Select Case Statements and Loops

• Lecture three covers Select Case statements and Loops.

Select Case

• Enables a variable to be tested for equality against a list of values.
• Each value constitutes a case.
• The variable being switched on is checked for each select case.
• Syntax:
  • Select Case expressionCase value1
  • Case value1
  • Statements
  • Case value2
  • Statements
  • Case Else
  • Statements
  • End Select
• Expression must evaluate to any elementary data type in VB.Net like Boolean, Byte, Char, Date, Double, Decimal, Integer, Long, Object, SByte, Short, Single, String, UInteger, ULong, and UShort.
• Expressionlist is a list of expression clauses, which represent matching values for the expression.
  • Multiple expression clauses are separated by commas.
• Statements are the code that runs following the case that runs if the select expression matches any clause in the expression list.
• Else statements are the code that runs following the default Case, if the select expression does not match any clause in the expressionlist of any of the Case statements.
• Example program creates a Windows Forms Application project called Select Demo.
  • Sets the Text property of the form to Select Case.
  • Adds a ListBox control named 1stData with IntegralHeight set to False and Dock to Fill.
  • Fills the 1stData listbox with the names: Omer, Ahmed, Ali, and Noor.
  • The program displays a message box indicating the person's favorite color depending on the name selected in the listbox.
• VB.NET code example:
  • Private Sub lstData_SelectedIndexChanged(sender As Object, e As EventArgs) Handles lstData.SelectedIndexChanged
  • Dim strName, strFavoriteColor As String
  • strName = lstData.Items(lstData.SelectedIndex).ToString
  • Select Case strName
  • Case "Omer": strFavoriteColor = "Yellow"
  • Case "Ahmed": strFavoriteColor = "Blue"
  • Case "Ali": strFavoriteColor = "Green"
  • Case "Noor": strFavoriteColor = "white"
  • End Select
  • MessageBox.Show(strName & "'s favorite color is " & strFavoriteColor, "Select Demo")
  • End Sub
• The select case statement can also be used to display a person's gender depending on the name selected.
  • Example VB.NET code:
    • Select Case strName.ToLower
    • Case "Omer", “Ahmed", “Ali": MessageBox.Show("This person's gender is male.", "Select Demo")
    • Case “Noor", “Rana": MessageBox.Show("This person's gender is female.", "Select Demo")
    • End Select
• The Case Else statement can be used to catch all remaining unmatched cases.
  • Example VB.NET code:
    • Select Case strName
    • Case “Ahmed", “Omer": MessageBox.Show("This person's gender is male.", "Select Demo")
    • Case “Noor", “Teba", “Zainab": MessageBox.Show("This person's gender is female.", "Select Demo")
    • Case Else: MessageBox.Show("I don't know this person's gender.", "Select Demo")
    • End Select

Loops

• When writing computer software, you often need to perform the same task several times.
• For example: creating a telephone bill for all customers or reading in ten files from your computer's disk.
• For Loops: These loops occur a certain number of times, like ten times.
• Do Loops: These loops keep running until a certain condition is reached, like until all the data is processed.
• For...Next Loop: loops a specific number of iterations
  • Create a new Windows Forms Application project called Loops.
  • Add a ListBox and a Button control to the form.
  • Change the Name property of the ListBox to 1stData and its IntegralHeight property to False.
  • Change the Name property of the button to btnForNextLoop.
  • Set its Text property to For NextLoop.
  • Double-click the button to create its Click event handler and add the following code:
    • Private Sub btnForNextLoop_Click(sender As Object, e As EventArgs) Handles btnForNextLoop.Click
    • Dim intCount As Integer
    • lstData.Items.Clear()
    • For intCount = 1 Το 5
    • lstData.Items.Add("I'm item " & intCount.ToString & " in the list!")
    • Next
    • End Sub
• For...Next Loop with Step: loops a specific number of iterations with a specified increment.
  • Return to the Forms Designer for the Loops project.
  • Add a Button control to your form.
  • Set its Name property to btnForNextLoopWithStep.
  • Set its Text property to For Next Loop w/ Step.
  • Double-click the button and add the following code in the Click event handler:
    • Private Sub btnForNextLoopWithStep_Click(sender As Object, e As EventArgs) Handles btnForNextLoopWithStep.Click
    • lstData.Items.Clear()
    • For intCount As Integer = 4 Το 62 Step 7
    • lstData.Items.Add(intCount.ToString)
    • Next
    • End Sub
• Looping Backward: loops in reverse order
  • Return to the Forms Designer and add another Button control to your form.
  • Set its Name property to btnBackwardsForNextLoop.
  • Set its Text property to Backwards For Next Loop.
  • Double-click the button and add the following code in the Click event handler:
    • Private Sub btnBackwardsForNextLoop_Click(sender As Object, e As EventArgs) Handles btnBackwardsForNextLoop.Click
    • lstData.Items.Clear()
    • For intCount As Integer = 10 Το 1 Step -1
    • lstData.Items.Add(intCount.ToString)
    • Next
    • End Sub
• For Each Loop: loops through each item in a collection.
  • Return to the Forms Designer, add another Button control to your form.
  • Set its Name property to btnForEachLoop and its Text property to For Each Loop.
  • Double-click the button and add the following code in the Click event handler:
    • Private Sub btnForEachLoop_Click(sender As Object, e As EventArgs) Handles btnForEachLoop.Click
    • lstData.Items.Clear()
    • For Each strFolder As String In My.Computer.FileSystem.GetDirectories("C:")
    • 1stData.Items.Add(strFolder)
    • Next
    • End Sub
• Do Until...Loop: executes a block of code until a condition becomes true.
  • Return to the Forms Designer in the Loops project, add another Button control to your form.
  • set its Name property to btnDoUntilLoop.
  • set its Text property to Do Until Loop.
  • Double-click the button and add the following code to its Click event handler:
    • Private Sub btnDoUntilLoop_Click(sender As Object, e As EventArgs) Handles btnDoUntilLoop.Click
    • Dim objRandom As New Random
    • Dim intRandomNumber As Integer = 0
    • 1stData.Items.Clear()
    • Do Until intRandomNumber = 10
    • intRandomNumber = objRandom.Next(25)
    • 1stData.Items.Add(intRandomNumber.ToString)
    • Loop
    • End Sub
• Do While...Loop: executes a block of code as long as a condition is true.
  • Return to the Forms Designer and add another Button control to your form.
  • Set its Name property to btnDoWhileLoop.
  • Set Text property to Do While Loop.
  • Double-click the button and add the following code to the Click event handler:
    • Private Sub btnDoWhileLoop_Click(sender As Object, e As EventArgs) Handles btnDoWhileLoop.Click
    • Dim objRandom As New Random
    • Dim intRandomNumber As Integer = 0
    • ClearList()
    • Do While intRandomNumber < 15
    • intRandomNumber = objRandom.Next(25)
    • lstData.Items.Add(intRandomNumber.ToString)
    • Loop
    • End Sub