QUEUES

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QUEUES
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QUEUE OBJECT

• Introduction
• queue is another simple data structure that is
  easy to manipulate
• useful for programmers because it can be used to
  model real-world situations
• 1) simulation of a bank line
• 2) simulation of auto mechanic shop bey
• queues work on the FIFO principle
  (first-in-first-out)
• 1) use an array to implement the queue/list
• 2) need two indexes
• front where we take from the list
• back where we add to the list
• 3) also, a number to keep track of the current
  size of list

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INTRO TO QUEUES (cont.)

• 4) when first value added, front and rear point
  to same place
• 5) each additional add rear marker moved down
  one
• 6) each remove
• front marker moved down one
• problem - must make sure not to add past the end
  of the array
• one solution make the array circular (when reach
  the end, reset rear to point to slot 0)
• another solution make the array dynamic
• (solution we will use here)

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Conceptual Design of a Queue

• so, if we design the Queue as an object
• Queue Object
• Knows
• pointer to space in memory containing the
  list
• the front
• the rear
• the current size ( items in the queue)
• Can Do
• Retrieve an item (take out of front of
  queue)
• Add an item (add to end of queue)
• Tell which item is in front (without
  removing)
• Clear the queue
• Tell if the queue is empty

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• typedef int Item
• typedef Item ItemPtr
• class Queue
• public
• static const int DEFAULTSIZE 30
• Queue()
• Queue(const int QueueSize)
• Queue()
• Item First()
• Item Remove()
• int Size()
• void Insert(const Item anItem)
• bool IsEmpty() const
• void Clear()
• private
• ItemPtr MyQueue
• int Front
• int Rear
• int Total

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MORE MEMBER FUNCTIONS

• The Default Constructor - creates a queue
  dynamically of the DEFAULTSIZE
• calls the Clear() function which sets the values
  of front and rear and sets total to 0
• Queue Queue()
• MyQueue new ItemDEFAULTSIZE
• ArraySize DEFAULTSIZE
• Clear()
• The Constructor - creates a Queue of QueueSize
  size
• Queue Queue(const int QueueSize)
• ArraySize QueueSize
• MyQueue new ItemQueueSize
• Clear()

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MORE MEMBER FUNCTIONS

• The Deconstructor - simply deletes the queue
  from the heap
• Queue Queue()
• delete MyQueue
• The IsFull Function - returns true if total is gt
  DEFAULTSIZE
• bool Queue IsFull() const
• return (total gt ArraySize)
• The IsEmpty Function - will return true if the
  queue is empty (no items in it)
• bool Queue IsEmpty() const
• return (Total 0)

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MORE MEMBER FUNCTIONS

• The Size Function - will return the number of
  items currently in queue
• int Queue Size() const
• return(Total)
• The Insert Function - takes an item as input and
  adds it to the queue
• void Queue Insert(const Item anItem)
• if (Total gt ArraySize)
• cout ltlt "Queue is Full"
• else
• if (Rear ArraySize -1)
• Rear 0
• else
• Rear
• MyQueuerear anItem
• Total

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MORE MEMBER FUNCTIONS

• The Remove Function - returns the item in the
  front of the queue and moves the Front marker
  over one
• Item Queue Remove()
• Item temp
• if (IsEmpty())
• cout ltlt "Cannot remove - queue empty"
  ltlt endl
• return 999
• else
• temp MyQueueFront
• if (Front ArraySize -1)
• Front 0
• else
• Front
• Total--
• return temp

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MORE MEMBER FUNCTIONS

• The First Function - gives back the first item in
  queue but does not remove it
• Item Queue First() const
• if (IsEmpty())
• cout ltlt "No items in queue " ltlt endl
• return -999
• else
• return(MyQueueFront)
• The Clear Function - clears the queue by
  resetting indexes and counters values are still
  in the queue but they are ignored by the program
• void Queue Clear()
• Front 0
• Rear ArraySize - 1
• Total 0

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AN EXAMPLE OF USING THIS QUEUE

• int main()
• Queue TheQueue(3)
• int number, count 0
• do
• cout ltlt "enter a number to add to the
  queue "
• cin gtgt number
• TheQueue.Insert(number)
• count
• while (count lt 3)
• int number_in_queue TheQueue.Size()
• int value_in_queue, evencout 0
• for (int i 1 i lt number_in_queue i)
  
• value_in_queue TheQueue.Remove()
• if (value_in_queue 2 0)
• evencout
• cout ltlt "Total even numbers in queue " ltlt
  evencount ltlt endl

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QUESTIONS?

• Read section on LINKED QUEUES
• (P. 412 425)