Visual Basic 2008 Tutorial

Lesson 11: Looping

Visual Basic 2008 allows a procedure to be repeated as many times as long as the processor and memory could support. This is generally called  looping . Looping is required when we need to process something repetitively until a certain condition is met. For example, we can design a  program that adds a series of numbers until the sum exceeds a certain value, or a program that asks the user to enter data repeatedly until he/she keys in the word 'Finish'. In Visual Basic 2008, we have three types of Loops, they are the For.....Next loop, the Do loop. and the While.....End while loop  

11.1  For....Next Loop

The format is:  

For counter=startNumber to endNumber (Step increment)

    One or more VB statements

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. For its application, you can refer to example 11.1 d.

Example 11.1 a

Dim counter as Integer

For  counter=1 to 10  

ListBox1.Items.Add (counter)

  Next

* The program will enter number 1 to 10 into the list box.

Example 11.1b

Dim counter , sum As Integer

For counter=1  to 100 step 10  

sum+=counter

ListBox1.Items.Add (sum)

 Next

* The program will calculate the sum of  the numbers as follows:

 sum=0+10+20+30+40+.....

Example 11.1c

 Dim counter, sum As Integer
sum = 1000
For counter = 100 To 5 Step -5
sum - = counter
ListBox1.Items.Add(sum)
Next

*Notice that increment can be negative.

The program will compute the subtraction as follow:

1000-100-95-90-.......

Example 11.1d

Dim n as Integer

For n=1 to 10

If n>6 then

Exit For

End If

Else

ListBox1.Items.Add ( n)

 Next

End If

Next

The process will stop when n is greater than 6.

11.2  Do Loop

The formats are

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

* Exiting the Loop

Sometime we need exit to exit a loop prematurely because of a certain condition is fulfilled. The syntax to use is known as Exit Do. Lets examine the following example

Example 11.2(a)

       Do while counter <=1000

             TextBox1.Text=counter

             counter +=1

       Loop

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

The above example can be rewritten as

        Do

               TextBox1.Text=counter

               counter+=1

       Loop until counter>1000

Example 11.2(b)

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

Dim sum, n As Integer

Do

n += 1

sum += n

ListBox1.Items.Add(n & vbTab & sum)

If n = 100 Then

Exit Do

End If

Loop Sub

In the above  example, we find 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 Add method to populate the ListBox. The statement ListBox1.Items.Add(n & vbTab & sum) will display the values of n  and sum and  uses the vbTab function to create a space between the headings n and sum.  

11.3  While ...End While Loop

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

 While condition

        Statements

End While

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. 

Example 11.3

Dim sum, n  As Integer

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim sum, n As Integer
While n <> 100
n += 1
sum = sum + n
ListBox1.Items.Add(n & vbTab & sum)
End While

End Sub

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