Visual Basic 2012 allows a procedure to be repeated as many times as long as the processor memory could support. This process is known as looping . We use the Looping procedure 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 2012, we have three types of Loops, they are the For.....Next loop, the Do loop. and the While.....End while loop
The syntax is:
For counter=startNumber to endNumber (Step increment) One or more VB statements Next<
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: sum=0+10+20+30+40+......
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 Do Loop syntaxes are
Do While condition Block of one or more Visual Basic 2012 statements Loop
Do Block of one or more Visual Basic 2012 statements Loop While condition
Do Until condition Block of one or more Visual Basic 2012 statements Loop
Do Block of one or more Visual Basic 2012 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
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 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 statementListBox1.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 statementListBox1.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 11.3Dim 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