notes data structure - queue

Course Objectives

At the end of the lesson students are expected to be able

to:

• Understand queue concepts and applications.

• Understand queue structure and operations that can be

done on queue.

• Understand and know how to implement queue using

array and linked list : linear array, circular array, linear

link list and circular list.

1.0 Introduction to Queue

Introduction to Queue

• New items enter at the back, or rear, of the

queue

• Items leave from the front of the queue

• First-in, first-out (FIFO) property

– The first item inserted into a queue is the

first item to leave

– Middle elements are logically inaccessible

Introduction to Queue

• Important in simulation & analyzing the

behavior of complex systems

Queue Applications

• Real-World Applications

– Cashier lines in any store

– Check out at a bookstore

– Bank / ATM

– Call an airline

Queue Applications

• Computer Science Applications

– Print lines of a document

– Printer sharing between computers

– Recognizing palindromes

– Shared resource usage (CPU, memory

access, …)

Queue Applications

• Simulation

– A study to see how to reduce the wait

involved in an application

Queue implementation

Add/

Enqueue

Remove/

Dequeue

Back/RearFront/Head

A B C

Basic Structure of a Queue:

•Data structure that hold the queue

•head

•rear

Queue implementation

Add/

Enqueue

RearHead

A B C D

Insert D into Queue (enQueue) : D is inserted at rear

RearHead

B C D

Remove/

Dequeue A

Delete from Queue (deQueue) : A is removed

Queue operations

• Queue operations

– Create an empty queue

– Destroy a queue

– Determine whether a queue is full

– Add a new item to the queue (enQueue)

– Determine whether a queue is empty

– Remove the item that was added earliest(deQueue)

– Retrieve at Front(getFront)

– Retrieve at Back the item that was added earliest(getRear)

Queue Implementation

Implementation:

– Array-based (Linear or Circular)

– Pointer-based : Link list (Linear or

Circular)

2.0 Queue Implementation Using

Array(Linear)

Queue Implementation Using

Array(Linear)

• Number of elements in Queue are fixed

during declaration.

• Need isFull() operation to determine

whether a queue is full or not.

Queue Implementation Using Array(Linear)

• Queue structure need at least 3 elements:

1) Element to store items in Queue

2) Element to store index at head

3) Element to store index at rear

Create Queue Operation

• Declare

– front & back are indexes in the array

– Initial condition: front =0 & back = -1

– Size of an array in queue

0 1 2 3 Max size

Queue

0

front

-1

back

Create Queue operation

Example Code 1

#include <iostream>

using namespace std;

#define max 5

int front = 0, back = -1;

char item[max], newitem;Create Queue

0 1 2 3 4

item

0

front

-1

back

Continue…

Front refer to index 0

enQueue operationvoid enQueue(){

cout<<"\n\t#################\n";cout<<"\n\t1. enQueue\n";//check queue is full if(back == max - 1){

cout<<"\n\tQueue Is Full, Cannot Add Item In Queue\n";}else{

cout<<"\n\t\tEnter Item:";cin>>newitem;back++;item[back]=newitem;cout<<endl;

}}

enQueue

Continue…

0

front

0

back

0 1 2 3 4

A

item

back = -1+1back = 0

From back/rearitem[back] = newitem

back++


enQueue operation

Continue…

0

front

1

back

0 1 2 3 4

A B

item

back = 0 +1back = 1


0

front

2

back

0 1 2 3 4

A B C

item

back = 1 +1back = 2


back++

back++



enQueue operation

Continue…

0

front

3

back

0 1 2 3 4

A B C D

item

back = 2 +1back = 3


0

front

4

back

0 1 2 3 4

A B C D E

item

back = 3 +1back = 4


back++

back++



deQueue operationvoid deQueue(){

cout<<"\n\t#################\n";cout<<"\n\t2.deQueue\n";if(back < front){

cout<<"\n\tThere is no data to remove from queue\n";}else{

char itemdeleted;itemdeleted=item[front];item[front] = NULL;cout<<"\n\tItem Remove From Queue:"<<itemdeleted<<endl;front++;

}cout<<endl;

