1. VB.Net Loops

2. Loop FOR index – start TO end [STEP step] [statements] [EXIT FOR]
NEXT index
DO [{WHILE| UNTIL} condition]
[EXIT DO]
LOOP

3. Do While/Do Until Private Sub Command1_Click() Dim counter As Integer
Do While counter <= 5
Debug.Write(counter)
counter = counter + 1
Loop
Text1.Text = counter
End Sub
Private Sub Button2_Click()
Do Until counter > 5

4. Loop Demo:Future Value
Dim output As String = "Year" & vbTab & "FutureValue" & vbCrLf
Dim output2 As String = "Year" & vbTab & "FutureValue" & vbCrLf
Dim amount, principal, iRate, year As Single
Dim yearIndex As Integer
principal = CSng(TextBox1.Text)
iRate = CSng(TextBox2.Text)
year = CSng(TextBox3.Text)
For yearIndex = 1 To CInt(year)
amount = principal * (1 + iRate) ^ yearIndex
output = output & yearIndex & vbTab & FormatCurrency(amount) & vbCrLf
output2 = output2 & yearIndex & vbTab & String.Format("{0:c}", amount) & vbCrLf
Next
MessageBox.Show(output, "FutureValue", MessageBoxButtons.OK)
MessageBox.Show(output2, "FutureValue", MessageBoxButtons.OK)
Note: Use the Do While, Do Until loops.
Note: vbCrLf is a VB keyword for a new line.

5. Example: Display characters entered in textbox1one at a time
Dim index As Integer
index = 0
For index = 0 To TextBox1.TextLength - 1
MessageBox.Show(TextBox1.Text.Substring(index, 1))
Next
Problem 1: How many “a” entered in textbox1?
Problem 2: How many words entered in textbox1?

6. Dim i, j, wordCount As Integer
Dim trimedText As String
trimedText = TextBox1.Text.Trim()
wordCount = 0
If trimedText.Length = 0 Then
Else
For i = 0 To trimedText.Length - 1
If trimedText.Substring(i, 1) = Chr(Keys.Space) Then
wordCount += 1
For j = i To trimedText.Length - 1
If Not trimedText.Substring(j, 1) = " " Then
i = j
Exit For
End If
Next
MessageBox.Show(wordCount.ToString & "words")

7. Dim i As Integer = 0
Dim j As Integer
Dim wordCount As Integer = 0
Dim searchText As String
searchText = TextBox1.Text
If searchText.Length > 0 Then
Do While True
j = searchText.IndexOf(" ", i)
If j < 0 Then
wordCount += 1
Exit Do
End If
If j = i Then
i += 1
Else
i = j + 1
If i >= searchText.Length Then
Loop
MessageBox.Show(wordCount.ToString & "words")

8. List Box Useful properties Methods Events: SelectedIndexChanged event
Items: The items in the listBox. It is a collection strcture. Items can be entered at the design time or entered with code.
SelectionMode: one or multi selection
SelectedItem(s), SelectedIndex
Methods
Add
Clear
Events: SelectedIndexChanged event

9. List Box Example
Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Load
TextBox1.Clear()
TextBox2.Clear()
ListBox1.Items.Clear()
ListBox1.Items.Add("Apple")
ListBox1.Items.Add("Orange")
ListBox1.Items.Add("Banana")
ListBox1.Items.Add("Strawberry")
TextBox2.Text = ListBox1.Items.Count.ToString
End Sub
Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
TextBox1.Text = ListBox1.SelectedItem

10. Selected Item’s Value
Demo:
Select interest rate from a list box.

11. ListBox Multiple Selections
Note: The SelectionMode property must set to multi selection.
extBox2.Text = ListBox1.SelectedItems.Count.ToString
Dim allSel As String
Dim i As Integer
For i = 0 To ListBox1.SelectedItems.Count - 1
allSel = allSel & ListBox1.SelectedItems.Item(I)
Or allSel = allSel & ListBox1.SelectedItems(I)
Next
‘**** Or use the For Each loop****
Dim item As String
For Each item In ListBox1.SelectedItems
allSel = allSel & item
TextBox3.Text = allSel

