Visual Basic 2008 allows a procedure to be repeated as many times as long as the processor and memorycould 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 the word 'Finish'.
In Visual Basic 2008, there are three types of Loops, they are the For.....Next loop, the Do loop. and the While.....End while loop. We shall examine the structure of each of the loops in details as well as demonstrating them with examples.
The structure of the For....Next Loop is as follows:
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… statement. For its application, you can refer to example 11.1 d.
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.
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:
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-..........
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.
The are a few ways we can write the code for the do loop
Do While condition Block of one or more VB statements Loop
Do Block of one or more VB statements Loop While condition
Do Until condition Block of one or more VB statements Loop
Do Block of one or more VB statements Loop Until condition
Sometimes we need 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 example
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim sum, n As Integer Do n += 1 sum += n ListBox1.Items.Add(n & vbTab & sum) If n = 100 Then Exit Do End If Loop while n<=1000 End Sub
The above example generates the summation of 1+2+3+4+... until n=100.
The structure of a While...End While is very similar to the Do Loop. Its structure 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.
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