UNIT II Queue. Syllabus Contents Concept of queue as ADT Implementation using linked and sequential...
-
Upload
pauline-wheeler -
Category
Documents
-
view
223 -
download
0
Transcript of UNIT II Queue. Syllabus Contents Concept of queue as ADT Implementation using linked and sequential...
UNIT II
Queue
2
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
3
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?
4
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
5
Model for an abstract data type Inside the ADT are two different parts of the model: data structure and operations (public and private).
6
Definition of a Queue• A queue is a data structure that models/enforces the first-
come 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”.)
7
A Graphic Model of a Queue
rear:All new items are added on this end
front:All items are deleted from this end
8
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
• isEmpty( )– Returns true if the queue has no items
• isFull()– Returns true if the queue is full
• size( )– Returns the number of items in the queue
9
objects: a finite ordered list with zero or more elements.methods: for all queue Queue, item element, max_ queue_ size positive integer Queue createQ(max_queue_size) ::= create an empty queue whose maximum size is max_queue_size Boolean isFullQ(queue, max_queue_size) ::= if(number of elements in queue == max_queue_size) return TRUE else return FALSE
Queue ADT
10
Queue Enqueue(queue, item) ::= if (IsFullQ(queue)) queue_full else insert item at rear of queue and return queue Boolean isEmptyQ(queue) ::= if (queue ==CreateQ(max_queue_size)) return TRUE else return FALSE
Element dequeue(queue) ::= if (IsEmptyQ(queue)) return else remove and return the item at front of queue.
Queue ADT (cont’d)
11
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
12
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(); };
13
Linear Queue Operation
• Explain all operations with example
14
Linear queue array implementationtemplate <class Item>class QUEUE { private: Item *q; int N, front, rear; public: QUEUE(int maxN) { q = new Item[maxN+1]; N = maxN+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 == maxN; } };
If front = rear, then empty; if put would make them equal, then full. Array is 1 larger to allow checks.
15
EMPTY QUEUE[1] [2] [1] [2]
[0] [3] [0] [3]
[5] [4] [5] [4]
front = 0 front = 0 rear = 0 rear = 2
J2
J1
J3
Circular Queue Operations
16
FULL QUEUE FULL QUEUE
[1] [2] [1] [2]
[0] [3][0] [3]
[5] [4] [5] [4]
front =0rear = 4
front =4rear =2
J2 J3
J1 J4
J5 J6 J5
J7
J8 J9
J6
J10
17
Circular Queue Operations
• Explain all operations with example
18
Circular queue array implementationtemplate <class Item>class QUEUE { private: Item *q; int N, front, rear; public: QUEUE(int maxN) { q = new Item[maxN]; N = maxN; front = maxN; 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.
19
Linear Queue using Linked List
• Explain all operations with example
20
Linear queue linked-list implementationtemplate <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; } };
21
Circular Queue using Linked List
• Explain all operations with example
22
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; } };
23
Algorithm Analysis
• enqueue O(1)• dequeue O(1)• size O(1)• isEmpty O(1)• isFull O(1)
24
multiqueue
More than one queue in a single array or Linked listeg. Patient Queue in a Hospital
Multiple Queue handle by multiple arrays eg. Multiple priority Queue for processes
25
26
double ended queueIt is called as dequeue (which is verb meaning to “remove element from Queue)Make use both the ends for insertion & deletion of the elementscan use it as a Queue & Stack
Explain with Example
27
Priority queue
Elements associated with specific ordering Two types
⁻ Ascending priority queue⁻ Descending priority queue
Application⁻ Priority scheduling in OS⁻ N/W communication
28
Model of Priority Queue
29
Implementation of Priority Queuetemplate <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; }
30
Implementation of Priority Queue ( continue…)
void put(Item x int p) { link temp , prev , nPatient = new node(x); nPatient->priority = p; if (head == 0) head = nPatient; else { temp = prev = head; while( temp->priority >= p & temp != 0) { prev = temp; temp = temp->next;} if( temp == head ) { nPatient->next = head ; head = nPatient; } else { prev->next = nPatient; nPatient->next = temp; } } };
31
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)
32
Thank You !