Queue ADT

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Queue ADT

• IC312
• A Linear Data Type

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Queue ADT

• Queue might bring to mind something like this

What can we do to a Queue like this?
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Queue ADT Assumptions

• Given the operations (behaviors) what are the
  assumptions/restrictions on the data?
• All data is of same type
• Data is in some specific order
• First added is at front, last at back
• What assumptions have we made about data storage?
• Relative order of data is maintained

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Queue ADT

• We can think of our list like this

a line where the first one in is the first one
out
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Queue ADT Operations

• Things we can do with a queue
• Add entries to back
• Remove from front
• Display entry at front
• Display ALL entries
• Get size of Queue
• Find out if Queue is empty

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Specifications (modifiers)

• Enqueue (add)
• Adds element to back of Q unless Q is full
• Returns nothing
• Dequeue (remove)
• Removes element from front of Q unless Q is empty
• Returns element from front

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Specifications (accessers)

• front
• Returns element from front of Q
• getSize
• Returns number of elements in Q
• isEmpty
• Returns true if Q is empty

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Adding Entries

• We always add (enqueue) to the back

Back
Front
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Removing Entries

• We always remove (dequeue) from the front

Back
Front
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Different Situations

• We must consider different possible situations
• For Example
• Is enqueueing onto an empty queue different than
  a populated queue?
• What about dequeueing?

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Problem Situations

• We must address potential problem situations
• Dequeueing from an empty queue is meaningless
• Displaying the front of an empty queue is
  meaningless
• What if queue has a fixed size and is full?

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Interface

• Defines methods a class must implement

interface QueueltEgt bool enqueue(E entry) E
dequeue() E front() int getSize() bool
isEmpty()
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Implementation

• Must implement all methods in interface
• Need underlying data structure Something to
  hold the data
• Can use array or list

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Array Implementation

• We may have problem called right drift
• If we do not shift elements in queue

Front
Back
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Right Drift

• We can have an empty array with no place to put a
  new element
• Head has drifted to the right
• Solutions?
• Shift elements
• Virtually shift elements

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Circular Array

• Turn this
• into this

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How

• Wrap the index around
• front (front1)MAX_SIZE
• rear (rear1)MAX_SIZE

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Array-based Queue

• Use an array of size N in a circular fashion
• Two variables keep track of the front and rear
• f index of the front element
• r index immediately past the rear element
• Array location r is kept empty

normal configuration
wrapped-around configuration
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Queue Operations

• We use the modulo operator (remainder of division)

Algorithm size() return (N - f r) mod
N Algorithm isEmpty() return (f r)
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Queue Operations (cont.)
Algorithm enqueue(o) if size() N ? 1
then throw FullQueueException else Qr ?
o r ? (r 1) mod N

• Operation enqueue throws an exception if the
  array is full
• This exception is implementation-dependent

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Queue Operations (cont.)
Algorithm dequeue() if isEmpty() then throw
EmptyQueueException else o ? Qf f ? (f
1) mod N return o

• Operation dequeue throws an exception if the
  queue is empty
• This exception is specified in the queue ADT

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Implementation

• Javas Queue Interface
• Javas AbstractQueue
• Must implement
• offer(E)
• peek()
• poll()
• size()
• iterator()

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Uses

• Keeping track of jobs
• To-do lists
• Printers