objects, classes, and hierarchies 2 container classes

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objects, classes, and hierarchies 2container
classes
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Types and subtypes
Object

• Types often have subtypes
• Integers are a subtype of numbers
• Boxes are a subtype of Pictures
• All types are subtypes of the magic type Object
• Subtyping means objects can have many types
• All Integers are also Numbers
• All Boxes are also Pictures
• And all objects are Objects

List
Number
Array
Integer
Float
Picture
Box
Line
Group
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Example 2The container class hierarchy
Collection

• An important area of computer science is the
  study of data structures
• Roughly how we put data objects together to make
  them useful
• A very important kind of data structure is the
  container
• A kind of object that holds a collection of
  other objects
• There are many kinds of containers
• Most of the container classes well be using are
  subtypes of List
• All the collection classes in .NET (and so in
  Meta) are in the namespace System.Collections

List
Dictionary
Array
ArrayList
Stack
Hashtable
Object String
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Lists

• Have a bunch of elements
• Have them in a definite order
• You can ask them how many elements they have
• You can ask for an element at a specific position
• You can change the element at a specific position

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Basic list operations

• length list or list.CountTells you how
  many items are in the list
• item list positionReturns the element of list
  at position
• set-item list position new-valueor item
  list position ? new-valueChanges the element
  of list at position

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The simplest list type the array

• An array is a fixed-length list
• Very fast and efficient
• Cant add/remove element after creation
• Although you can change elements
• The list args procedure returns an array
• Technically, the type Object
• Which means an array of arbitrary objects
• There are other kinds of specialized arrays too

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The ArrayList a more powerful list type

• ArrayLists can change size, but are less
  efficient
• using System.CollectionsAccess collection
  classes
• new ArrayListMakes an ArrayList
• x.Add object
• Add object to end of ArrayList x
• x.Insert position object
• Adds element at the specified position
• x.Remove object
• Removes all occurrences of object
• x.RemoveAt position
• Remove element at position
• x.ClearRemoves all elements from x

• Some results omitted to save space
• ?using System.Collections
• ?define mylist new ArrayList
• ? mylist.Add 98
• ? mylist.Add 38
• ? mylist.Add 4
• ? mylist
• 98 38 4
• ? mylist.Remove 38
• ? mylist
• 98 4
• ? mylist.Insert 1 test
• ? mylist
• 98 test 4
• ?

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The stack a simpler collection class

• Stacks can only be changes at their top or
  beginning
• new StackMakes a new stack
• s.Push objectAdds a new object to the top of
  the stack s
• s.PopRemoves the item at the top of s and
  returns it
• s.PeekReturns the item at the top of s without
  removing it
• s.ClearRemoves all data from s

• ? define s new Stack
• System.Collections.Stack
• ? s.Push 1
• ? s.Push 2
• ? s.Push 3
• ? s.Peek
• 3
• ? s.Peek
• 3
• ? s.Pop
• 3
• ? s.Pop
• 2
• ? s.Pop
• 1
• ? s.Pop
• Error Stack empty.

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Dictionary classes

• Dictionaries are objects that store associations
  between pairs of objects
• One object is called the key
• The other the value
• The most common dictionary class is the Hashtable

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The Hashtable class

• new HashtableMakes a new hashtable
• lookup dict key
• Returns the object listed in dict under key
• Or null if key doesnt appear in dict
• store-dictionary dict key valueor lookup
  dict key ? value
• Adds or changes the entry for key in dict to
  value
• dict.ClearRemoves all entries from dict
• dict.Contains keyChecks whether dict contains
  an entry for key

• ? define h new Hashtable
• ? lookup h test
• null
• ? store-dictionary h test
  got it
• "got it"
• ? store-dictionary h test2 something
  different
• "something different"
• ? lookup h test
• "got it"
• ? store-dictionary h test
  changed it
• "changed it"
• ? lookup h test
• "changed it"