}

deQueue

Continue…

0

front

4

back

0 1 2 3 4

A B C D E

item

back = 3 + 1back = 4itemdeleted = item[front]

front = 0

From front/headitem[front] = NULL


1

front

4

back

0 1 2 3 4

NULL B C D E

item

back = 3 + 1back = 4

front = 0 + 1front = 1

front++

Continue…

1

front

4

back

0 1 2 3 4

NULL B C D E

item


itemdeleted = item[front]front = 1


2

front

4

back

0 1 2 3 4

NULL NULL C D E

item



front++




deQueue operation

Continue…

2

front

4

back

0 1 2 3 4

NULL NULL C D E

item




3

front

4

back

0 1 2 3 4

NULL NULL NULL D E

item



front++



deQueue operation

Continue…

3

front

4

back

0 1 2 3 4

NULL NULL NULL D E

item




4

front

4

back

0 1 2 3 4

NULL NULL NULL NULL E

item



front++



deQueue operation

Continue…

4

front

4

back

0 1 2 3 4

NULL NULL NULL NULL E

item




5

front

4

back

0 1 2 3 4

NULL NULL NULL NULL NULL

item



front++


deQueue operation

Retrieve at front(getFront) operation

void getFront(){cout<<"\n\t#################\n";cout<<"\n\t3.getFront\n";if(back < front){

cout<<"\n\tThere is no data to at front\n";}else{

cout<<"\n\tItem At Front:"<<item[front]<<endl;

}}

Continue…

Retrieve at back(getRear) operation

void getRear(){cout<<"\n\t#################\n";cout<<"\n\t4.getRear\n";if(back < front){

cout<<"\n\tThere is no data to at rear\n";}else{

cout<<"\n\tItem At Rear:"<<item[back]<<endl;

}}

Continue…

destroyQueue operation

void destroyQueue(){

delete [] item;

}

Continue…

displayQueue operation

void displayQueue(){cout<<"\n\tDisplay Item In Queue\n";if(back < front){

cout<<"\n\tThere is no data in queue to be displayed\n";}else{

cout<<"\t";for(int i=0; i < max; i++ ){

cout<<"\t"<<item[i];}cout<<endl;

}

}

Continue…

Queue Implementation Using Array(Linear)int main(){int selection;menu:

cout<<"\nPlease Choose Your Selection\n";cout<<"\n1\tenQueue\n";cout<<"\n2\tdeQueue\n";cout<<"\n3\tGetFront\n";cout<<"\n4\tGetRear\n";cout<<"\n5\tDestroyQueue\n";cout<<"\n6\tDisplay\n";cout<<"\n\tSelection is:";cin>>selection;

Continue…


Continue…

switch(selection){case 1: enQueue();

displayQueue();goto menu;break;

case 2: deQueue();displayQueue();goto menu;break;

case 3: getFront();displayQueue();goto menu;break;


case 4: getRear();displayQueue();goto menu;break;

case 5: destroyQueue();displayQueue();goto menu;break;

case 6: displayQueue();goto menu;break;

default:cout<<"\n\tWrong Selection\n";}

return 0;}


• Problem: Rightward-Drifting:

• After a sequence of additions & removals,

items will drift towards the end of the array

• enQueue operation cannot be performed

on the queue below, since back = max – 1.

5

front

4

back

0 1 2 3 4

NULL NULL NULL NULL NULL

item



front++


• Rightward drifting solutions

– Shift array elements after each deletion

• Shifting dominates the cost of the

implementation


– Use a circular array: When Front or Back

reach the end of the array, wrap them around

to the beginning of the array

• Problem:

– Front & Back can't be used to

distinguish between queue-full & queue-

empty conditions


• Solution:

– Use a counter

– Count == 0 means empty queue

– Count == MAX_QUEUE means full

queue


Array(Circular)


Array(Circular)

• Number of elements in Queue are fixed

during declaration.

• Need isFull() operation to determine

whether a queue is full or not.


Array(Circular)

• Queue structure need at least 3 elements:

1) Element to store items in Queue

2) Element to store index at head

3) Element to store index at rear

4) Element to store index in counter

Create Queue Operation

• Declare

– front & back are indexes in the array

– count to store index

– Initial condition: front =0 , back = -1, count = 0

– Size of an array in queue

Queue Implementation Using Array(Circular)

– The Wrap-around effect is obtained by using

modulo arithmetic (%-operator)front = 0

back = -1

0

1

2

34

5

6

7

count = 0


– enQueue

• Increment back, using modulo arithmetic

• Insert item

• Increment count

– deQueue

• Increment front using modulo arithmetic

• Decrement count

– Disadvantage

• Overhead of maintaining a counter or flag


front = 0

back = -1

0

1

2

34

5

6

7

count = 0

Example Code 2:

#include <iostream>


#define max 8

char queue[max], newitem;

int front = 0, back = -1, count = 0;

Continue…

queue


void enQueue(){

cout<<"\n\t#### enQueue Circular ####\n";

if(count == max){

cout<<"\n\tQueue Circular Is Full!!!\n";

}else{

cout<<"\n\tfront:"<<front<<"\t"<<"back:"<<back<<"\tcount:"<<count<<“\tmax:”<<max<<"\n";

cout<<"\n\tEnter Item:";

cin>>newitem;

back = (back + 1)% max;

queue[back] = newitem;

count++;

