Conditional Statements and Loops in Visual Basic
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
- Create a new Visual Basic project called "Constructs"
- Right-click the form and click View Code
- Select Form1 Events using the drop-down list at the top of the code editor window
- Select the Click event using the drop-down list at the top of the code editor window.
- Implement the event handler as follows:
Private Sub Form1_Click (sender As Object, e As EventArgs) Handles Me.Click
If BackColor <> Color.Red Then
BackColor = Color.Red
Exit Sub
End If
If BackColor = Color.Red Then
BackColor = Color.Blue
Exit Sub
End If
End Sub
Run the application and test it by clicking anywhere in the form to toggle the form's background colour between blue and red.
The If . . . Then . . . Else statement
This construct 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
Reopen the Constructs project if you have closed it, and change the form's Click event as follows:
Private Sub Form1_Click (sender As Object, e As EventArgs) Handles Me.Click
If BackColor = Color.Red Then
BackColor = Color.Blue
Else
BackColor = Color.Red
End If
End Sub
This code does exactly the same as the previous example, but in a slightly different way. Run the application and test it by clicking anywhere in the form to toggle its colour.
The If . . . Then . . . ElseIf statement
This construct is similar to 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 is found, and its statement has been executed, the program terminates the conditional search at End If. If none of the conditions are true, a default condition can be provided by adding a final Else section. The default condition must be the last in the list, and its associated statement will be executed only if none of the other conditions are true.
Exercise
Reopen the Constructs project if you have closed it, and change the form's Click event as follows:
Private Sub Form1_Click (sender As Object, e As EventArgs) Handles Me.Click
If BackColor = Color.Red Then
BackColor = Color.Blue
ElseIf BackColor = Color.Blue Then
BackColor = Color.Green
ElseIf BackColor = Color.Green Then
BackColor = Color.Black
Else
BackColor = Color.Red
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
Reopen the Constructs project if you have closed it, and change the form's Click event as follows:
Private Sub Form1_Click (sender As Object, e As EventArgs) Handles Me.Click
Select Case BackColor
Case Color.Red
BackColor = Color.Blue
Case Color.Blue
BackColor = Color.Green
Case Color.Green
BackColor = Color.Black
Case Else
BackColor = Color.Red
End Select
End Sub
Run the application and test it by clicking anywhere in the form to cycle its colour.
The Do While . . . Loop statement
A loop is a programming construct that is used to carry out an action repeatedly. Visual Basic provides a number of loop constructs. The first loop construct we will examine is the Do While . . . Loop, which examines a condition and executes the statement (or statements) within the loop if the condition is true, after which it will examine the condition once more.
As long as the condition remains true, the statement will continue to be executed and the condition re-examined. If the condition is false when the loop begins, the statement will never execute. Otherwise, once the condition becomes false, the program will exit the loop and the statement will not be executed again. Care must be taken to ensure that a false condition can exist at some point, or the program will be stuck in a loop it cannot break out of.
The construction of this loop is as follows:
Do While Condition
Statement (s)
Loop
The Do . . . Loop While statement
The Do . . . Loop While construct is similar to the Do While . . . Loop construct, except that the statement(s) within the loop will always execute at least once, because the condition is not tested until the end of the loop.
The construction of this loop is as follows:
Do
Statement (s)
Loop While Condition
Exercise
Reopen the Constructs project if you have closed it, and change the form's code as shown below. The code demonstrates the use of both the Do While . . . Loop construct and the Do . . . loop While construct. Note that we have added a timer subroutine that takes a number of ticks as its argument. Microsoft defines a tick as an interval of 100 nanoseconds.
Sub myTimer (Interval As Long)
Dim StartTime As New DateTime ()
Dim EndTime As New DateTime ()
Dim TimeElapsed As Long
TimeElapsed = 0
StartTime = DateTime.Now ()
Do While TimeElapsed < Interval
EndTime = DateTime.Now ()
'get time elapsed in 100 nanosecond intervals
TimeElapsed = EndTime.Ticks () - StartTime.Ticks ()
Loop
End Sub
Private Sub Form1_Click (sender As Object, e As EventArgs) Handles Me.Click
Dim GreyScale As Integer
BackColor = Color.Black
Do
BackColor = Color.FromArgb (GreyScale, GreyScale, GreyScale)
Me.Refresh ()
GreyScale = GreyScale + 1
myTimer (1000000) 'one tenth of a second
Loop While GreyScale < 255
MsgBox ("Cycle complete.")
End Sub
Run the application and test it by clicking anywhere in the form. The form's background color should slowly fade from black to white. Note that the value passed to the myTimer() function represents a multiple of 100 nanoseconds and determines the time taken to complete the loop.
The Do Until . . . Loop statement
The Do Until . . . Loop construct is virtually identical to the Do While . . . Loop construct except that the statement(s) within the loop are executed only while the condition is false.
The construction of this loop is as follows:
Do Until Condition
Statement(s)
Loop
The Do . . . Loop Until statement
The Do . . . Loop Until construct is virtually identical to the Do . . . Loop While construct, except that the statement(s) within the loop are executed only while the condition is false. As with the Do . . . Loop While construct, the statement(s) within the loop will always execute at least once, because the condition is not tested until the end of the loop.
The construction of this loop is as follows:
Do
Statement(s)
Loop Until Condition
The For ... To ... Next loop
The keyword For indicates the start of a counting loop. Counting loops are used to execute a statement (or series of statements) a specific number of times. The first kind of counting loop we shall look at is the For . . . To . . . Next loop.
The construction of the For . . . To . . . Next loop is as follows:
For Counter = Start To End
Statement(s)
Next
Execution of the loop begins with Counter set to the value specified by Start, and examines whether this value is greater than the value specified by End. If not, any statements within the loop are executed, and the value of Counter is incremented by 1.
This process continues until the value of Counter becomes equal to the value of End, at which point execution of the loop stops. A variation on the counting loop uses the Step keyword to set the value by which Counter is incremented each time through the loop by a specified value, as shown below.
For Counter = Start To End Step Increment
Statement(s)
Next Counter
Exercise
Reopen the Constructs project if you have closed it, and change the form's code as shown below. The code does exactly the same thing as the previous example, but this time it uses the For . . . To . . . Next counting loop.
Sub myTimer (Interval As Long)
Dim StartTime As New DateTime ()
Dim EndTime As New DateTime ()
Dim TimeElapsed As Long
TimeElapsed = 0
StartTime = DateTime.Now ()
Do While TimeElapsed < Interval
EndTime = DateTime.Now ()
'get time elapsed in 100 nanosecond intervals
TimeElapsed = EndTime.Ticks () - StartTime.Ticks ()
Loop
End Sub
Private Sub Form1_Click (sender As Object, e As EventArgs) Handles Me.Click
Dim GreyScale As Integer
BackColor = Color.Black
For GreyScale = 0 To 255
BackColor = Color.FromArgb (GreyScale, GreyScale, GreyScale)
Me.Refresh ()
myTimer (1000000) 'one tenth of a second
Next GreyScale
MsgBox ("Cycle complete.")
End Sub