Chapter 11 Using Collections

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Chapter 11Using Collections

• Advanced Programming Using Visual Basic.NET

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Referencing Collection Items
A collection may consist of a group of
individual items or a collection of objects.

• To reference an individual table in the Tables
  collection
• Assuming that DsStoreSales1 is a dataset based on
  an .xsd file (a strongly typed dataset)

The words collection, list, and data structure
are used interchangeably
OR
DsStoreSales1.Tables("Stores") 'The Stores table
DsStoreSales1.Tables(0) 'Also refers to the
Stores table, assuming that it is the first table
DsStoreSales1.Stores
Referencing Collection Items
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System.Collections Namespace

• The System.Collections namespace provides classes
  for several different types of collections.
• Some types of collections are based on the way
  items are handled.
• A queue is like a line the first item in should
  be the first one out (FIFO)

System.Collections Namespace
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System.Collections Namespace

• In a stack collection, the last one in is the
  first one out (LIFO)
• A dictionary collection consists of a key and
  value pair
• Hashtable
• SortedList
• All of the collections inherit from the same base

System.Collections Namespace
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System.Collections Namespace
Collection Class
System.Collections Namespace
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System.Collections Namespace
Collection Class
System.Collections Namespace
 
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Collection Classes Demo
Dim mstrLanguage As String() "English",
"Spanish", "Chinese", "Italian", "German",
"French", "Tagalog"
Dim mstrList As SortedList Dim mhshTable As
Hashtable Dim mstkList As Stack Dim mqueList As
Queue
System.Collections Namespace
 
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Stacks
An unsorted list, like a stack is not the best
type of list for performing searches.

• An unsorted list
• LIFO
• Use the Push method to add an item to the list.
• Use the Pop method to remove an item from the
  list.
• Use the Peek method to look at the last item
  without removing it.

'Add an item to the stack mstkList.Push(strItem)
'View the last item without removing it strItem
mstkList.Peek()   'Remove the last item from the
list mstkList.Pop()
Using Stacks
 
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Using Queues
'Add an item to the queue mqueList.Enqueue(strItem
)
'View the next item to be removed without
removing it strItem mqueList.Peek()   'Remove
the first item from the list mqueList.Dequeue()

• An unsorted list
• LIFO
• Use the Enqueue method to add items to the queue
• Use the Dequeue method to remove items from the
  queue

Using Stacks
 
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Using Hash Tables

• A dictionary-type collection of key/value pairs
• Fastest type of list for searching
• Keys are calculated using an algorithm that
  produces the same key with every calculation.
• Use the GetHashCode method to calculate the key.
• Use Add and Remove methods to place/remove items
  from the list.

'Calculate the hash code for the key strKey
strItem.GetHashCode.ToString 'Add an item to the
hash table mhshTable.Add(strKey,
strItem)   'Calculate the hash code of a selected
item strItem lstLanguages.SelectedItem.ToString
strKey strItem.GetHashCode.ToString 'Remove the
selected item by key mhshTable.Remove(strKey)
Using Stacks
 
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Using Hash Tables

• Dont use a hash table when
• Data must be in a specific order
• When there may be duplicate keys
• Refer to the collection of items in a hash table
  using the Values property

'Reload the list from the hash table
collection lstLanguages.Items.Clear() For Each
strItem In mhshTable.Values
lstLanguages.Items.Add(strItem) Next
Using Stacks
 
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Sorted Lists
'Create the key strKey strItem.Substring(0,
3) 'Add the key and item to the sorted
list msrtList.Add(strKey, strItem)

• A list of key and value pairs automatically
  sorted by the key
• Use the Add, Remove, and RemoveAt methods
• Access elements by key, by value, or by index
• Keys must be unique and may be created from a
  hash code calculation

'Retrieve the selected item to be removed strItem
lstLanguages.SelectedItem.ToString 'Set the key
as the first three characters of the item strKey
strItem.Substring(0, 3) 'Remove the item by
key msrtList.Remove(strKey)
Using Stacks
'Remove the item by index msrtList.RemoveAt(intInd
ex)
 
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Sorted Lists

• Sorted lists work well when you need to
  add/remove items from the middle of the list.
• Items can be removed from a list by index (but
  indexes change as items are added or deleted).
• Refer to the collection of items in a sorted list
  using the Values property.

