NGC Training Programme

8/10/2019 NGC Training Programme

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QUEUE


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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.


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1.0 Introduction to Queue


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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


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Introduction to Queue

Important in simulation & analyzing the

behavior of complex systems


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Queue Applications

Real-World Applications

Cashier lines in any store

Check out at a bookstore

Bank / ATM

Call an airline


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Queue Applications

Computer Science Applications

Print lines of a document

Printer sharing between computers

Recognizing palindromes

Shared resource usage (CPU, memory

access, )


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Queue Applications

Simulation

A study to see how to reduce the wait

involved in an application


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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


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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


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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)


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Queue Implementation

Implementation:

Array-based (Linear or Circular)

Pointer-based : Link list (Linear or

Circular)


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2.0 Queue Implementation Using

Array(Linear)


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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.


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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


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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


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Create Queue operation

Example Code 1

#include

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


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enQueue operation

void enQueue(){

cout


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enQueue operation

Continue

0

front

1

back

0 1 2 3 4

A B

item

back = 0 +1

back = 1

From back/rearitem[back] = newitem

0

front

2

back

0 1 2 3 4

A B C

item

back = 1 +1

back = 2

From back/rear

item[back] = newitem

back++

back++




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enQueue operation

Continue

0

front

3

back

0 1 2 3 4

A B C D

item

back = 2 +1

back = 3

From back/rearitem[back] = newitem

0

front

4

back

0 1 2 3 4

A B C D E

item

back = 3 +1

back = 4

From back/rear

item[back] = newitem

back++

back++




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deQueue operation

void deQueue(){

cout


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1

front

4

back

0 1 2 3 4

NULL B C D E

item

back = 3 + 1

back = 4

front = 0 + 1

front = 1

front++

Continue

1

front

4

back

0 1 2 3 4

NULL B C D E

item

back = 3 + 1

back = 4

itemdeleted = item[front]

front = 1From front/head

item[front] = NULL

2

front

4

back

0 1 2 3 4

NULL NULL C D E

item

back = 3 + 1

back = 4

front = 1 + 1

front = 2

front++




deQueue operation


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Continue

2

front

4

back

0 1 2 3 4

NULL NULL C D E

item

back = 3 + 1

back = 4


front = 2

From front/head

item[front] = NULL

3

front

4

back

0 1 2 3 4

NULL NULL NULL D E

item

back = 3 + 1

back = 4

front = 2 + 1

front = 3

front++



deQueue operation


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Continue

3

front

4

back

0 1 2 3 4

NULL NULL NULL D E

item

back = 3 + 1

back = 4


front = 3

From front/headitem[front] = NULL

4

front

4

back

0 1 2 3 4

NULL NULL NULL NULL E

item

back = 3 + 1

back = 4

front = 3 + 1

front = 4

front++



deQueue operation


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Continue

4

front

4

back

0 1 2 3 4

NULL NULL NULL NULL E

item

back = 3 + 1

back = 4


front = 4

From front/head

item[front] = NULL

5

front

4

back

0 1 2 3 4

NULL NULL NULL NULL NULL

item

back = 3 + 1

back = 4

front = 4 + 1

front = 5

front++


deQueue operation


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Retrieve at front(getFront) operation

void getFront(){

cout


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Retrieve at back(getRear) operation

void getRear(){

cout


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destroyQueue operation

void destroyQueue(){

delete [] item;

}

Continue


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displayQueue operation

void displayQueue(){

cout


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Queue Implementation Using Array(Linear)

int main()

{int selection;

menu:

cout


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Continue

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

displayQueue();

goto menu;

break;

case 2: deQueue();

displayQueue();goto menu;

break;

case 3: getFront();

displayQueue();

goto menu;break;


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case 4: getRear();displayQueue();

goto menu;

break;

case 5: destroyQueue();

displayQueue();

goto menu;

break;

case 6: displayQueue();

goto menu;

break;

default:cout


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Rightward drifting solutions

Shift array elements after each deletion

Shifting dominates the cost of the

implementation


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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


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Solution:

Use a counter

Count == 0 means empty queue

Count == MAX_QUEUE means full

queue


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Array(Circular)


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Array(Circular)

Number of elements in Queue are fixed

during declaration.

Need isFull() operation to determine

whether a queue is full or not.


