DS Lec 4 Queue

8/20/2019 DS Lec 4 Queue

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Linear Data Structures

QueueLecture # 4



Problem to be SolvedIt is so often necessary to wait one’s turn before having access to something.We may want to simulate a real life situation of a waiting line, like

A line of eo le waiting to urchase tickets, where the first erson in line is thefirst erson serve!.

With in a com uter system there may be lines of tasks

Waiting for the rinter

Waiting for access to !isk storage

"r in a time sharing system for use of the $%.&he !ata structures use! to solve this ty e of roblems is calle!Queue



The QueueA 'ueue is an or!ere! collection of items from which items maybe !elete! at one en! (front of the queue) an! into which itemsmay be inserte! at the other en! (rear of the queue).

Insertion(enqueue) occurs at the rear location of a 'ueue.(eletion (dequeue) occurs at the front location of a 'ueue.Queues are first)in)first)out(FIFO)or first)come)first serve!(FCFS) lists.&he general 'ueue mo!el is that elements are inserte! at oneen! an! !elete! from other en!.

Queue QDequeue () Enqueue (x)



The Queue

Assume an initially em ty 'ueue an! followingse'uence of insertions an! !eletions*



The Queue



Implementation of QueueThe Physical Model:+ Im lement a 'ueue by using an array.+ We must kee track of both the front an! the rear of the 'ueue.+ "ne metho! woul! be to kee the front of the 'ueue always in the first

location of the array.+ An entry coul! be a en!e! to the 'ueue sim ly by increasing the counter

showing the rear.10 12 7

(elete an entry ointer by the front, but after the fist entry was serve!,all the remainingentries woul! nee! to be move! one osition u the 'ueue to fill in the vacancy, which woul! be an e ensive rocess.

12 7



Implementation of QueueLinear I !le entation+ In this im lementation an array with two in!ices will be use!.+ "ne in!e in!icates front of 'ueue an! secon! in!e in!icates rear of

'ueue.+ %sing front an! rear in!ices there is no nee! of moving any entries.+ &o en'ueue an entry to the 'ueue, we sim ly increase the rear by one an!

ut the entry in that osition

12 7

&o !e'ueue an entry, we take it from the osition at the front an! then increase the front by one.

7 8

12 7 8



Queue Operations

-our common o erations which are associate! with a 'ueue are*

initiali e* Initiali es the 'ueue to be em ty.en'ueue* Inserts an element at the rear of the'ueue.!e'ueue* /emoves the front element from the'ueue an! returns its value.is0m ty* /eturns 1true2 if 'ueue contains noelements3 otherwise returns 1false2.



QUEUE (queue h!#include<iostream.h> template<class Type> class Queue{

private:

Type *data; 5 &his ointer oints to the array which behaves as 'ueue, the s ace for this array isallocate! !ynamically 5int front; 5 &his variable is use! to in!icate front of the 'ueue 5int rear; 5 &his variable is use! to in!icate rear of the 'ueue5const int size; 5 &his variable is use! to in!icate 'ueue si e5

public:

Queue int!; 5&his constructor creates a 'ueue5"Queue !; 5&his !estructor frees the !ynamically allocate! s ace 5void en ueue Type!; 5 inserts an element, if 'ueue is not full 5

Type de ueue !; 5 !eletes an element, if 'ueue is not em ty5 bool is$mpty !; 5 returns &/%0 if the 'ueue is em ty5

%; 5en! of class5



QUEUE (queue h!5&his constructor creates a 'ueue5

te !late"class Ty!e#Queue"Ty!e#::Queue(int qsi$e):front(%&)'rear(%&)'si$e(qsi$e)

data ne* Ty!e+si$e,-

5&his !estructor frees the !ynamically allocate! s ace 5te !late"class Ty!e#

Queue"Ty!e#::/Queue()

delete+, data-



QUEUE (queue h!01 inserts an ele ent' if queue is not full 10te !late"class Ty!e#2oid Queue"Ty!e#::enqueue(Ty!e ite )

if(rear si$e%&)cout""3Queue Full3""endl-

else

if(front %&)front 4-data+55rear, ite -



QUEUE (queue h!5 !eletes an element, if 'ueue is not em ty5

te !late"class Ty!e#Ty!e Queue"Ty!e#::dequeue()

Ty!e te !-if(front %&)

cout""3Queue 6 !ty3""endl- return 78LL- else

te ! data+front,-if (front rear)

front %&- rear %&-elsefront55-

return te !-



QUEUE (driver cpp!;include"iostrea .h#;include3queue.h32oid ain()

int s-

cout""36nter the queue si$e: 3-cin##s-Queue"int# qint(s)-qint.enqueue(<)-qint.enqueue(=)-qint.enqueue(>)-

cout""qint.dequeue()""endl-cout""qint.dequeue()""endl-cout""qint.dequeue()""endl-

OutputEnter the queue size: 3

5

8

2



Implementations of QueuesCircular ?rrays+ &hinking of the array as a circle rather than a straight line.+ As entries are a!!e! an! remove! from the 'ueue, the hea!

will continually chase the tail aroun! the array.+ &o im lement a circular array as an or!inary array, we think of

the ositions aroun! the circle as numbere! from 6 tosize)7, where size is the total number of entries in the array.

