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VB2010 Lesson 11- Looping

Visual Basic 2010 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 2010, we have three types of Loops, they are the For.....Next loop, the Do loop. and the While.....End while loop

 

11.1  For....Next Loop

The format is:  

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.....sstatement. 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  Do Loop

 

The formats are

 

a)   Do While condition

 

            Block of one or more VB statements

 

      Loop

 

b)   Do 
          

            Block of one or more VB statements 


      Loop While condition

 

c)    Do Until condition 


              Block of one or more VB statements


       Loop

 

d)    Do 
            

          Block of one or more VB 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

 

In the above  example, we find the arithmatic 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 arithmatic summation

 

The output is displayed below:

 

 

 

11.3  While ...End While Loop

 

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

 

the following format:

 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

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