chap07 - queue data structure

8/3/2019 Chap07 - Queue Data Structure

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Data Structures Using Java 1

Chapter 7

Queues


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Chapter Objectives

Learn about queues

Examine various queue operations

Learn how to implement a queue as an array

Learn how to implement a queue as a linked list

Discover priority queues

Discover queue applications


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Queues

Definition: data structure in which the

elements are added at one end, called the

rear, and deleted from the other end, calledthe frontor first

First In First Out (LIFO) data structure


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Basic Operations on a Queue

initializeQueue: Initializes the queue to an

empty state

isEmptyQueue: Determines whether the

queue is empty. If the queue is empty, it

returns the value true; otherwise, it returns

the value false


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Basic Operations on a queue

isFullQueue: Determines whether the queueis full. If the queue is full, it returns the

value true; otherwise, it returns the valuefalse

front: Returns the front (first) element of thequeue; the queue must exist

back: Returns the front (first) element of thequeue; the queue must exist


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Basic Operations on a queue

addQueue: Adds a new element to the rear

of the queue; the queue must exist and must

not be full

deleteQueue: Removes the front element of

the queue; the queue must exist and must

not be empty


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Queue Exception Class

Adding an element to a full queue, and

removing an element from an empty queue,

generates errors and exceptions calledqueue overflow and queue underflow

exception

Exception classes designed to handle theseexceptions


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Implementation of Queues as

Arrays Initially queue is empty; queueFront and queueRear

point directly to first and last elements of queue

To implement a queue as an array we need: An array

The variables queueFront and queueRear to keep track

of the first and last elements of the queue

The variable maxQueueSize to specify the maximum

size of the queue


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Arrays


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Arrays


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Circular Queue

Possible problem: If a sequence ofoperations eventually sets index queueRear

to point to last array position, it gives theimpression that the queue is full.

However, the queue has only two or threeelements and front of the array is empty(see Figure 7-4).


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Circular Queue


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Circular Queue


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Circular Queue


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ArraysCase 1: Suppose that after certain operations, the array

containing the queue is as shown below


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ArraysdeleteQueue operation results in an empty queue


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ArraysCase 2: Let us now consider the queue shown below


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ArraysResulting array in Figure 7-11 represents a full queue


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Full Queue vs. Empty Queue

Problem: distinguishing between an empty and a

full queue

Arrays in Figures 7-9 and 7-11 have identicalvalues for queueFront and queueRear

Solutions:

Keep a count

Let queueFront indicate index of array position

precedingfirst element of queue, rather than index of

actual first element itself (see Figure 7-12)


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UML Diagram of the

class QueueClass


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Initialize Queue

public void initializeQueue()

{

for(int i = queueFront; i < queueRear;

i = (i + 1) %maxQueueSize)

list[i] = null;

queueFront = 0;

queueRear = maxQueueSize - 1;

count = 0;

}


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Empty Queue and Full Queue

public boolean isEmptyQueue()

{

return (count == 0);}

public boolean isFullQueue()

{return (count == maxQueueSize);

}


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front

public DataElement front() throws

QueueUnderflowException

{

if(isEmptyQueue())

throw new QueueUnderflowException();

DataElement temp = list[queueFront].getCopy();

return temp;

}


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back

public DataElement back() throws

QueueUnderflowException

{

if(isEmptyQueue())


DataElement temp = list[queueRear].getCopy();

return temp;

}


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Add Queue

public void addQueue(DataElement queueElement)

throws QueueOverflowException

{

if(isFullQueue())

throw new QueueOverflowException();

queueRear = (queueRear + 1) % maxQueueSize; //use the mod

//operator to advance queueRear

//because the array is circular

count++;

list[queueRear] = queueElement.getCopy();}


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Delete Queue

public void deleteQueue() throws QueueUnderflowException

{

if(isEmptyQueue())


count--;list[queueFront] = null;

queueFront = (queueFront + 1) % maxQueueSize; //use the mod

//operator to advance queueFront

//because the array is circular

}


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Constructor

Constructor

creates an array of the size specified by the user

Default value is 100

Initializes queueFront queueRear to indicate that

the queue is empty


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Linked Queue as an ADT


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Empty and Full Queue

Queue is empty if queueFront is NULL

Queue is full only if we run out of memory


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addQueue

Adds a new element to the end of the queue

Access the reference variable queueRear to

implement addQueue


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Front, Back, and Delete Queue

If queue is nonempty:

operation front returns the first element of the queue

operation back returns the last element of the queue operation deleteQueue removes the first element of the

queue

If queue is empty: method front terminates the program

method back terminates the program


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Priority Queue

FIFO rules of a queue are relaxed

Customers or jobs with higher priority are

pushed to front of queue

To implement:

use an ordinary linked list, which keeps the

items in order from the highest to lowest priority

use a treelike structure


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Application of Queues

Simulation: technique in which one system modelsthe behavior of another system; used when it is tooexpensive or dangerous to experiment with real

systems

Simulation examples: wind tunnels used to experiment with design of car

bodies

flight simulators used to train airline pilots

Computer simulations: objects being usuallyrepresented as data


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Theater Problem

The manager of a local movie theater is hearing complaintsfrom customers about the time they have to wait in line to

buy tickets. The theater currently has only one cashier.

Another theater is preparing to open in the neighborhoodand the manager is afraid of losing customers. The manager

wants to hire enough cashiers so that a customer does nothave to wait too long to buy a ticket, but does not want tohire extra cashiers on a trial basis and potentially waste timeand money.

One thing that the manager would like to know is the

average time a customer has to wait for service. The manager wants someone to write a program to simulate

the behavior of the theater.


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Queuing System

Server:object that provides the service

Customer: object receiving the service

transaction time: service time; time it takesto serve a customer

time-driven simulation:clock isimplemented as a counter and the passageof time (e.g. 1 minute) can be implementedby incrementing the counter (by 1)


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iti C t Q


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waitingCustomerQueueclass WaitingCustomerQueue extends QueueClass

{

//default constructor

public WaitingCustomerQueue(){

super();

}

//constructor with a parameter

public WaitingCustomerQueue(int size)

{

super(size);}

//copy constructor

public WaitingCustomerQueue(WaitingCustomerQueue otherQ)

{

super(otherQ);

}

//Method to increment the waiting time of each

//customer in the queue by one time unit.//Postcondition: The waiting time of each customer in

// the queue is incremented by one time unit.

public void updateWaitingQueue()

{

//Definition as given below.

}

}


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Poisson Distribution

Need to know the number of customers arriving at a given

time unit and how long it takes to serve each customer.

Use Poisson distribution from statistics, which saysprobability ofy events occurring at a given time is given by:

where is the expected value thaty events occur at that time.


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Chapter Summary

Queue Data Structure

Restricted Version of arrays and linked

list

Basic operations

First In First Out (FIFO)

Queues Implemented as Arrays


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Chapter Summary

Queues Implemented as Linked Lists

Priority Queues


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