+ @hen *e increase an indeA !astsize

%&' *e start o2eraBain at 4.

7 8 9 12 7 8 9



Implementations of Queues

we can increase an in!e rear by 7 in a circulararray by writingif8rear 9: si e)7;

rear : 63else

rear<<3



"ne ossible 'ueue im lementation is an array.0lements are a!!e! to the en! of the 'ueue, but&hey may be remove! from its beginning, thereby releasing array

cells.&hese cells shoul! not be waste!. &herefore,&hey are utili e! to en'ueue new elements,Whereby the en! of the 'ueue may occur at the beginning of the

array.&his situation is icture! in following figure

84

2

151110

6

l st !irst

"he queue is !ull i! the !irst ele#enti##e$i tel% pre&e$es in the

&'unter&l'& ise $ire&ti'n the l st ele#ent*




=owever, because a circular array is im lemente! with a 1normal2array,

&hequeue is full if either theFirst ele ent is in the first cell and the last ele ent is in the last cell .

4 2 15 11 10 6 8

!irst l st

> or if the first ele ent is riBht after the last

10 6 8 4 2 15 11

l st !irst




?imilarly,enqueue() an! dequeue() have to consi!er the ossibilityof wra ing aroun! the array when a!!ing or removing elements.0.g. enqueue() can be viewe! as o erating on a circular array, butin reality, it is o erating on a one !imensional array. &herefore,If the last element is in the last cell an! if any cells are available atthe beginning of the array,a new element is lace! there.

2 4 8

!irst l stenqueue(6)

6 2 4 8

!irstl st




If the last element is in any other osition, then the new element isut after the last, s ace ermitting.

2 4 8

!irst l stenqueue(6)

2 4 8 6

!irst l st

&hese two situations must be !istinguishe! when im lementing a'ueue viewe! as a circular array.

2

4

8l st

!irst2

4

8 6

l st

!irst

enqueue(6)




"I#"U $# QUEUE (queue h!#include<iostream.h> template<class Type> class &ircularQueue{

private:

Type *data; 5 &his ointer oints to the array which behaves as 'ueue, the s ace for this array is allocate!!ynamically 5

int front; 5 &his variable is use! to in!icate front of the 'ueue 5int rear; 5 &his variable is use! to in!icate rear of the 'ueue5const int size; 5 &his variable is use! to in!icate 'ueue si e5

public:

&ircularQueue unsi'ned int!; 5&his constructor creates a 'ueue5"&ircularQueue !; 5&his !estructor frees the !ynamically allocate! s ace 5void en ueue Type!; 5 inserts an element, if 'ueue is not full 5Type de ueue !; 5 !eletes an element, if 'ueue is not em ty5

bool is$mpty !; 5 returns &/%0 if the 'ueue is em ty5%; 5en! of class5



"I#"U $# QUEUE (queue h!5&his constructor creates a 'ueue5

te !late"class Ty!e#CircularQueue"Ty!e#:: CircularQueue(unsiBned int qsi$e):front(%&)'rear(%&)

si$e qsi$e-data ne* Ty!e+si$e,-

5&his !estructor frees the !ynamically allocate! s ace 5te !late"class Ty!e#CircularQueue "Ty!e#::/ CircularQueue()

delete+, data-



01 inserts an ele ent' if queue is not full 10te !late"class Ty!e#2oid CircularQueue"Ty!e#::enqueue(Ty!e ite )

if((rear si$e%& front 4)DD(rear5& front))cout""3Queue Full3""endl-

else

if(front %&)front 4-

if(rear si$e%&)rear 4-

elserear55-

data+rear, ite -

"I#"U $# QUEUE (queue h!



01 deletes an ele ent' if queue is not e !ty10te !late"class Ty!e#Ty!e CircularQueue"Ty!e#::dequeue()

Ty!e te !-

if(front %&)cout""3Queue 6 !ty3""endl- return 78LL-

elsete ! data+front,-

if (front rear)

front rear %&-elseif(front si$e%&)

front 4- elsefront55-

return te !-




01 returns T986 if the queue is e !ty10te !late"class Ty!e#

ool CircularQueue"Ty!e#::is6 !ty()

if(rear %&)return true-

else

return false-




"I#"U $# QUEUE (driver cpp!;include"iostrea .h#;include3queue.h32oid ain()

int s-cout""36nter the queue si$e: 3-

cin##s-CircularQueue"int# qint(s)-qint.enqueue(<)-qint.enqueue(=)-qint.enqueue(>)-cout""qint.dequeue()""endl-

qint.enqueue(E)-cout""qint.dequeue()""endl-cout""qint.dequeue()""endl-cout""qint.dequeue()""endl-

Output

Enter the queue size: 3

5

8

2

4

DS Lec 4 Queue

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