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 11.1b
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-..........
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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.
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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
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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.
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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.
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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 |
