Queues

1
Queues

• ITI 1121
• N. El Kadri

2
Definitions

• A queue is a linear abstract data type such that
  insertions are made at one end, called the rear,
  and removals are made at the other end, called
  the front.
• Queues are sometimes called FIFOs first-in
  first-out.
• enqueue() ? Queue ? dequeue()
• The two basic operations are
• enqueue adds an element to the rear of the
  queue.
• dequeue removes and returns the element at the
  front of the queue.
• ?Software queues are similar to physical ones
  queuing at the supermarket, at the bank, at
  cinemas, etc.

3
Applications

• Shared resources management
• (system programming)
• Access to the processor
• Access to the peripherals such as disks and
  printers.
• Application programs
• Simulations
• Generating sequences of increasing length over a
  finite size alphabet
• Navigating through a maze.

4
Example

• class Simple
• public static void main( String args )
• Queue queue new QueueImplementation()
• for ( int i0 ilt10 i )
• queue.enqueue( new Integer( i ) )
• while ( ! queue.isEmpty() )
• System.out.println( queue.dequeue() )
• ? What does it print?

5

• q new QueueImpl() q ?
• q.enqueue(a) q ? a
• q.enqueue(b) q ? a,b
• q.enqueue(c) q ? a,b,c
• q.enqueue(d) q ? a,b,c,d
• q.dequeue() ? a q ? b,c,d
• q.dequeue() ? b q ? c,d
• q.enqueue(e) q ? c,d,e
• q.dequeue() ? c q ? d,e
• q.dequeue() ? d q ? e
• ? Elements of a queue are processed in the same
  order as the they are inserted into the queue,
  here a was the first element to join the queue
  and it was the first to leave the queue
  first-come first-serve.

6
Implementations

• Just like stacks, there are two families of
  implementations.
• Array-based
• Linked-structure.

7
Todays Topics

• Queue
• Implementation of Queue
• Usage of Queue

8
Queue

• Queue First In First Out (FIFO)
• Toll Station
• Car comes, pays, leaves
• Check-out in Big Y market
• Customer comes, checks out and leaves
• More examples Printer, Office Hours,

Output
Input
9
More Examples of Queue

• In our daily life
• Airport Security Check
• Cinema Ticket Office
• Bank, ATM
• Anything else ?

10
What Is Queue

• Queue is an abstract data type
• Adding an entry at the rear
• Deleting an entry at the front

11
Abstract Data Types

• Queue
• Operating on both ends
• Operations EnQueue(in), DeQueue(out)

12
Queue

• Queue is FIFO ( First-In First-Out)
• A queue is open at two ends. You can only add
  entry (enqueue) at the rear , and delete entry
  (dequeue) at the front.
• Note that you cannot add/extract entry in the
  middle of the queue.

13
Applications of Queue

• Printing Job Management
• Packet Forwarding in Routers
• Message queue in Windows
• I/O buffer

14
Printing Job Management

• Many users send their printing jobs to a public
  printer
• Printer will put them into a queue according to
  the arrival time and print the jobs one by one
• These printing documents are A.doc, B.doc, C.doc
  and D.doc

15
Printing Queue

• A.doc B.doc C.doc arrive to printer.

Now printing A.doc
A.doc is finished. Now printing B.doc
Now still printing B.doc
D.doc comes
B.doc is finished. Now printing C.doc
C.doc is finished. Now printing D.doc
16
First-in First-out (FIFO)
The first one enqueued is the first one dequeued.
(FIFO)
When we enqueue entries in the queue and then
dequeue them one by one, we will get the items in
the same order.
A, B, C come out
17
Question

• Queue is an abstract data structure
• Item can be Integer, Double, String, Employee,
  Faculty
• How to implement a general queue for all those
  types?