12. ListBox Items Collections
Methods:
ADD: ListBox1.Items.Add("Apple")
Item: Retrieve an object from Items
ListBox1.Items.Item(Index)
ListBox1.Items(Index)
0-based index
Insert: ListBox.Items.Insert(Index, item)
Remove: Delete an object with a position index or key.
ListBox.Items.Remove(Item)
ListBox.Items.RemoveAt(Index)
Listbox1.Refresh()
Clear: ListBox.Items.Clear()
Count: Return the number of objects in a collection.

13. Using ListBox to Display Output
ListBox1.Items.Clear()
ListBox1.Items.Add("Year" & vbTab & "FutureValue")
For yearIndex = 1 To CInt(year)
amount = principal * (1 + iRate) ^ yearIndex
ListBox1.Items.Add(yearIndex.ToString & vbTab & FormatCurrency(amount))
Next

14. ComboBox Allows the user to type text directly into the combo box.
Use the Text property to get entered item:
ComboBox1.Text
The index for an entered item is –1.
SelectedItem may be different from the Text property.
Search an item in the list: ComboBox1.Items.IndexOf(“search text”)
Found: return the index of the search text.
Not found: return –1.
How to add an entered item to the list?

15. Unstructured Error Handling
On Error GoTo errorhandler (a label)
Built-in Err object properties:
Number – Error number
Source – Name of VB ile in which error occurred
Description – error message
Continue the program execution:
Resume: returns execution to the statement that caused the error.
Resume Next
Resume label: Jumps to the line containing the label.
Exit Sub

16. Error Handling Example
Private Sub cmdDivide_Click()
On Error GoTo DivideErrorHandler
lalAnswer.Caption=CStr(CDbl(Text1.text)/CDbl(Text2.Text))
Exit Sub
DivideErrorHandler:
MsgBox(CStr(Err.Number) & “: “ & Err.Description)
End Sub

17. Structured Error Handling
Try
result = Val(TextBox1.Text) / Val(TextBox2.Text)
TextBox3.Text = result.ToString
Catch except As DivideByZeroException
MessageBox.Show(except.Message)
Catch except As Exception
Finally
MessageBox.Show("I get exdecuted, no matter what")
End Try

18. Collections Collections are used to store lists of objects.
More flexible than array:
No need to declare the number of objects in a collection.
Objects can be added, deleted at any position.
Object can be retrieved from a collection by a key.
Can store different types of objects.
A collection’s name usually end with a “s”.

19. Using Collections Define a collection: Methods:
Ex. Dim Pets as New Collection
Methods:
ADD: Add object to a collection
Pets.Add(“dog”)
Add an object with a key:
Pets.Add(“Dog”, “D”)
Item: Retrieve an object from a collection with a position index (base 1) or with a key.
petName = Pets.Item(1)
petName = Pets.Item(“D”)
Count: Return the number of objects in a collection.
Remove: Delete an object with a position index or key.

20. Iterating Through a Collection
Dim Pets as New Collection …
Dim Indx as Long
For Indx = 1 to Pets.Count
…operations …
Next Indx
For Each pet in Pets
… operations …
Next pet

21. Enumerations
Provide a way to associate meaningful names with a sequence of constant values.
Define an enumeration using an Enum statement in classes, modules, and structures.
Private Enum seasonOfYear
Spring = 1
Summer = 2
Fall= 3
Winter = 4
End Enum

22. Enumerations
Provide a way to associate meaningful names with a sequence of constant values.
Enum enumTest
spring = 1
summer
fall
winter
End Enum
Dim etest As enumTest
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
etest = enumTest.summer
TextBox1.Text = Val(etest).ToString