Visual Basic 2008 Looping

Visual Basic 2008 memungkinkan prosedur yang akan diulang sebanyak yang selama prosesor dan memori yang dapat mendukung. Hal ini umumnya disebut perulangan. Looping diperlukan bila kita perlu untuk memproses sesuatu yang berulang-ulang sampai kondisi tertentu terpenuhi. Sebagai contoh, kita dapat merancang sebuah program yang menambahkan serangkaian nomor sampai jumlah yang melebihi nilai tertentu, atau program yang meminta pengguna untuk memasukkan data berulang kali hingga kunci dia di kata 'Selesai'. Dalam Visual Basic 2008, kami memiliki tiga jenis 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… 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  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 example	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 Do n += 1 sum += n ListBox1.Items.Add(n & vbTab & sum) If n = 100 Then Exit Do End If Loop Sub In the above  example, we find 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 Add method to populate the ListBox. The statement ListBox1.Items.Add(n & vbTab & sum) will display the values of n  and sum and  uses the vbTab function to create a space between the headings n and sum.
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