lesson 15 Looping

A procedure in a computer program that runs repeatedly until meeting the certain condition is called looping. A loop can go on repetitively as long as the processor and memory could support.  For example, a program that adds a series of numbers until the sum exceeds a certain value or a program that prompts the user to enter data repeatedly until he or she enters the word ‘Finish’.

In Visual Basic 2013, there are three methods of Looping,  the For…..Next loop, the Do loop, and the While…..End While loop. All methods produce the same repetitive effects.

15.1 Looping using For...Next Loop

The most common looping method in VB 2013 is the For....Next loop. The structure of a For...Next loop is as shown below:

For counter=startNumber to endNumber Step increment
 One or more statements
Next

To exit a For…..Next Loop, you can place the Exit For statement within the loop; it is normally used together with the If….Then statement. For its application, you can refer to example 15.1 d.

Example 15.1 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 15.1b

Dim counter , sum As Integer
For counter=1 to 100 step 10
 sum+=counter
 ListBox1.Items.Add (sum)
Next

* The programme will calculate the sum of the numbers as follows:

sum=0+10+20+30+40+……

Example 15.1c

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 15.1d

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.

15.2 Looping using Do Loop

The Do Loop structures are

a)

Do While condition
Block of one or more statements
Loop

b)

Do
Block of one or more statements
Loop While condition

c)

Do Until condition
Block of one or more statements
Loop

d)

Do
Block of one or more statements
Loop Until condition

Sometimes we need exit to exit a loop prematurely because a certain condition is fulfilled. The syntax to use is Exit Do. Let's examine the following examples:

Example 15.2(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 15.2(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

* The loop in the above example can be replaced by the following loop:

Do Until n = 10
 n += 1
 sum += n
 ListBox1.Items.Add(n & vbTab & sum)
Loop

The output is as shown in Figure 15.1

15.3 Looping using While....End While Loop

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

While conditions

 Visual Basic 2013 statements

End While

Example 15.3

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim sum, n As Integer
 ListBox1.Items.Add("n" & vbTab & "sum")
 ListBox1.Items.Add("———————-")
While n <>10
 n += 1
 sum += n
 ListBox1.Items.Add(n & vbTab & sum)
End While
End Sub