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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


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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


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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


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enQueue

Increment back, using modulo arithmetic

Insert item

Increment count

deQueue

Increment frontusing modulo arithmetic

Decrement count

Disadvantage

Overhead of maintaining a counter or

flag


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front = 0

back = -1

0

1

2

34

5

6

7

count = 0

Example Code 2:

#include


#define max 8

char queue[max], newitem;

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

Continue

queue


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void enQueue(){

cout


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enQueue Implementation Using Array(Circular)

Continue

front = 0

back = 1

0

1

2

34

5

6

7

count = 2

A

back = (0 + 1) % 8

back = 1 % 8

back = 1

queue[1] = B

count = 1 + 1

count = 2

0

0

1

8 1

B

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


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Continue

front = 0

back = 2

0

1

2

34

5

6

7

count = 3

A

back = (1 + 1) % 8

back = 2 % 8back = 2

queue[2] = C

count = 2 + 1

count = 3

0

0

2

8 2

B

C

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

queue


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Continue

front = 0

back = 3

0

1

2

34

5

6

7

count = 4

A

back = (2 + 1) % 8

back = 3 % 8

back = 3

queue[3] = D

count = 3 + 1

count = 4

0

0

3

8 3

B

C

D



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Continue

front = 0

back = 4

0

1

2

34

5

6

7

count = 5

A

back = (3 + 1) % 8

back = 4 % 8

back = 4

queue[4] = E

count = 4 + 1

count = 5

0

0

4

8 4

B

C

DE



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Continue

front = 0

back = 5

0

1

2

34

5

6

7

count = 6

A

back = (4 + 1) % 8

back = 5 % 8

back = 5

queue[5] = F

count = 5 + 1

count = 6

0

0

5

8 5

B

C

DE


F


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Continue

front = 0

back = 6 0

1

2

34

5

6

7

count = 7

A

back = (5 + 1) % 8

back = 6 % 8

back = 6

queue[6] = G

count = 6 + 1

count = 7

0

0

6

8 6

B

C

DE


F

G


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Continue

front = 0back = 7

0

1

2

34

5

6

7

count = 8

A

back = (6 + 1) % 8

back = 7 % 8

back = 7

queue[7] = H

count = 7 + 1

count = 8

0

0

7

8 7

B

C

DE

From previous slide: front = 0, back = 6, count = 7queue

F

G

H


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deQueue Implementation Using Array(Circular)

void deQueue(){

cout


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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] = NULL

front = (1 + 1) % 8

front = 2% 8

front = 2

count = 7 - 1

count = 6

0

0

2

8 2

C

DE

queue

F

GH


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cout


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int main(){

int selection;

menu:

cout


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switch(selection){

case 1: enQueue();

displayQueue();

goto menu;

break;

case 2: deQueue();

displayQueue();

goto menu;

break;


goto menu;

break;

Continue


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default:cout


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Linked List(Linear)


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Queue Implementation Using Linked List(Linear)

Pointer-Based Implementation More straightforward than array-based

Need Two external pointer (Front & Back) which frontto

trace deQueue operation and backto trace deQueueoperation.

C t Q I l t ti U i Li k d


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Create Queue Implementation Using Linked

List(Linear)

Example Code 1:

#include


struct nodeQueue{

char name;

int age;

nodeQueue *next;

};

name age next

Compiler get the initial illustrated structure of node

Continue



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List(Linear)

nodeQueue *back_ptr = NULL;

nodeQueue *front_ptr=NULL;

Continue

NULL

NULL

back_ptr

front_ptr

enQueue Implementation Using Linked


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List(Linear)

void enQueue(){

//create new node

nodeQueue *newnode;

newnode = new nodeQueue;

coutage;

newnode->next = NULL;

Continue

Ali 29 NULL

newnode

0110

0110



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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

0110newnode

Ali 29 NULL

age

0110

name next

Insertion to an empty queue

0110

back_ptr



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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



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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


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Continue

deQueue Implementation Using Linked List(Linear)

void deQueue(){


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cout


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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

temp

if(front_ptr->next == NULL){

front_ptr = NULL;

back_ptr = NULL;

delete temp;

}else{

}

NULL

front_ptr

NULL

back_ptr


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Continue

}else{

front_ptr = front_ptr->next;

delete temp; }

Tina 30 NULL

0111

name age next

0111

back_ptr

0111

front_ptr

Ali 29 0111

age

0110

name next

0110

temp

Tina 30 NULL

0111

name age next

0111

back_ptr

0111

front_ptr

displayQueue Implementation Using Linked


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Continue

displayQueue Implementation Using LinkedList(Linear)


cout


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Continue


int main()

{

int selection;

menu:

cout


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switch(selection){

case 1: enQueue();displayQueue();

goto menu;

break;

case 2: deQueue();

displayQueue();

goto menu;

break;


goto menu;

break;

default:cout

Date post:	02-Jun-2018
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