} }

Continue…

front = 0

back = 00

1

2

34

5

6

7

count = 1

A

back = (-1 + 1) % 8back = 0 % 8back = 0queue[0] = A

count = 0 + 1count = 1

0

0

0

8√ 0

enQueue Implementation Using Array(Circular)

Continue…

front = 0

back = 1

0

1

2

34

5

6

7

count = 2

A

back = (0 + 1) % 8back = 1 % 8back = 1queue[1] = B


0

0

1

8√ 1

B

From previous slide: front = 0, back = 0, count = 1 queue


Continue…

front = 0

back = 2

0

1

2

34

5

6

7

count = 3

A

back = (1 + 1) % 8back = 2 % 8back = 2queue[2] = C


0

0

2

8√ 2

B

C

From previous slide: front = 0, back = 1, count = 2

queue


Continue…

front = 0

back = 3

0

1

2

34

5

6

7

count = 4

A

back = (2 + 1) % 8back = 3 % 8back = 3queue[3] = D


0

0

3

8√ 3

B

C

D

From previous slide: front = 0, back = 2, count = 3queue


Continue…

front = 0

back = 4

0

1

2

34

5

6

7

count = 5

A

back = (3 + 1) % 8back = 4 % 8back = 4queue[4] = E


0

0

4

8√ 4

B

C

DE



Continue…

front = 0

back = 5

0

1

2

34

5

6

7

count = 6

A

back = (4 + 1) % 8back = 5 % 8back = 5queue[5] = F


0

0

5

8√ 5

B

C

DE


F


Continue…

front = 0

back = 6 0

1

2

34

5

6

7

count = 7

A

back = (5 + 1) % 8back = 6 % 8back = 6queue[6] = G


0

0

6

8√ 6

B

C

DE


F

G


Continue…

front = 0back = 7

0

1

2

34

5

6

7

count = 8

A

back = (6 + 1) % 8back = 7 % 8back = 7queue[7] = H


0

0

7

8√ 7

B

C

DE


F

G

H

deQueue Implementation Using Array(Circular)

void deQueue(){

cout<<"\n\t#### deQueue Circular ####\n";

if(count == 0){

cout<<"\n\tQueue Circular Is Empty, No Data To Be Deleted!!!\n";

}else{

queue[front] = NULL;

front=(front + 1) % max;

count--;

}

}

Continue…

front = 1

back = 7

0

1

2

34

5

6

7

count = 7

queue[0] = NULLfront = (0 + 1) % 8front = 1 % 8front = 1

count = 8 - 1count = 7

0

0

1

8√ 1

B

C

DE

queue

F

G

H

deQueue Implementation Using Array(Circular)

Continue…

From previous slide: front = 1, back = 7 , count = 7

front = 2

back = 7

0

1

2

34

5

6

7

count = 6

queue[1] = NULLfront = (1 + 1) % 8front = 2% 8front = 2

count = 7 - 1count = 6

0

0

2

8√ 2

C

DE

queue

F

G

H


void displayQueue(){

cout<<"\n\t#### Display Queue Circular ####\n";

cout<<"\n\tfront:"<<front<<"\t"<<"back:"<<back<<"\tcount:"<<count<<“\tmax:”<<max<<"\n";

if(count == 0){

cout<<"\n\tQueue Circular Is Empty, No Data To Be Display\n";

}else{

cout<<"\n\tItem In Queue Circular\n";

for(int i = 0; i < max; i++){

cout<<"\t"<<queue[i];

}

}

}

Continue…


int main(){

int selection;

menu:

cout<<"\n\nPlease Choose Your Selection\n";

cout<<"\n1\tenQueue Circular\n";

cout<<"\n2\tdeQueue Circular\n";

cout<<"\n3\tDisplay Queue\n";

cout<<"\n\tSelection is:";

cin>>selection;

Continue…


switch(selection){

case 1: enQueue();

displayQueue();

goto menu;

break;

case 2: deQueue();

displayQueue();

goto menu;

break;

case 3: displayQueue();

goto menu;

break;

Continue…


default:cout<<"\n\tWrong Selection\n";

}

return 0;

}


Linked List(Linear)

Queue Implementation Using Linked List(Linear)

Pointer-Based Implementation

• More straightforward than array-based

• Need Two external pointer (Front & Back) which front to

trace deQueue operation and back to trace deQueue

operation.

