Control and Loop Structures in VB 6.0

 CONTROL STRUCTURES IN VB 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, NestedIf...Then...Else and Select...Case

 

i. 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

Example

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

 

ii. 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

Example

If average>50 Then
txtGrade.Text = "Pass"
Else
txtGrade.Text = "Fail"
End If

 

iii. 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

Example

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

 

iv. 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

Example

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 VB 6.0

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


i. Do While... Loop Statement

The Do While...Loop is used to execute statements until a certain condition is met.

Example

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.


ii. While... Wend Statement

A While...Wend statement behaves like the Do While...Loop statement.

Example

The following While...Wend counts from 1 to 100

Dim number As Integer
number = 1
While number <=100
number = number + 1
Wend


iii. Do...Loop While Statement

The Do...Loop While statement first executes the statements and then test the condition after each execution.

Example

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.


iv. 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.

Example

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.


v. 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.

Example

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 to 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

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