Visual Basic 2010 Lesson 11- Looping

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In VB programming, a  sequence of instructions that is repeated until a certain condition is met is called looping.  For example, we can write VB code that adds a series of numbers until the sum exceeds a certain value, or a VB program that asks the user to enter data repeatedly until he or she enters 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 Looping using the For….Next Loop

The syntax for looping using the For…Next loop is:

For counter=startNumber to endNumber
One or more Visual Basic 2010 statements
Next

To exit a For…..Next Loop, you can place the Exit For statement within the loop; and it is typically 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
NextThe process will stop when n is greater than 6.
11.2 Do Loop
The Do Loop formats area) Do While condition
Block of one or more Visual Basic 2010 statements
Loopb) Do
Block of one or more Visual Basic 2010 statements
Loop While conditionc) Do Until condition
Block of one or more Visual Basic 2010 statements
Loop

d) Do
Block of one or more Visual Basic 2010 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. Let’s examine the following examples
 

Example 11.2(a)
Do while counter <=1000TextBox1.Text=counter
counter +=1Loop* 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.ClickDim sum, n As IntegerListBox1.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
LoopEnd Sub

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