Stacks, Queues, and Bucket Sort

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Stacks, Queues, and Bucket Sort

• COMP 114
• Thursday April 11

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Announcements

• Program 2 grading
• If you used a String in a class like Reference to
  store the data, see TA for grade change
• 10 pt. Instead of 20 pt.
• Program 4 due Tuesday

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Topics

• Last Time
• Lists

• This Class
• Stacks
• Queues

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Stacks

• A stack is a last-in, first-out data structure.
• Think of a stack of paper or books
• put paper on top
• remove paper from top
• These operations are called push and pop
• push onto the stack
• pop from the stack

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Processor Stack

• Stack used to store return locations for calls to
  methods
• Note how return follows stack order
• New call, push to stack
• Return, always top of stack
• More info later

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Interface

• push(Object p)
• Same as add(Object p)
• Object pop()
• Same as remove()
• Object peek() Optional
• int size() Optional

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Implementations

• Several standard ways
• List
• Vector/Array
• Combination

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List
Head of list is latest element.
Head
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List pushing to stack
Head
10
List pushing to stack
Head
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Popping stack
Head
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Popping stack
Head
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Popping stack
Head
Return data.
data5
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Vector Implementation
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Size 6
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Push
0
Size 6
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Push
0
Size 7
1
2
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Pop
0
Size 7
1
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Pop
0
Return ref. to data
Size 6
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data6
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Real Implementations

• Would you implement a processors stack like
  this?
• Advantages
• Disadvantages
• Normally with hardware assist
• Stack pointer

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List of Stacks
Theyre more than just arrays. Have to keep size
and next location.
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What Good Are Stacks?

• Used to implement method (function) calls
• Call frame
• Program counter
• Parameters
• Local variables
• Have you run out of stack space?
• Of course can be used to do recursion manually

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Queues

• First in, first out, or FIFO
• Line at movie or grocery-store checkout
• Print queue
• Operations
• enqueue
• dequeue

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Interface

• enqueue(Object p)
• Same as add(Object p)
• Object dequeue()
• Same as Object remove()
• Object peek()

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Implementations

• List based
• Vector based
• Circular array
• Fixed size

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List (doubly linked)
Tail of list is latest element.
Tail
Head
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Enqueue
Tail
Head
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Enqueue
Tail
Head
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Dequeue
Remove oldest element
Tail
Head
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Dequeue
Remove oldest element
Tail
Head
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Dequeue
Return
Tail
data1
Head
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Vector-Based Queue
0
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Queue new entries to end.
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Vector-Based Queue
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Dequeue entries from beginning.
Whats the cost?
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Array-Based Queues

• If we have an upper bound, an array has multiple
  advantages
• We can wrap around

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Array-Based Queue
0
1
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Enqueue
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Array-Based Queue
0
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Array-Based Queue
0
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Enqueue another??
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Array-Based Queue
0
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Warp around!
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Array-Based Queue
0
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2
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How do you compute wrap?

• Mod operator
• location location size_of_stack
• Like remainder
• Divides by size of stack, return remainder
• Examples
• 4 5 ? 4
• 5 5 ? 0
• 6 5 ? 1

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What if you run out?

• Some queues are fixed size
• Old keyboard queue in DOS
• Tossed out newer keystrokes
• You could toss older data in other apps
• Maybe for a temperature monitor
• Alternative Copy

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Priority Queues

• A hospital
• Triage
• Used to schedule jobs/processes
• By priority
• Assign a range of priorities
• Keep a separate list-based queue for each

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Many Uses in Computers

• Most hardware has queues
• Called FIFOs
• Characters input
• Ethernet

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Summary

• Stacks
• Used for subroutine calls, local variables
• Queues
• Used to buffer data

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Problem

• In creating something like a Linked List class,
  you need to access LinkedListNode
• But want to keep it private from rest of program
• Can use package and keyword protected
• How to make package
• Then how to use protected

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How to Make Package

• First make empty VJ project with same name as
  package.
• I named example lastra

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Create or Copy Files

• For my example, Ill copy ReadURL and Keyboard

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Make Classes part of Package

• Add package lastra to top of files.

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Compile (Build)

• A folder names lastra will be created in project

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Copy Folder

• Copy the folder to a convenient place
• I used c\java
• Youll use this as item in CLASSPATH

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Use New Package

• Create new project
• Add c\java to classpath tab in Project
  Properties

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Add Import to Classes

• Add import directive, just as when using other
  packages
• Can now use package classes

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Dont Forget!

• If you make changes, you must copy again to
  package folder
• Alternatively, you could import from location of
  VJ project

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Protected

• You can now make the ListNode constructor
  protected
• Only accessible from classes in package

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Inner Classes

• Alternative way to hide LinkedListNode
• public class LinkedList
• private LinkedListNode head
• ...
• private class LinkedListNode
• private Object data
• private LinkedListNode next
• private LinkedListNode(...)
• data v
• next n

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Inner Classes vs. Package

• Package allows access from a number of classes
• Maybe necessary if you plan to have a lot of
  functionality
• Maybe some type of sort
• Different types of Linked Lists that use common
  code base

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Examples

• On the class code page
• MinimalLinkedList_Inner.zip
• MinimalLinkedList_Package.zip

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Next Time

• Trees
• more complex data structure
• often built on lists
• Good for searching