Lesson 9: Looping

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In lesson 7 and lesson 8, we have learned how to handle decisions making process using If...Then...Else and also Select Case program structures in Visual Basic. Another procedure that involves decisions making is looping. Visual Basic allows a procedure to be repeated many times until a condition or a set of conditions is fulfilled. This is generally called looping . Looping is a very useful feature of Visual Basic because it makes repetitive works easier. There are  two kinds of loops in Visual Basic,  the Do...Loop and the For.......Next loop.

9.1 Do Loop

The Do Loop statements have three different forms, as shown below:

a) Do While condition

     Block of one or more VB statements

     Loop


b)   Do
    Block of one or more VB statements
     Loop While condition

 

c)    Do Until condition
              Block of one or more VB statements
       Loop


d)    Do
     Block of one or more VB statements

       Loop Until condition

Example 9.1

Do while 
counter <=1000 
num.Text=counter
counter =counter+1
Loop 

* The above example will keep on adding until counter > 1000

The above example can be rewritten as

Do 
counter=counter+1 
Loop until counter>1000 
 

9.2 Exiting the Loop

Sometime we need exit to exit a loop earlier when a certain condition is fulfilled. The keyword to use is  Exit Do. You can examine Example 9.2 for its usage.

Example 9.2

Dim sum, n As Integer
Private Sub Form_Activate()
List1.AddItem "n" & vbTab & "sum"									
Do
n = n + 1									
Sum = Sum + n										
List1.AddItem n & vbTab & Sum

If n = 100 Then Exit Do End If Loop End Sub

Explanation

In the above example, we compute the summation of 1+2+3+4+……+100.  In the design stage, you need to insert a ListBox into the form for displaying the output, named List1. The program uses the AddItem method to populate the ListBox. The statement List1.AddItem "n" & vbTab & "sum" will display the headings in the ListBox, where it uses the vbTab function to create a space between the headings n and sum.

9.3  For....Next Loop

The For....Next Loop event procedure is written as follows:
 

For counter=startNumber to endNumber (Step increment)

    One or more VB statements

Next

Example 9.3 a


For counter=1 to 10
display.Text=counter
Next

Example 9.3 b

For counter=1 to 1000 step 10
counter=counter+1
Next

Example 9.3 c

For counter=1000 to 5 step -5
counter=counter-10
Next

Sometimes the user might want to get out from the loop before the whole repetitive process is executed, the command to use is Exit For. To exit a For….Next Loop, you can place the Exit For statement within the loop; and it is normally used together with the If…..Then… statement. Its usages is shown in Example 9.3 d.

Example 9.3 d

Private Sub Form_Activate( )
For n=1 to 10
If n>6 then
Exit For
End If
Else
Print n
End If
End Sub

 

9.4 Nested For...Next  Loop

When you have a loop within a loop, then you have created a nested loop. You can actually have as many loops as you want in a nested loop provided the loops are not the never-ending type. For a nested loop that consists of two loops, the first cycle of the outer loop will be processed first, then it will process the whole repetitive process of the inner loop, then the second cycle of the outer loop will be processed and again the whole repetitive process of the inner loop will be processed. The program will end when the whole cycle of the outer loop is processed.  

The Structure of a nested loop is : 

For counter1=startNumber to endNumber (Step increment)

    For counter2=startNumber to endNumber (Step increment)

        One or more VB statements

    Next counter2

Next  counter1

Example 9.4

Private Sub Form_Activate ( )
For firstCounter= 1to 5
Print "Hello"
For secondCounter=1 to 4
Print "Welcome to the VB tutorial"

Next secondCounter Next firstCounter Print"Thank you"

End Sub

Figure 9.1

The output of the above program is shown in Figure 9.1. As the outer loop has five repetitions, it will print the word “Hello” five times. Each time after it prints the word “Hello”, it will print four lines of the “Welcome to the VB tutorial” sentences as the inner loop has four repetitions.

9.5 The While….Wend Loop

The structure of a While….Wend Loop is very similar to the Do Loop. it takes the following form:

 While condition

        Statements

Wend

The above loop means that while the condition is not met, the loop will go on. The loop will end when the condition is met. Let’s examine the program listed in example 9.4.

Example 9.5

Dim sum, n  As Integer
Private Sub Form_Activate()
List1.AddItem "n" & vbTab & "sum"
While n <> 100
n = n + 1
Sum = Sum + n
List1.AddItem n & vbTab & Sum
Wend
End Sub

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