18
Abstract Data Type

• Same as Stack, we use Object data type instead of
  int or double or String or other data type
• Use an array in queue, which stores all items
  come in.
• Object Queue
• Other implementations are possible

19
Array Implementation of Queue

20
Operations

• enqueue
• add a new item at the rear
• dequeue
• remove a item from the front
• isEmpty
• check whether the queue is empty or not
• isFull
• check whether the queue is full or not
• size
• return the number of items in the queue
• peek
• return the front item

21
Problem

• An array has limited size, once rear is at the
  end of this array, and there is new item coming
  in, what can we do?

1
0
n-1
3
2
Y
X


front
rear
22
Two Solutions

• Shifting all items to front in the array when
  dequeue operation. ( Too Costly )
• Wrapped around array ---- Circular Array

23
Circular Array

• Wrapped around array

24
EnQueue DeQueue In Circular Array

• DeQueue
• front (front 1) MOD n

• EnQueue
• rear (rear 1) MOD n

25
Empty/Full In Circular Array

• When rear equals front, Queue is empty
• When (rear 1) MOD n equals front, Queue is full
• Circular array with capacity n at most can hold
  n-1 items.

26
Implementation for Queue

• public class ArrayQueuePT
• private final static int DEFAULT_CAPACITY 100
• // suppose the default capacity for this queue is
  100.
• private Object queue
• // The array that holds the items
• private int rear, front
• // Index of rear, front item in the queue

27
ArrayQueuePT Constructor

• // Creates a queue with the default capacity
• public ArrayQueuePT ()
• this(DEFAULT_CAPACITY)
• // Creates a queue with a user-specified capacity
• public ArrayQueuePT (int capacity)
• if (capacity lt 2)
• throw new IllegalArgumentException ("Capacity
  must be gt 1")
• queue new Objectcapacity
• rear front 0

28
ArrayQueuePT --- Size()

• How many items currently in the queue?
• public int size()
• if( rear gt front )
• return rear front
• else
• return queue.length rear front

29
ArrayQueuePT --- isEmpty/isFull

• // check whether the queue is empty
• public boolean isEmpty()
• return rear front
• // check whether the queue is full
• public boolean isFull()
• return size() queue.length-1

30
ArrayQueuePT --- enqueue()

• public void enqueue(Object item)
• if (item null)
• throw new IllegalArgumentException ("Item is
  null")
• if (isFull())
• throw new IllegalStateException (Queue is
  full")
• queuerear item
• rear (rear 1)queue.length

31
ArrayQueuePT --- dequeue()

• public Object pop( )
• if (isEmpty())
• throw new IllegalStateException (Queue is
  empty")
• Object frontItem queuefront
• queuefront null
• front (front 1)queue.length
• return frontItem

32
ArrayQueuePT -- peek()

• public Object peek()
• if (isEmpty())
• throw new IllegalStateException (Queue is
  empty")
• return queuefront

33
Interface

• Users dont need to know how Queue is
  implemented.
• Users only need to know how they can operate the
  Queue.
• There are many ways to implement Queue, but all
  of them have the same interfaces
• enqueue, dequeue, peek, isFull, isEmpty

34
Interface and Implementation Class

• Interface contains a skeleton for public
  operations
• No real code for each method
• Like function prototype
• A class implements interface
• Implements every method
• Statement
• public class myclass implements interface

35
Interface for Queue

• public interface QueuePT
• public boolean isEmpty()
• public boolean isFull()
• public Object peek()
• public Object dequeue()
• public void enqueue(Object item)
• public int size()

36
Implementation of ArrayQueuePT Class

• public class ArrayQueuePT implements QueuePT
• private final static int DEFAULT_CAPACITY 100
• private Object queue // The array
  holds the queue
• private int rear, front
• // Index of rear, front items in queue

37
Create an object of ArrayQueuePT

• // create a queue with default capacity
• QueuePT myqueue new ArrayQueuePT()
• // create a queue with 1024 elements
• QueuePT registry new ArrayQueuePT(1024)

38
Using ArrayQueuePT

• enqueue and dequeue characters in Queue myque
• myque.enqueue( new Character(a) )
• char ch ((Character)myque.dequeue()).charValue()
  
• enqueue and dequeue Employee in Queue registry
• registry.enqueue( new Employee(kyle, 5456) )
• Employee emp (Employee) registry.dequeue()

