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Lesson 11 : Looping


Visual Basic 2012 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 2012, we have three types of Loops, they are the For.....Next loop, the Do loop. and the While.....End while loop

11.1 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; 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 Do Loop syntaxes are

a)

Do While condition
	Block of one or more Visual Basic 2012 statements
Loop

b)

Do
	Block of one or more Visual Basic 2012 statements
Loop While condition

c)

Do Until condition
	Block of one or more Visual Basic 2012 statements
Loop

d)

Do
	Block of one or more Visual Basic 2012 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
	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
	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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