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VB2010 Lesson 11- Looping

Visual Basic 2010 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 2010, 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


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.....sstatement. 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)


* 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  


ListBox1.Items.Add (sum)


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


Example 11.1c



Dim counter, sum As Integer

sum = 1000

For counter = 100 To 5 Step -5

sum - = counter





*Notice that increment can be negative.

The program will compute the

subtraction as follow:



Example 11.1d


Dim n as Integer


For n=1 to 10


If n>6 then


Exit For


End If




ListBox1.Items.Add ( n)




End If




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




b)   Do 

            Block of one or more VB statements 

      Loop While condition


c)    Do Until condition 

              Block of one or more VB statements



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 examples





Example 11.2(a)


       Do while counter <=1000




             counter +=1



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


The above example can be rewritten as






       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


ListBox1.Items.Add("n" & vbTab & "Sum")



n += 1

sum += n

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


If n = 100 Then

Exit Do

End If



End Sub


In the above  example, we find the arithmatic summation of 1+2+3+4+......+100. In


the design stage, you need to insert a ListBox into the form for displaying the output 


The program uses the Add method to populate the ListBox. The statement


ListBox1.Items.Add("n" & vbTab & "sum") will display the headings in the ListBox,


where it uses the vbTab function to create a space between the heading n and sum.


The statement  ListBox1.Items.Add(n & vbTab & sum) will list the number n and the


 values of the arithmatic summation


The output is displayed below:




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



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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