39
Printing Job Management

• 4 documents are ready to print
• Print the printer status
• new document comes
• which document is finished
• Using ArrayQueuePT

40
New Printing Jobs come

• public void printstatus()
• QueuePT que new ArrayQueuePT( ) //Create a
  new queue
• System.out.println("Printing job Null ")
  //print the printer status
• // new printing jobs come, a.doc, b.doc,
  c.doc, d.doc
• System.out.println("New printing job
  a.doc")
• que.enqueue( "a.doc" )
• System.out.println("New printing job
  b.doc")
• que.enqueue( "b.doc" )
• System.out.println("New printing job
  c.doc")
• que.enqueue( "c.doc" )
• System.out.println("New printing job
  d.doc")
• que.enqueue( "d.doc" )

41
Finishing Printing Jobs

• // print the printer status
• System.out.println("\nPrinting job there
  are "
• que.size() " jobs in the queue")
• System.out.println(" " que.dequeue() " is
  Finished")
• System.out.println(" " que.dequeue() " is
  Finished")
• System.out.println(" " que.dequeue() " is
  Finished")
• System.out.println(" " que.dequeue() " is
  Finished")

42
Customer Service In Royal Bank

• Suppose there is only one customer service
  available in Royal Bank on Saturday morning
• In every 3 minutes, a new customer arrives at the
  end of waiting line
• Each customer will need 5 minutes for the service
  
• Print out the information after the first 30
  minutes
• The time of arriving and leaving for each
  customer
• How many customers are in the line?
• Who is the current serving customer?

43
Customer Service Queue

• public void run( )
• // Create a new queue
• QueuePT que new ArrayQueuePT(100)
• int time 0 // in minutes
• int incustomer 0 // secquence of
  customers
• int servicetime 0 // service times

44
Customer In Service

• // what's going on in 30 minutes
• while ( time lt 30 )
• // if queue is not empty, one customer
  service is working
• if( que.size()!0 )
• servicetime servicetime 1
• // customer leaves when finishing the
  service, the service time is 5 minutes
• if( servicetime 5 )
• String name (String)que.dequeue(
  )
• System.out.println("ltlt " name
  " leaves at time " time)
• // start to service time for next
  customer
• servicetime 0

45
New Customer Comes

• // every 3 minutes, there is a new customer
  coming.
• if( time30 )
• incustomer incustomer 1
• String name "CUSTOMER " incustomer
• que.enqueue( name )
• System.out.println("gtgt " name "
  arrives at time " time)
• time time 1

46
Print Status After 30 Minutes

• // print the status after 30 minutes
• System.out.println("\n
  " )
• if( que.size()!0 )
• System.out.println("There are "
  que.size() " customers in the line" )
• System.out.println("The current serving
  customer is " que.peek() )
• else
• System.out.println("There are no
  customers in the line" )

47
Priority Queue --- Air Travel

• Only one check-in service in Air Canada at
  airport
• Two waiting lines for passengers
• one is First class service
• the other is Economy class service
• Passengers in the first-class waiting line have
  higher priority to check in than those in the
  economy-class waiting line.

48
Priority Queue

• Two queues
• one is high priority queue
• the other is low priority queue
• Service rules
• First serve the people in high priority queue
• If no passengers are in high priority queue,
  serve the passengers in low priority queue

49
Two Queues

• High Priority Queue, will come in hpQue
• Low Priority Queue, will come in lpQue

High Priority Queue
Check In
Customers coming in
D
C
H
G
F
E
B
A
Low Priority Queue
50
Pseudocode For Arrival

• Passengers Arrival
• if( new Passenger comes )
• if( is First Class)
• hpQue.enqueue( new Passenger )
• else
• lpQue.enqueue( new Passenger )

51
Pseudocode For Service

• Check-In Service
• if( hpQue is not empty )
• serve the passenger from high
  priority queue,
• hpQue.dequeue()
• else
• serve the passenger from low priority
  queue,
• lpQue.dequeue()