Visual Basic 2008 Tutorial

Lesson 10: Select Case Control Structure

 In the previous lesson, we have learned how to control the program flow using the If...ElseIf control structure. In this chapter, you will learn  another way to control the program flow, that is, the Select Case control structure. However, the 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 many different conditions because using If...Then..ElseIf statements might become too messy.

10.1 The Select Case...End Select Structure

The format of the Select Case control structure is show below: Select Case test expression    Case expression list 1         Block of one or more VB statements    Case expression list 2         Block of one or more VB Statements    Case expression list 3         Block of one or more VB statements    Case expression list 4         .         .         .    Case Else         Block of one or more VB Statements End Select

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-"       Label2.Text="Distinction"   Case "B"         Label3.Text="Credit"   Case "C"         Label4.Text="Pass"   Case Else         Label5.Text="Fail"   End Select

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 could 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 = mrk.Text
 
Select Case mark

 Case 0 to 49
 
     Label1.Text = "Need to work harder"
 

Case 50 to 59
 
    Label2.Text = "Average"
 

Case 60 to 69

   Label3.Text= "Above Average"

Case 70 to 84

Label4.Text = "Good"

Case Else

Label5.Text= "Excellence"

End Select

End Sub

 
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