Queues

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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, called the front or
  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 queue is
  full. If the queue is full, it returns the value
  true otherwise, it returns the value false
• front Returns the front (first) element of the
  queue the queue must exist
• back Returns the front (first) element of the
  queue 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 called queue overflow and queue
  underflow exception
• Exception classes designed to handle these
  exceptions

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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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Implementation of Queues as Arrays
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Implementation of Queues as Arrays
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Circular Queue

• Possible problem If a sequence of operations
  eventually sets index queueRear to point to last
  array position, it gives the impression that the
  queue is full.
• However, the queue has only two or three elements
  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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Implementation of Queues as Arrays
Case 1 Suppose that after certain operations,
the array containing the queue is as shown below
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Implementation of Queues as Arrays
deleteQueue operation results in an empty queue
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Implementation of Queues as Arrays
Case 2 Let us now consider the queue shown below
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Implementation of Queues as Arrays
Resulting 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 identical
  values for queueFront and queueRear
• Solutions
• Keep a count
• Let queueFront indicate index of array position
  preceding first element of queue, rather than
  index of actual first element itself (see Figure
  7-12)

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UML Diagram of theclass QueueClass
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Initialize Queue

• public void initializeQueue()
• for(int i queueFront i lt queueRear
• i (i 1)
  maxQueueSize)
• listi 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 listqueueFront.getCopy()
  
• return temp

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back

• public DataElement back() throws
  QueueUnderflowException
• if(isEmptyQueue())
• throw new QueueUnderflowException()
• DataElement temp listqueueRear.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
• listqueueRear queueElement.getCopy()

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Delete Queue

• public void deleteQueue() throws
  QueueUnderflowException
• if(isEmptyQueue())
• throw new QueueUnderflowException()
• count--
• listqueueFront 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 models
  the behavior of another system used when it is
  too expensive 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 usually
  represented as data

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Theater Problem

• The manager of a local movie theater is hearing
  complaints from 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
  neighborhood and the manager is afraid of losing
  customers. The manager wants to hire enough
  cashiers so that a customer does not have to wait
  too long to buy a ticket, but does not want to
  hire extra cashiers on a trial basis and
  potentially waste time and 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 takes to
  serve a customer
• time-driven simulation clock is implemented as a
  counter and the passage of time (e.g. 1 minute)
  can be implemented by incrementing the counter
  (by 1)

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Application of Queues
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Application of Queues
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Application of Queues
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waitingCustomerQueue
class WaitingCustomerQueue extends QueueClass
//default constructor public
WaitingCustomerQueue() super()
//constructor with a parameter
public WaitingCustomerQueue(int size)
super(size) //copy constructor
public WaitingCustomerQueue(WaitingCustomerQueu
e 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 says probability of y events
occurring at a given time is given by
where is the expected value that y 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
• Application of Queues