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Lesson 11: Looping


Visual Basic 2008 allows a procedure to be repeated as many times as long as the processor and memorycould 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 the word 'Finish'.

In Visual Basic 2008, there are  three types of Loops, they are the For.....Next loop, the Do loop. and the While.....End while loop. We shall examine the structure of each of the loops in details as well as demonstrating them with examples.

11.1 The For....Next Loop

The structure of the For....Next Loop is as follows:

 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 The Do Loop

The are a few ways we can write the code for the do loop

11.2.1

Do While condition 
 Block of one or more VB statements 
Loop

11.2.2

 Do 
   Block of one or more VB statements
 Loop While condition  

11.2.3

Do Until condition 
 Block of one or more VB statements 
Loop  

11.2.4

Do
 Block of one or more VB statements  
Loop Until condition 

11.3 Exiting the Loop

Sometimes we need 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

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 while n<=1000
End Sub

The above example generates the summation of 1+2+3+4+... until n=100.


11.3  While ...End While Loop

The structure of a While...End While is very similar to the Do Loop. Its structure is as follows

 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

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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