Chapter 10- Using Select Case

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In the previous lesson, we have learned how to control the program flow using the If…ElseIf control structure. In this lesson, you will learn another way to control the program flow in Visual Basic 2010, that is, the Select Case control structure.
Select Case control structure is slightly different from the If….ElseIf control structure . The difference is that the Select Case control structure basically only make decision on one expression or dimension (for example the examination grade) while the If …ElseIf statement control structure may evaluate only one expression, each If….ElseIf statement may also compute entirely different dimensions. Select Case is preferred when there exist multiple conditions because using If…Then..ElseIf statements will become too messy.
10.1 The Select Case…End Select Structure

The format of the Select Case control structure is as follows: Select Case test expression
Case expression list 1
Block of one or more Visual Basic 2010 statements
Case expression list 2
Block of one or more Visual Basic 2010 Statements
Case expression list 3
Block of one or more Visual Basic 2010 statements
Case expression list 4
.
.
.
Case Else
Block of one or more Visual Basic 2010 Statements
End Select

The usage of Select Case is shown in the following examples

Example 10.1

‘ Examination Grades
Dim grade As String
Private Sub Compute_Click( )
grade=txtgrade.Text
Select Case grade
Case “A”
Label1.Text=”High Distinction”
Case “A-”
Label1.Text=”Distinction”
Case “B”
Label1.Text=”Credit”
Case “C”
Label1.Text=”Pass”
Case Else
Label1.Text=”Fail”
End Select
End Sub

Example 10.2

In this example, you can use the keyword Is together with the comparison operators.
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
‘Examination Marks
Dim mark As Single
mark = mrk.Text
Select Case mark
Case Is >= 85
Label1.Text= “Excellence”
Case Is >= 70
Label2.Text= “Good”
Case Is >= 60
Label3.Text = “Above Average”
Case Is >= 50
Label4.Text= “Average”
Case Else
Label5.Text = “Need to work harder”
End Select
End Sub

Example 10.3 Example 10.2 can be rewritten as follows:
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
‘Examination Marks
Dim mark As Single
mark = Textbox1.Text
Select Case mark
Case 0 to 49
Label1.Text = “Need to work harder”
Case 50 to 59
Label1.Text = “Average” s
Case 60 to 69
Label1.Text= “Above Average”
Case 70 to 84
Label1.Text = “Good”
Case 85 to 100
Label1.Text= “Excellence”
Case Else
Label1.Text= “Wrong entry, please reenter the mark”
End Select
End Sub

Example 10.4 Grades in high school are usually presented with a single capital letter such as A, B, C, D or E. The grades can be computed as follow:
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
‘Examination Marks
Dim mark As Single
mark = TextBox1.Text
Select Case mark
Case 0 To 49
Label1.Text = “E”
Case 50 To 59
Label1.Text = “D”
Case 60 To 69
Label1.Text = “C”
Case 70 To 79
Label1.Text = “B”
Case 80 To 100
Label1.Text = “A”
Case Else
Label1.Text = “Error, please reenter the mark”
End Select
End Sub

The output of Example 10.4