'Reload the list box from the sorted list
collection lstLanguages.Items.Clear() For Each
strItem In msrtList.Values
lstLanguages.Items.Add(strItem) Next
Using Stacks
 
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Using Array Lists

• Dynamically increase in size as new elements are
  added
• Use the Capacity property to set the size of the
  list
• Capacity is automatically increased in chunks as
  elements are added
• Use the TrimToSize method to reduce the size of
  the collection

Using Stacks
 
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Using Array Lists
Using Stacks
 
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Using Array Lists
Using Stacks
 
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Creating a Collection of Objects

• Create a collection class and add objects to the
  collection
• Choose the collection class type based on needs
  for speed, sorting, retrieval by value, index, or
  key
• Refer to the member of a collection in two ways
• Specify an index number
• Give each object a unique string key (PIN,
  customer number, account number)

Creating a Collection of Objects
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A Collection of Student Objects

• Create and access a collection of student objects

Creating a Collection of Objects
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Declaring a Collection

• Declare and instantiate the collection at the
  module level.
• Example uses a sorted list and a hash code of the
  name as the key

Dim colStudents As New SortedList()
Creating a Collection of Objects
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Adding Objects to a Collection

• Create the key using the GetHashCode method
• Convert the key to a string
• Add the object to the collection

'Declare and instantiate a new student. Dim
objStudent As New Student(txtName.Text,
CDec(txtGPA.Text)) 'Calculate a key strKey
objStudent.Name.GetHashCode.ToString 'Add to the
collection colStudents.Add(strKey, objStudent)
Creating a Collection of Objects
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Removing an Element from a Collection

• Remove method deletes by key
• RemoveAt removes by index

With lstStudents If .SelectedIndex ltgt -1
Then 'Get the key from the selected
student name Dim strKey As String
.SelectedItem.GetHashCode.ToString
colStudents.Remove(strKey)
DisplayList() Else
MessageBox.Show("Select a student from the
list.", _ "A Collection of
Students") End If End With
Creating a Collection of Objects
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Retrieving an Element from a Collection

• Use the Item property to retrieve an object from
  a list.
• Dictionary-type collections (sorted lists and
  hash tables) return an object of DictionaryEntry
  data type.
• Cast the element to the object type needed.

Dim objStudent As Student   'Get the key for the
selected name Dim strKey As String
.SelectedItem.GetHashCode.ToString objStudent
CType(colStudents.Item(strKey), Student)
Creating a Collection of Objects
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Using For Each / Next

• Use a For Each/Next structure to access each
  object in a collection

'Loop through the collection and display the
items in a list box Dim objStudent As Student Dim
deStudent As DictionaryEntry   lstStudents.Items.C
lear() For Each deStudent In colStudents
objStudent CType(deStudent.Value, Student)
lstStudents.Items.Add(objStudent.Name) Next
Creating a Collection of Objects
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Using an Items Collection

• The Items collection of a text box or combo box
  is a collection of objects.
• If necessary, write a ToString method to override
  the base class

'Procedure in the Client class Public Overrides
Function ToString() As String 'Return the
Name property for this object's ToString method
Return mstrName End Function
Using an Items Collection
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Using an Items Collection
Using an Items Collection

• Add an object to a list box Items collection
• Convert the DictionaryEntry data type to the
  correct type
• Retrieve, convert, and reference individual
  properties of the object.

lstClients.Items.Add(objClient)
objClient CType(.Items(.SelectedIndex), Client)
objClient CType(.Items(.SelectedIndex),
Client) MessageBox.Show("The phone number for "
objClient.Name " is " _ objClient.PhoneNumbe
r)