Control Structures In Visual Basic 6.0

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
statement
End If

e.g.: If average>75 Then
txtGrade.Text = "A"
End If

If...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 structure

You can use Nested If either of the methods as shown above

Method 1

If < condition 1 > Then
statements
ElseIf < condition 2 > Then
statements
ElseIf < condition 3 > Then
statements
Else
Statements
End If

Method 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

Loops (Repetition Structures) In Visual Basic 6

A repetition structure allows the programmer to that an action is to be repeated until given condition is true.

Do While... Loop Statement

The Do While...Loop is used to execute statements until a certain condition is met. The following Do Loop counts from 1 to 100.

Dim number As Integer
number = 1
Do While number <= 100
number = number + 1
Loop

A variable number is initialized to 1 and then the Do While Loop starts. First, the condition is tested; if condition is True, then the statements are executed. When it gets to the Loop it goes back to the Do and tests condition again. If condition is False on the first pass, the statements are never executed.

While... Wend Statement

A While...Wend statement behaves like the Do While...Loop statement. The following While...Wend counts from 1 to 100

Dim number As Integer

number = 1
While number <=100
number = number + 1
Wend

Do...Loop While Statement

The Do...Loop While statement first executes the statements and then test the condition after each execution. The following program block illustrates the structure:

Dim number As Long
number = 0
Do
number = number + 1
Loop While number < 201

The programs executes the statements between Do and Loop While structure in any case. Then it determines whether the counter is less than 501. If so, the program again executes the statements between Do and Loop While else exits the Loop.

Do Until...Loop Statement

Unlike the Do While...Loop and While...Wend repetition structures, the Do Until... Loop structure tests a condition for falsity. Statements in the body of a Do Until...Loop are executed repeatedly as long as the loop-continuation test evaluates to False.

An example for Do Until...Loop statement. The coding is typed inside the click event of the command button

Dim number As Long
number=0
Do Until number > 1000
number = number + 1
Print number
Loop

Numbers between 1 to 1000 will be displayed on the form as soon as you click on the command button.

The For...Next Loop

The For...Next Loop is another way to make loops in Visual Basic. For...Next repetition structure handles all the details of counter-controlled repetition. The following loop counts the numbers from 1 to 100:

Dim x As Integer
For x = 1 To 50
Print x
Next

In order to count the numbers from 1 yo 50 in steps of 2, the following loop can be used

For x = 1 To 50 Step 2
Print x
Next

The following loop counts numbers as 1, 3, 5, 7..etc

The above coding will display numbers vertically on the form. In order to display numbers horizontally the following method can be used.

For x = 1 To 50
Print x & Space$ (2);
Next

To increase the space between the numbers increase the value inside the brackets after the & Space$.

Following example is a For...Next repetition structure which is with the If condition used.

Dim number As Integer
For number = 1 To 10
If number = 4 Then
Print "This is number 4"
Else
Print number
End If
Next

In the output instead of number 4 you will get the "This is number 4".

Exit For And Exit Do Statement In Visual Basic 6

A For...Next loop condition can be terminated by an Exit For statement. Consider the following statement block.

Dim x As Integer
For x = 1 To 10
Print x
If x = 5 Then
Print "The program exited at x=5"
Exit For
End If
Next

The preceding code increments the value of x by 1 until it reaches the condition x = 5. The Exit For statement is executed and it terminates the For...Next loop. The Following statement block containing Do...While loop is terminated using Exit Do statement.

Dim x As Integer
Do While x < 10
Print x
x = x + 1
If x = 5 Then
Print "The program is exited at x=5"
Exit Do
End If
Loop

With...End With statement

When properties are set for objects or methods are called, a lot of coding is included that acts on the same object. It is easier to read the code by implementing the With...End With statement. Multiple properties can be set and multiple methods can be called by using the With...End With statement. The code is executed more quickly and efficiently as the object is evaluated only once. The concept can be clearly understood with following example.

With Text1
.Font.Size = 14
.Font.Bold = True
.ForeColor = vbRed
.Height = 230
.Text = "Hello World"
End With

In the above coding, the object Text1, which is a text box is evaluated only once instead of every associated property or method. This makes the coding simpler and efficient.