Conditional Statements
The If ... Then statement
The If ... Then statement evaluates whether or not an expression is true. The construct is used as follows:
If ConditionToCheck Then Statement
The program examines a condition (ConditionToCheck), which can be a simple expression or a combination of expressions. If ConditionToCheck evaluates to true, the program will execute Statement. Any number of statements can be executed following the Then keyword. If only one statement is to be executed, the If ... Then statement can be written on one line, as shown above. If a number of statements are to be executed, each statement should appear on a separate line, as shown below.
If ConditionToCheck Then
Statement 1
Statement 2
...........
Statement n
End If
Note the use of the End If keywords to terminate this construct.
Exercise
- Start Visual Basic and select Standard EXE by pressing Enter
- Right-click the form and click View Code
- In the Code Editor, click the arrow of the Object combo box and select Form
- Click the arrow of the Procedure combo box and select Click
- Implement the events as follows:
Private Sub Form_Click()
If BackColor = vbRed Then BackColor = vbBlue
End Sub
Private Sub Form_Load()
BackColor = vbRed
End Sub
Run the application and test it by clicking anywhere in the form to change its colour.
The If ... Then ... Else statement
This constructs offers two alternative courses of action. The first alternative is chosen if the specified condition is true, otherwise the second alternative is chosen. The construct is used as follows:
If ConditionToCheck Then
Statement1
Else
Statement 2
End If
Exercise
- Change the form's Click event as follows:
Private Sub Form_Click()
If BackColor = vbRed Then
BackColor = vbBlue
Else
BackColor = vbRed
End If
End Sub
Run the application and test it by clicking anywhere in the form to toggle its colour.
The If ... Then ... ElseIf statement
This construct is like If ... Then ... Else, except that it offers a number of choices. The construct is used as follows:
If Condition1 Then
Statement1
ElseIf Condition2 Then
Statement2
.
.
ElseIf Conditionk Then
Statementk
End If
The program will examine each condition in turn until it finds a condition that is true. Once a true condition has been found, and its statement executed, the program terminates the conditional search at EndIf. If none of the stated conditions are true, a default condition can be provided by adding a last Else section. This must be last in the list of conditions, and its associated statement would be executed only if none of the other conditions were true.
Exercise
- Change the form's Click event as follows:
Private Sub Form_Click()
If BackColor = vbRed Then
BackColor = vbBlue
ElseIf BackColor = vbBlue Then
BackColor = VBGreen
ElseIf BackColor = vbGreen Then
BackColor = VBBlack
Else
BackColor = vbRed
End If
End Sub
Run the application and test it by clicking anywhere in the form to cycle its colour.
The Select Case Statement
If there are a large number of conditions to be tested, an alternative approach is to use the Select Case statement. The construct used is as follows:
Select Case Expression
Case Expression1
Statement1
Case Expression2
Statement2
Case Expressionk
Statementk
End Select
The condition described by Expression is evaluated, and compared with each of the expressions (Expression1, Expression2 etc.) that follow. Once a match is found, the corresponding statement is executed. If none of the stated conditions are true, a default condition can be provided by adding a Case Else statement at the end of the list.
Exercise
- Change the form's Click event as follows:
Private Sub Form_Click()
Select Case BackColor
Case vbRed
BackColor = vbBlue
Case vbBlue
BackColor = VBGreen
Case vbGreen
BackColor = VBBlack
Case Else
BackColor = vbRed
End Select
End Sub
Run the application and test it by clicking anywhere in the form to cycle its colour.
