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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. We can also write a program that prompts the user to enter data repeatedly until he or she enters the word ‘Finish’. In Visual Basic 2012, there are three types of Loops, they are the For…..Next loop, the Do loop. and the While…..End while loop
11.1 Looping using the For….Next Loop
The syntax is:
For counter=startNumber to endNumber (Step increment) One or more VB statements Next
To exit a For…..Next Loop, you can place the Exit For statement within the loop, please 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
The process will stop when n is greater than 6.
11.2 The Do Loop
The Do Loop structures are
a)
Do While condition Block of one or more statements Loop
b)
Do Block of one or more statements Loop While condition
c)
Do Until condition Block of one or more statements Loop
d)
Do Block of one or more statements Loop Until condition
* 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. Let’s examine the following examples
Example 11.2(a)
Do while counter
* 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
ListBox1.Items.Add("n" & vbTab & "Sum")
ListBox1.Items.Add("----------------------")
Do
n += 1
sum += n
ListBox1.Items.Add(n & vbTab & sum)
If n = 100 Then
Exit Do
End If
Loop
End Sub
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