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VB2010 Lesson 10- Using Select Case

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 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 followss:

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

10.2 The usage of Select Case is shown in the following exampless

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

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

 

 

 

 

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