Lesson 11 : Looping

In Visual Basic 2010 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 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
 Next

The process will stop when n is greater than 6.

11.2 The Do Loop

The Do Loop structures are

Do While condition
Block of VB 2010 statements
Loop

Do
 Block of VB2010 statements
Loop While condition

Do Until condition
 Block of VB2010 statements
Loop

Do
 Block of VB 2010 statements
Loop Until condition

Sometimes we need to exit a loop prematurely because a condition is fulfilled. The keyword to use is known as Exit Do.Let's examine the following examples

Example 11.2(a)

Do while counter <=1000
 TextBox1.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.Click

Dim 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
Loop
End Sub

In the above example, we find the arithmetic 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 arithmetic summation. The output is displayed below:

11.3 The While...End While Loop

The structure of a While....End While is very similar to the Do Loop. The syntax 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

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