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Queues

Lecture-14

Umar Manzoor

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Queues

A Queueis a special kind of list, where

items are inserted at one end (the rear)

And deleted at the other end (the front)

Other Name:

First In First Out (FIFO)

Difference from Stack:

Insertion go at the end of the list, rather thanthe beginning of the list.

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Common Operations on Queues

(Queue ADT)

1. MAKENULL(Q): Makes Queue Q be an empty list.

2. FRONT(Q): Returns the first element on Queue Q.

3. ENQUEUE(x,Q): Inserts element x at the end of Queue Q.

4. DEQUEUE(Q): Deletes the first element ofQ.

5. EMPTY(Q): Returns true if and only if Q is an empty queue.

Example:

Line of customers in a bank

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Applications of Queues

Operating systemmulti-user/multitasking environments, where

several users or task may be requesting the sameresource simultaneously.

Communication Software queues to hold information received over networks

and dial up connections. (Information can betransmitted faster than it can be processed, so is

placed in a queue waiting to be processed)Some other?

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Implementation

StaticQueue is implemented by an array, and size of

queue remains fix

Dynamic A queue can be implemented as a linked list,

and expandorshrinkwith each enqueue or

dequeue operation.

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A pointer Implementation of Queues

Keep two pointers:

FRONT: A pointer to the first element of the queue.

REAR: A pointer to the last element of the queue.

x y .zFront

Rear

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A pointer Implementation of Queues

Q.front

Q.Rear

MAKENULL(Q)

Q.front

Q.Rear

ENQUEUE(x,Q)

.x

NULL

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Q.frontQ.Rear

ENQUEUE(y,Q)

x .y

Q.front

Q.Rear

DEQUEUE(Q)

.y

A pointer Implementation of Queues

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A class for Dynamic Queue implementation

class DynIntQueue

{

private:

struct QueueNode

{

int value;

QueueNode *next;

};

QueueNode *front;QueueNode *rear;

int numItems;

public:

DynIntQueue(void);

~DynIntQueue(void);

void enqueue(int);int dequeue(void);

bool isEmpty(void);

void makeNull(void);

};

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Implemenaton

//************************

// Constructor *

//************************

DynIntQueue::DynIntQueue(void)

{

front = NULL;

rear = NULL;

numItems = 0;

}

//************************

// Destructor *

//************************

DynIntQueue::~DynIntQueue(void){

makeNull();

}

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//********************************************

// Function enqueue inserts the value in num *

// at the rear of the queue. *

//********************************************

void DynIntQueue::enqueue(int num)

{

QueueNode *newNode;

newNode = new QueueNode;

newNode->value = num;

newNode->next = NULL;

if (isEmpty())

{

front = newNode;

rear = newNode;

}

else{

rear->next = newNode;

rear = newNode;

}

numItems++;

}

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//**********************************************

// Function dequeue removes the value at the *

// front of the queue, and copies it into num. *

//**********************************************

int DynIntQueue::dequeue(void)

{

QueueNode *temp;

int num;

if (isEmpty())

cout value;

temp = front->next;

delete front;

front = temp;

numItems--;

}

return num;

}

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//*********************************************

// Function isEmpty returns true if the queue *

// is empty, and false otherwise. *

//*********************************************

bool DynIntQueue::isEmpty(void)

{

if (numItems)

return false;

else

return true;}

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//********************************************

// FunctionmakeNull dequeues all the elements *

// in the queue. *

//********************************************

void DynIntQueue::makeNull(void)

{

while(!isEmpty())

dequeue();

}

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Program

// This program demonstrates the DynIntQeue class

void main(void)

{

DynIntQueue iQueue;

cout

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Program Ouput

Enqueuing 5 items...

The values in the queue were:

0

1

2

3

4

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Array Implementation

First Element

Last Element

maxlength

Front

Second

Element.

.

Rear

When queue is empty both front and rear are set to -1

While enqueueing increment rear by 1, and while dequeueing

increment front by 1

When there is only one value in the Queue, both rear and front

have same index

Can we implement Queue by using only one

index variable Front or Rear??

YES, by moving elements of array to neighboring

locations like we did in STACK but this is in-efficient

Why it is inefficient?

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Array Implementation

5 4 6 7 8 7 6

0 1 2 3 4 5 6 7 8

Front=0

Rear=6

8 7 6

0 1 2 3 4 5 6 7 8

Front=4

Rear=6

7 6 12 67

0 1 2 3 4 5 6 7 8

Front=5

Rear=8How can we insert more elements? Rear index can

not move beyond the last element.

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Solution: Using circular queue

Allow rear to wrap around the array.

if(rear == queueSize-1)

rear = 0;else

rear++;

Or use module arithmeticrear = (rear + 1) % queueSize;

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7 6 12 67

0 1 2 3 4 5 6 7 8

Front=5

Rear=8

Enqueue 39 Rear=(Rear+1) mod Queue Size = (8+1) mod 9 = 0

39 7 6 12 67

0 1 2 3 4 5 6 7 8

Front=5

Rear=0

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How to determine empty and full

Queues?

It is some tricky

Number of approaches A counter indicating number of values in the

queue can be used (We will use this approach)

We will see another approach as well at the end

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Implementationclass IntQueue

{

private:

int *queueArray;

int queueSize;

int front;

int rear;

int numItems;public:

IntQueue(int);

~IntQueue(void);

void enqueue(int);

int dequeue(void);

bool isEmpty(void);

bool isFull(void);void clear(void);

};

Note, the member function clear, which clears the queue by resetting the

front and rear indices, and setting the numItems to 0.

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IntQueue::IntQueue(int s) //constructor

{

queueArray = new int[s];

queueSize = s;front = -1;

rear = -1;

numItems = 0;

}

IntQueue::~IntQueue(void) //destructor

{

delete [] queueArray;

}

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//********************************************

// Function enqueue inserts the value in num *

// at the rear of the queue. *

//********************************************

void IntQueue::enqueue(int num)

{

if (isFull())

cout

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//*********************************************

// Function dequeue removes the value at the *

// front of the queue, and copies t into num. *

//*********************************************

int IntQueue::dequeue(void)

{

if (isEmpty())

cout

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//********************************************

// Function isFull returns true if the queue *// is full, and false otherwise. *

//********************************************

bool IntQueue::isFull(void)

{

if (numItems < queueSize)return false;

else

return true;

}

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//*******************************************

// Function clear resets the front and rear *

// indices, and sets numItems to 0. *

//*******************************************

void IntQueue::clear(void)

{

front = - 1;

rear = - 1;

numItems = 0;

}

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//Program demonstrating the IntQueue class

void main(void)

{

IntQueue iQueue(5);

cout

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Program Output

Enqueuing 5 items...

Now attempting to enqueue again...

The queue is full.The values in the queue were:

0

1

2

3

4

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Another implementation of Queues using

Arrays

class CQueue{

int Data*,QueueSize,Front,Rear;

public:

CQueue(int size);~CQueue(int size);

bool IsFull();

bool IsEmpty();

void Enqueue(int num);int Dequeue();

void MakeNull;

};

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CQueue::CQueue(int size)

{

Front=Rear=-1;Data=new int[size];

}

void CQueue ::Enqueue(int num);

{ if (IsFull()) { cout

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int CQueue ::Dequeue(int num);{

if (IsEmpty()) { cout

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bool CQueue::IsEmpty()

{

if (Front==-1) return true;else return false;

}

bool CQueue::IsFull()

{

If (((Rear+1)%QueueSize)==Front)

return true;

else return false;}