Q[N] and 3 variables: int front, int rear, int noItems as follows Q front = 3 rear = 3 An Empty Queue Front and rear are equal to each other and noItems = 0 in an empty Queue noItems = 0 1 2 3 4 56 0 Q front = 3 8 6 15 rear = 6 A partially- full Queue 2 4 7 Front points to the first element in the Queue Rear points to the next slot after the last element in the Queue 9 noItems = 3 1 2 3 4 56 0 Q front = 3 8 6 15 rear = 3 A Full Queue 2 4 7 Front and rear are equal to each other and noItems = N=7 in a full Queue 9 noItems = 7 1 2 3 4 56 0

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7 Array Implementation of Queues Q front = 3 8 6 15 rear = 1 After 20 is inserted 420 1 2 3 4 56 0 Q front = 3 8 6 15 rear = 6 Initial Queue Q front = 3 8 6 15 rear = 0 4 After 4 is inserted Q front = 4 8 6 rear = 1 420 0 1 2 3 4 5 6 6 8 4 Rear = 1 Front = 4 Conceptual view of the final queue: circular array 1 2 3 4 56 0 1 2 3 4 56 0 1 2 3 4 56 0 noItems = 3 noItems = 4 noItems = 5 After 15 is removed noItems = 4

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8 Queue Using Arrays: Declarations & Operations class Queue { private: static int N = 100; // size of the queue int Q[]; // Array holding the queue elements (ints) int front; // front of the queue int rear; // rear of the queue int noItems; // # of items in the queue public: Queue(); bool isEmpty(); bool isFull(); int Enqueue(int item); int Dequeue(); };

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9 Queue Operations: Constructor, isEmpty, isFull // Constructor Queue(){ Q = new int[N]; front = rear = noOfItems = 0; } //end-Queue // Returns true if the Q is empty bool isEmpty(){ return noOfItems == 0; } //end-isEmpty // Returns true if the Q is full bool isFull(){ return noOfItems == N; } //end-isFull

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10 Queue Operations: Enqueue // Inserts a new item into the queue // Returns 0 on success, -1 on failure int Enqueue(int newItem){ if (isFull()){ println(“Queue is full”); return -1; } //end-if Q[rear] = newItem; // Put the new item at the end rear++; if (rear == N) rear = 0; // Now move rear noItems++; // One more item in the queue return 0; } //end-EnQueue

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11 Queue Operations: Dequeue // Removes and returns the item at the front of the queue // If the queue is empty, returns -1 (an error) int Dequeue(){ int idx = -1; if (isEmpty()){ println(“Queue is empty”); return -1; } //end-if idx = front; // This is where the first item is. front++; if (front == N) front = 0; // Move front noItems--; // One less item in the Queue return Q[idx]; // Return the item } //end-Dequeue

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12 Queue Usage Example main(){ Queue q = new Queue(); if (q.isEmpty()) println(“Queue is empty”); // Q empty now q.Enqueue(49); q.Enqueue(23); println(“Front of the Q is: ” + q.Dequeue()); // prints 49 q.Enqueue(44); q.Enqueue(22); println(“Front of the Q is: ” + q.Dequeue()); // prints 23 println(“Front of the Q is: ” + q.Dequeue()); // prints 44 println(“Front of the Q is: ” + q.Dequeue()); // prints 22. println(“Front of the Q is: ” + q.Dequeue()); // prints -1 if (q.isEmpty()) println(“Queue is empty”); // Q empty now } //end-main

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13 Array Implementation of Queues: Last Word We can implement a queue of at most “N-1” elements with an array int Q[N] and 2 variables: int front, int rear Think about how you would define –An empty queue –A full queue Q front = 3 8 6 15 rear = 2 A Full Queue 2 4 7 Q front = 3 rear = 3 An Empty Queue Front and rear are equal to each other in an empty queue There is one empty space between rear and front in a full queue

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Linked-List implementation of Queues Initial Queue Enqueue(3) 15629 Front Rear Dequeue() 15 6 29 Front 3 Rear After 3 is enqueued Dequeue() 15 6 2 Front 3 Rear After 9 is dequeued 15 6 Front 3 Rear After 2 is dequeued

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15 Queue using Linked List: Declarations & Operations struct QueueNode { int item; QueueNode next; QueueNode(int e){item=e; next=null;} }; class Queue{ private: QueueNode front; // Ptr to the front of the Q QueueNode rear; // Ptr to the rear of the Q public: Queue(); bool isEmpty(); void Enqueue(int item); int Dequeue(); };

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16 Queue Operations: Constructor, isEmpty // Constructor Queue(){ front = rear = null; } //end-Queue // Returns true if the Q is empty bool isEmpty(){ return front == null; } //end-isEmpty

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17 Queue Operations: Enqueue // Inserts a new item into the queue void Enqueue(int newItem){ // Allocate a QueueNode for the item QueueNode node = new QueueNode(newItem); if (front == NULL){ front = rear = node; } else { rear.next = node; rear = node; } //end-else } //end-EnQueue

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18 Queue Operations: Dequeue // Removes and returns the item at the front of the queue // If the queue is empty, returns -1 (an error) int Dequeue(){ if (isEmpty()){ println(“Queue is empty”); return -1; } //end-if QueueNode tmp = front; // Keep a ptr to the front node front = front.next; // Remove the front node if (front == null) rear = NULL; // Empty Q? // Return the item return tmp.item; } //end-Dequeue

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19 Queue Usage Example main(){ Queue q = new Queue(); if (q.isEmpty()) println(“Queue is empty”); // Q empty now q.Enqueue(49); q.Enqueue(23); println(“Front of the Q is: ” + q.Dequeue()); // prints 49 q.Enqueue(44); q.Enqueue(22); println(“Front of the Q is: ” + q.Dequeue()); // prints 23 println(“Front of the Q is: ” + q.Dequeue()); // prints 44 println(“Front of the Q is: ” + q.Dequeue()); // prints 22. println(“Front of the Q is: ” + q.Dequeue()); // prints -1 if (q.isEmpty()) println(“Queue is empty”); // Q empty now } //end-main

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20 Applications of Queues File servers: Users needing access to their files on a shared file server machine are given access on a FIFO basis Printer Queue: Jobs submitted to a printer are printed in order of arrival Phone calls made to customer service hotlines are usually placed in a queue Expected wait-time of real-life queues such as customers on phone lines may be too hard to solve analytically use queue for simulating real-life queues

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