CSC 212

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CSC 212

• Stacks Queues

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Announcement

• Daily quizzes accepted electronically only
• Submit via one or other Dropbox
• Cannot force you to compile test them
• Can make it much less work
• Please, please, please use this as an excuse to
  practice your Java
• Next homework assignment is on web

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Working Together

• Goal is for students to learn
• Means mistakes are made, questions raised
• Peers useful when learning material
• But the goal is for YOU to learn material
• Talk about homework, daily quizzes all you want
• Leave conversation with only memories, nothing
  written, typed, dictated, displayed on a screen
• Wait at least 15 minutes before continuing with
  homework

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Queues

• Elements may be inserted at any time
• Only element which has been in queue the longest
  may be removed
• Items enqueued into rear of queue
• Items dequeued from front of queue

cashier
Front of line Dequeue here
Back of line Enqueue here
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Queue vs. Stack

• Stack follows LIFO policy
• Last In-First Out
• Objects removed in reverse order of addition
• Queue follows FIFO policy
• First In-First Out
• Objects removed in order they were added

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Queue ADT

• Queue supports two key methods
• enqueue(obj)Insert obj at the rear of the queue
• dequeue()Returns and removes the object from
  front of queue sends error when queue is empty

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Queue ADT

• Queue also defines other methods
• size()Number of objects in the queue
• isEmpty()Returns if the queue is empty.
• front()Return but do not remove object from
  front of queue send error when queue is empty

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Array-based Queue

• Queue uses array in circular manner
• Also specifies maximum size of queue
• The queue consists of
• N-element array, q
• f, index of the front element
• r, index of the rear element

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Array-based Queue Pictorial
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Array-Based Queue Pseudocode

• int size() return (N - f r) mod N
• boolean isEmpty() return(f r)
• Object front()
• if isEmpty() then
• throw QueueEmptyExceptionreturn q f

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Array-Based Queue Pseudocode

• Object dequeue()
• if isEmpty() then throw QueueEmptyExceptiontem
  p ? qfqf ? null
• f ? (f 1) mod Nreturn temp

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Array-Based Queue Pseudocode

• Object enqueue(obj)
• if size() N - 1 then
• throw QueueFullException
• qr ? objr ? (r 1) mod N

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Daily Quiz 1

• Show the output (if any) and what q, f, r would
  equal after each of the following calls (assume N
  5)
• enqueue(5)
• dequeue()
• enqueue(3)
• enqueue(4)
• enqueue(7)
• isEmpty()
• front()
• dequeue()
• enqueue(1)
• enqueue(2)
• size()
• enqueue(9)

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Linked Lists

• Linked lists are dynamically allocated collection
  of nodes
• Pseudocode of Node class
• public class Node Object element Node
  next // constructors, get set methods

next
node
elem
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Adding to Linked Lists

• Maintain a head field/variable
• To insert new node into linked list
• Create initialize new nodenewNode ? new
  Node(elem)
• Set new nodes next fieldto current value of
  headnewNode.setNext(head)
• Assign head to the new nodehead ? newNode

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Deleting from Linked List

• Easy to delete head node
• Save value of headreturnMe ? head
• Move head to next nodehead ? head.getNext()
• Return previous headreturn returnMe

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Using Linked Lists

• What data structure does a linked list resemble?
• What is the maximum size the linked list can hold?

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Using Linked Lists

• What is complexity of insert and remove?
• How would you design isEmpty()?
• What is its complexity?

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Using Linked List

• algorithm size
• Node trav ? head
• int count 0
• while (trav ! null)
• count
• trav ? trav.getNext()
• endwhile
• return count
• What is this complexity?

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Linked List and Queues

• What is needed to implement a queue with a linked
  list?
• How would we write this algorithm?

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DLNode

• public class DLNode
• / prev links to the previous DLNode in the
  linked list /
• protected DLNode prev
• / next links to the next DLNode in the linked
  list /
• protected DLNode next
• / element is the data stored at this node /
• protected Object element
• // Not shown here constructors, get set //
  routines

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Deque

• Pronounced deck
• Avoids mistaking it for dequeue method
• Stands for double ended queue
• Enables inserting and removing items at both
  front and rear
• Uses doubly-linked list
• So uses DLNode rather than Node

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Deque ADT

• Defines the following methods
• insertFirst(Object e), insertLast(Object e)
• Object removeFirst(), Object removeLast()
• Object first(), Object last()
• int size()
• boolean isEmpty()
• What exceptions should be thrown?

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Daily Quiz 2

• Use the Queue interface, Node,
  EmptyQueueException classes found on the quiz web
  page
• Implement a linked-list based queue
• You do not need to do any javadoc documentation
• But you can (it is worth 30 of programming
  assignment grades)!