UNIT II Queue Syllabus Contents Concept of queue

UNIT II Queue

Syllabus Contents • Concept of queue as ADT • Implementation using linked and sequential organization. – linear – circular queue • Concept – multiqueue – double ended queue – priority queue • Applications of queue 2

Abstract Data Type • What does ‘abstract’ mean? • From Latin: to ‘pull out’—the essentials – To defer or hide the details – Abstraction emphasizes essentials and defers the details, making engineering artifacts easier to use • I don’t need a mechanic’s understanding of what’s under a car’s hood in order to drive it – What’s the car’s interface? – What’s the implementation? 3

ADT = properties + operations • An ADT describes a set of objects sharing the same properties and behaviors – The properties of an ADT are its data (representing the internal state of each object • double d; -- bits representing exponent & mantissa are its data or state – The behaviors of an ADT are its operations or functions (operations on each instance) • sqrt(d) / 2; //operators & functions are its behaviors • Thus, an ADT couples its data and operations – OOP emphasizes data abstraction 4

Model for an abstract data type Inside the ADT are two different parts of the model: data structure and operations (public and private). 5

Definition of a Queue • A queue is a data structure that models/enforces the firstcome first-serve order, or equivalently the first-in first-out (FIFO) order. • That is, the element that is inserted first into the queue will be the element that will deleted first, and the element that is inserted last is deleted last. • A waiting line is a good real-life example of a queue. (In fact, the British word for “line” is “queue”. ) 6

A Graphic Model of a Queue rear: All new items are added on this end front: All items are deleted from this end 7

Operations on Queues • Insert(item): (also called enqueue) – It adds a new item to the rear of the queue • Remove( ): (also called delete or dequeue) – It deletes the front item of the queue, and returns to the caller. If the queue is already empty, this operation returns NULL • getfront( ): – Returns the value in the front element of the queue • getrear( ): – Returns the value in the rear element of the queue • is. Empty( ) – Returns true if the queue has no items • is. Full() – Returns true if the queue is full • size( ) – Returns the number of items in the queue 8

Queue ADT objects: a finite ordered list with zero or more elements. methods: for all queue Queue, item element, max_ queue_ size positive integer Queue create. Q(max_queue_size) : : = create an empty queue whose maximum size is max_queue_size Boolean is. Full. Q(queue, max_queue_size) : : = if(number of elements in queue == max_queue_size) return TRUE else return FALSE 9

Queue ADT (cont’d) Queue Enqueue(queue, item) : : = if (Is. Full. Q(queue)) queue_full else insert item at rear of queue and return queue Boolean is. Empty. Q(queue) : : = if (queue ==Create. Q(max_queue_size)) return TRUE else return FALSE Element dequeue(queue) : : = if (Is. Empty. Q(queue)) return else remove and return the item at front of queue. 10

Array-based Queue Implementation • As with the array-based stack implementation, the array is of fixed size – A queue of maximum N elements • Slightly more complicated – Need to maintain track of both front and rear Implementation 1 Implementation 2 11

FIFO queue ADT interface template <class Item> class QUEUE { private: // Implementation-dependent code public: QUEUE(int); int empty(); void put(Item); Item get(); int full(); }; 12

Linear Queue Operation • Explain all operations with example 13

Linear queue array implementation template <class Item> class QUEUE { private: Item *q; int N, front, rear; public: QUEUE(int max. N) { q = new Item[max. N+1]; N = max. N+1; front = 0; rear = 0; } int empty() const { return front % N == rear; } void put(Item item) { q[rear++] = item; rear = rear % N; } Item get() { front = front % N; return q[front++]; } int full() const { return rear == max. N; } }; If front = rear, then empty; if put would make them equal, then full. Array is 1 larger to allow checks. 14

Circular Queue Operations EMPTY QUEUE [2] [1] [2] J 2 [0] [3] [5] front = 0 rear = 0 [4] J 3 [0]J 1 [3] [5] [4] front = 0 rear = 2 15

FULL QUEUE [1] [2] J 2 [0] J 3 J 1 J 4 J 6 [4] front =0 rear = 4 [2] J 9 [3][0]J 7 J 10 J 6 J 5 [5] [1] J 8 [3] J 5 [5] [4] front =4 rear =2 16

Circular Queue Operations • Explain all operations with example 17

Circular queue array implementation template <class Item> class QUEUE { private: Item *q; int N, front, rear; public: QUEUE(int max. N) { q = new Item[max. N]; N = max. N; front = max. N; rear = 0; } int empty() const { return front % N == rear; } void put(Item item) { q[rear++] = item; rear = rear % N; } Item get() { front = front % N; return q[front++]; } int full() const { return (rear+1)%N == front; } }; If front = rear, then empty; if put would make them equal, then full. Array is 1 larger to allow checks. 18

Linear Queue using Linked List • Explain all operations with example 19

Linear queue linked-list implementation template <class Item> class QUEUE { private: struct node { Item item; node* next; node(Item x) { item = x; next = 0; } }; typedef node *link; link head, tail; public: QUEUE(int) { head = 0; } int empty() const { return head == 0; } void put(Item x) { link t = tail; tail = new node(x); if (head == 0) head = tail; else t->next = tail; } Item get() { Item v = head->item; link t = head->next; delete head; head = t; return v; } }; 20

Circular Queue using Linked List • Explain all operations with example 21

Circular queue linked-list implementation template <class Item> class QUEUE { private: struct node { Item item; node* next; node(Item x) { item = x; next = 0; } }; typedef node *link; link head, tail; public: QUEUE(int) { head = 0; } int empty() const { return head == 0; } void put(Item x) { link t = tail; tail = new node(x); if (head == 0) { head = tail; head->next=head; } else { t->next = tail; tail->next = head; } } Item get() { Item v = head->item; link t = head->next; delete head; head = t; tail->next = head; return v; } }; 22

Algorithm Analysis • • • enqueue. O(1) dequeue O(1) size O(1) is. Empty O(1) is. Full O(1) 23

multiqueue ØMore than one queue in a single array or Linked list eg. Patient Queue in a Hospital ØMultiple Queue handle by multiple arrays eg. Multiple priority Queue for processes 24

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double ended queue ØIt is called as dequeue (which is verb meaning to “remove element from Queue) ØMake use both the ends for insertion & deletion of the elements Øcan use it as a Queue & Stack Ø Explain with Example 26

Priority queue Ø Elements associated with specific ordering Ø Two types ⁻ Ascending priority queue ⁻ Descending priority queue Ø Application ⁻ Priority scheduling in OS ⁻ N/W communication 27

Model of Priority Queue 28

Implementation of Priority Queue template <class Item> class QUEUE { private: struct node { Item item; int priority; node* next; node(Item x) { item = x; next = 0; } }; typedef node *link; link head; public: QUEUE(int) { head = 0; } int empty() const { return head == 0; } item get() { item patient = head->item ; head = head->next; return patient; } 29

Implementation of Priority Queue ( continue…) void put(Item x int p) { link temp , prev , n. Patient = new node(x); n. Patient->priority = p; if (head == 0) head = n. Patient; else { temp = prev = head; while( temp->priority >= p & temp != 0) { prev = temp; temp = temp->next; } if( temp == head ) { n. Patient->next = head ; head = n. Patient; } else { prev->next = n. Patient; n. Patient->next = temp; } } }; 30

Queues Applications • An electronic mailbox is a queue – The ordering is chronological (by arrival time) • A waiting line in a store, at a service counter, on a one-lane road, Patient in a Hospital • Applications related to Computer Science – Threads – Job scheduling (e. g. Round-Robin algorithm for CPU allocation) 31

Thank You ! 32