Control Statements are used to control the flow of program's execution. Visual Basic supports control structures such as if... Then, if...Then ...Else, Select...Case, and Loop structures such as Do While...Loop, While...Wend, For...Next etc method.
If...Then selection structure
The If...Then selection structure performs an indicated action only when the condition is True; otherwise the action is skipped.
Syntax of the If...Then selection
If <condition> Then
statementEnd Ife.g.: If average>75 ThentxtGrade.Text = "A"End IfIf...Then...Else selection structure
The If...Then...Else selection structure allows the programmer to specify that a different action is to be performed when the condition is True than when the condition is False.
Syntax of the If...Then...Else selection
If <condition > Then
statements
Else
statements
End If
e.g.: If average>50 Then
txtGrade.Text = "Pass"
Else
txtGrade.Text = "Fail"
End If
Nested If...Then...Else selection structure
Nested If...Then...Else selection structures test for multiple cases by placing If...Then...Else selection structures inside If...Then...Else structures.Syntax of the Nested If...Then...Else selection structureYou can use Nested If either of the methods as shown aboveMethod 1If < condition 1 > ThenstatementsElseIf < condition 2 > ThenstatementsElseIf < condition 3 > ThenstatementsElseStatementsEnd IfMethod 2
If < condition 1 > Then
statements
Else
If < condition 2 > Then
statements
Else
If < condition 3 > Then
statements
Else
Statements
End If
End If
EndIf
e.g.: Assume you have to find the grade using nested if and display in a text box
If average > 75 Then
txtGrade.Text = "A"
ElseIf average > 65 Then
txtGrade.Text = "B"
ElseIf average > 55 Then
txtGrade.text = "C"
ElseIf average > 45 Then
txtGrade.Text = "S"
Else
txtGrade.Text = "F"
End If
Select...Case selection structure
Select...Case structure is an alternative to If...Then...ElseIf for selectively executing a single block of statements from among multiple block of statements. Select...case is more convenient to use than the If...Else...End If. The following program block illustrate the working of Select...Case.
Syntax of the Select...Case selection structure
Select Case Index
Case 0
Statements
Case 1
Statements
End Select
e.g.: Assume you have to find the grade using select...case and display in the text box
Dim average as Integer
average = txtAverage.Text
Select Case average
Case 100 To 75
txtGrade.Text ="A"
Case 74 To 65
txtGrade.Text ="B"
Case 64 To 55
txtGrade.Text ="C"
Case 54 To 45
txtGrade.Text ="S"
Case 44 To 0
txtGrade.Text ="F"
Case Else
MsgBox "Invalid average marks"
End Select
Note: In this example I have used a message box function. In later lessons you will learn how to use message box functions.