Create Queue Implementation Using Linked

List(Linear)Example Code 1:

#include <iostream>


struct nodeQueue{

char name;

int age;

nodeQueue *next;

};

name age next

Compiler get the initial illustrated structure of node

Continue…

Create Queue Implementation Using Linked

List(Linear)

nodeQueue *back_ptr = NULL;

nodeQueue *front_ptr=NULL;

Continue…

NULL

NULL

back_ptr

front_ptr

enQueue Implementation Using Linked

List(Linear)void enQueue(){

//create new node

nodeQueue *newnode;

newnode = new nodeQueue;

cout<<"\n\t####enQueue####\n";

//assign data field for name and age

cout<<"Enter Name:";

cin>>newnode->name;

cout<<"Enter Age:";

cin>>newnode->age;

newnode->next = NULL;

Continue…

Ali 29 NULL

newnode

0110

0110


List(Linear)

//insert newnode into queue

//check whether queue is empty

if((front_ptr == NULL) && (back_ptr == NULL)){

front_ptr = newnode;

back_ptr = newnode;

}else{

back_ptr->next = newnode;

back_ptr = newnode;

}

Continue…

0110

front_ptr

0110

newnode

Ali 29 NULL

age

0110

name next

Insertion to an empty queue

0110

back_ptr


List(Linear)

Continue…

Insertion to a non empty queue

back_ptr->next = newnode;

back_ptr=newnode;

Tina 30 NULL

0111

name age nextnewnode

0111

0110

back_ptr

0110

front_ptr

Ali 29 NULL

age

0110

name next


List(Linear)

Continue…

Insertion to a non empty queue

Tina 30 NULL

0111

name age next

0111

back_ptr

0110

front_ptr

Ali 29 0111

age

0110

name next

Continue…

deQueue Implementation Using Linked List(Linear)

void deQueue(){

cout<<"\n\t####deQueue####\n";

//check whether queue is empty


cout<<"\n\tQueue Is Empty!!!\n";

}else{

nodeQueue *temp;

temp = front_ptr;

if(front_ptr->next == NULL){

front_ptr = NULL;

back_ptr = NULL;

delete temp;

}else{

front_ptr = front_ptr->next;

delete temp; } } }

Continue…

If the queue contains one item only

Continue…

deQueue Implementation Using Linked List(Linear)

If the queue contains one item only to be deleted

nodeQueue *temp;

temp = front_ptr;

0110

front_ptr

Ali 29 NULL

age

0110

name next

0110

back_ptr

0110

tempif(front_ptr->next == NULL){

front_ptr = NULL;

back_ptr = NULL;

delete temp;

}else{

…}

NULL

front_ptr

NULL

back_ptr

deQueue Implementation Using Linked

List(Linear)

Continue…

If the queue contains more than one item

nodeQueue *temp;

temp = front_ptr;

Tina 30 NULL

0111

name age next

0111

back_ptr0110

front_ptr

Ali 29 0111

age

0110

name next

0110

temp

Continue…

…}else{

front_ptr = front_ptr->next;

delete temp; }

Tina 30 NULL

0111

name age next

0111

back_ptr0111

front_ptr

Ali 29 0111

age

0110

name next

0110

temp

Tina 30 NULL

0111

name age next

0111

back_ptr0111

front_ptr

Continue…

displayQueue Implementation Using Linked List(Linear)

void displayQueue(){

cout<<"\n\t####Display Queue####\n";


cout<<"\n\tQueue Is Empty!!!\n";

cout<<"\n\tfront_ptr :"<<front_ptr<<"\tback_ptr :"<<back_ptr<<endl;

}else{

nodeQueue *cursor;

cursor=front_ptr;

cout<<"\n\tThe Elements In Queue Are\n";

cout<<"\n\tfront_ptr :"<<front_ptr<<"\tback_ptr :"<<back_ptr<<endl;

int node=1;

while(cursor){

cout<<"\n\tNode :"<<node++<<"\tName :"<<cursor->name<<"\tAge :"<<cursor-

>age<<"\tcursor-next:"<<cursor->next<<endl;

cursor=cursor->next; } }

Continue…

Queue Implementation Using Linked List(Linear)

int main()

{

int selection;

menu:

cout<<"\n\nMenu Selection\n";

cout<<"\n1\tenQueue\n";

cout<<"\n2\tdeQueue\n";

cout<<"\n3\tDisplay Queue\n";

cout<<"\n\tSelection is:";

cin>>selection;

Continue…

Queue Implementation Using Linked List(Linear)switch(selection){

case 1: enQueue();

displayQueue();

goto menu;

break;

case 2: deQueue();

displayQueue();

goto menu;

break;

case 3: displayQueue();

goto menu;

break;

default:cout<<"\n\tWrong Selection\n"; }

return 0;

}

notes data structure - queue

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