In lesson 7 and lesson 8, we have learned how to
handle decisions making process using If...Then...Else and also Select Case
program structures in Visual Basic. Another procedure that involves decisions
making is looping. Visual Basic allows a procedure to be repeated many times
until a condition or a set of conditions is fulfilled. This is generally
called looping . Looping is a very useful feature of Visual Basic because
it makes repetitive works easier. There are two kinds of loops in Visual
Basic, the Do...Loop and
the For.......Next loop.
9.1 Do Loop
The Do Loop statements have three
different forms, as shown below:
a) Do While condition
Block of one or more VB statements
Loop
b) Do
Block of one or more VB statements
Loop While condition
Block of one or more VB statements
Loop While condition
c) Do Until condition
Block of one or more VB statements
Loop
Block of one or more VB statements
Loop
d) Do
Block of one or more VB statements
Block of one or more VB statements
Loop Until condition
Example 9.1
Do
while counter <=1000
num.Text=counter
counter =counter+1
Loop
* The above example will keep on adding until counter >1000.
The above example can be rewritten as
Do
num.Text=counter
counter=counter+1
counter=counter+1
Loop
until counter>1000
9.2 Exiting the Loop
Sometime we need exit to exit a
loop earlier when a certain condition is fulfilled. The keyword to use is Exit Do. You can examine
Example 9.2 for its usage.
Example 9.2
Dim sum, n
As Integer
Private
Sub Form_Activate()
List1.AddItem
"n" & vbTab & "sum"
Do
n = n + 1
Sum = Sum + n
List1.AddItem
n & vbTab& Sum
If n
= 100 Then
Exit
Do
End
If
Loop
End Sub
Explanation
In the above example, we compute 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 AddItem method to populate the ListBox. The statement List1.AddItem
"n" & vbTab & "sum" will display the headings in
the ListBox, where it uses the vbTab function to create a space between the
headings n and sum.
9.3 For....Next Loop
The For....Next Loop event
procedure is written as follows:
For counter=startNumber to
endNumber (Step increment)
One or more VB
statements
Next
Example 9.3 a
For counter=1 to 10
display.Text=counter
Next
Example 9.3 b
For counter=1 to 1000 step 10
counter=counter+1
Next
Example 9.3 c
For counter=1000 to 5 step -5
counter=counter-10
Next
*Notice that the increment can be negative
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. Its
usages is shown in Example 9.3 d.
Example 9.3
d
Private Sub
Form_Activate( )
For n=1 to
10
If n>6
then
Exit For
End If
Else
Print n
End If
End Sub
9.4 Nested For...Next Loop
When you have a loop within a loop,
then you have created a nested loop. You can actually have as many loops as you
want in a nested loop provided the loops are not the never-ending type. For a
nested loop that consists of two loops, the first cycle of the outer loop will
be processed first, then it will process the whole repetitive process of the
inner loop, then the second cycle of the outer loop will be processed and again
the whole repetitive process of the inner loop will be processed. The program
will end when the whole cycle of the outer loop is processed.
The Structure of a nested loop is
:
For counter1=startNumber to endNumber (Step
increment)
For counter2=startNumber to
endNumber (Step increment)
One
or more VB statements
Next counter2
Next counter1
Example 9.4
Private Sub Form_Activate ( )
For
firstCounter= 1to 5
Print “Hello”
For
secondCounter=1 to 4
Print “Welcome to the VB tutorial”
Next
secondCounter
Next firstCounter
Print” Thank you”
End
Sub
Figure 9.1
The output of the above program is
shown in Figure 9.1. As the outer loop has five repetitions, it will print the
word “Hello” five times. Each time after it prints the word “Hello”, it will
print four lines of the “Welcome to the VB tutorial” sentences as the inner
loop has four repetitions.
9.5 The While….Wend Loop
The structure of a While….Wend Loop is very similar to the Do
Loop. it takes the following form:
While condition
Statements
Wend
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. Let’s examine the program listed in example 9.4.
Example 9.5
Dim sum, n As Integer
Private Sub Form_Activate()
List1.AddItem "n" & vbTab & "sum"
While n <> 100
n = n + 1
Sum = Sum + n
List1.AddItem n & vbTab & Sum
Wend
End Sub
