Lesson 9: Looping

Visual Basic allows a procedure to be repeated many times as long as the processor until a condition or a set of conditions is fulfilled. This is generally called looping . Looping is a very useful feature of Visual Basic because it makes repetitive works easier. There are  two kinds of loops in Visual Basic,  the Do...Loop  and the For.......Next loop

9.1  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

Example 9.1

       Do while counter <=1000

             num.Text=counter

             counter =counter+1

       Loop

* The above example will keep on adding until counter >1000.

The above example can be rewritten as

        Do

               num.Text=counter
               counter=counter+1

       Loop until counter>1000

9.2 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. You can examine Example 9.2 for its usage.

Example 9.2

Dim sum, n As Integer

 Private Sub Form_Activate()

List1.AddItem "n" & vbTab & "sum"

Do

   n = n + 1

   Sum = Sum + n

 List1.AddItem n & vbTab & Sum

 If n = 100 Then

 Exit Do

 End If

  Loop

End Sub

Explanation

In the above  example, we compute the summation of 1+2+3+4+……+100.  In the design stage, you need to insert a ListBox into the form for displaying the output, named List1. The program uses the AddItem method to populate the ListBox. The statement List1.AddItem "n" & vbTab & "sum" will display the headings in the ListBox, where it uses the vbTab function to create a space between the headings n and sum.

 

Example 9.3 a For  counter=1 to 10   display.Text=counter   Next	Example 9.3 b For counter=1 to 1000 step 10   counter=counter+1  Next
Example 9.3 c   For counter=1000 to 5 step -5   counter=counter-10    Next *Notice that increment can be negative	Example 9.3 d Private Sub Form_Activate( ) For n=1 to 10 If n>6 then Exit For End If Else Print n End If End Sub

 http://www.vbtutor.net/vb6/vbtutor.html