In lesson 2, you have learned how to enter the
program code and run the sample VB programs but without much understanding
about the logics of VB programming. Now, let’s get down to learning some basic
rules about writing the VB program code.
To start writing an event procedure, you need
to double-click an object. For example, if you want to write an event procedure
when a user clicks a command button,
you double-click on the command button and an event procedure will appear as
shown in Figure 2.1. It takes the following format:
Private Sub Command1_Click
(Key in your program code here)
You then need to key-in the procedure in the
space between Private Sub Command1_Click............. End Sub. Sub actually stands for sub procedure that
made up a part of all the procedures in a program. The program code is made up
of a number of statements that set certain properties or trigger some actions.
The syntax of Visual Basic’s program code is almost like the normal English
language though not exactly the same, so it is very easy to learn.
he syntax to set the property of an object or
to pass certain value to it is :
Object.Property
Example 4.1
Private Sub Command1_click
Label1.Visible=false
Label2.Visible=True
Text1.Text=”You are
correct!”
End sub
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Example 4.2
Private Sub Command1_click
Label1.Caption=” Welcome”
Image1.visible=true
End sub
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Example 4.3
Private Sub Command1_click
Pictuire1.Show=true
Timer1.Enabled=True
Lable1.Caption=”Start
Counting
End sub
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In Example 4.1, clicking on the command button will make label1
become invisible and label2 become visible; and the text” You are correct” will
appear in TextBox1. In example 4.2, clicking on the command button will
make the caption label1 change to “Welcome” and Image1 will become
visible. In example 4.3 , clicking on the command button will make
Picture1 show up, timer starts running and the caption of label1 change to
“Start Counting”.
Syntaxes that do not involve setting of properties are
also English-like, some of the commands are Print,
If…Then….Else….End If, For…Next, Select Case…..End Select , End and Exit
Sub. For
example, Print
“ Visual Basic” is to
display the text Visual Basic on screen and End is to end the program. Other commands will be explained in
details in the coming lessons.
Program code that involve calculations is very easy to write,
you need to write them almost like you do in mathematics. However, in order to
write an event procedure that involves calculations, you need to know the basic
arithmetic operators in VB as they are not exactly the same as the normal
operators we use, except for + and - . For multiplication, we use *, for
division we use /, for
raising a number x to the power of n, we use x ^n and for square root, we use Sqr(x). VB offers many more advanced mathematical functions such as Sin, Cos,
Tan and Log, they
will be discussed in lesson 10. There are also two important functions that are
related to arithmetic operations, i.e. the functions Val and Str$ where
Val is to convert text entered into a textbox to numerical value and Str$ is to
display a numerical value in a textbox as a string (text). While the
function Str$ is as important as VB can display a numeric values as string
implicitly, failure to use Val will results in wrong calculation. Let’s examine
example 4.4 and example 4.5.
Example 4.4
Private Sub Form_Activate()
Text3.text=text1.text+text2.text
End Sub
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Example 4.5
Private Sub Form_Activate()
Text3.text=val(text1.text)+val(text2.text)
End Sub
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When you run the program in example 4.4 and enter 12 in textbox1
and 3 in textbox2 will give you a result of 123, which is wrong. It is because
VB treat the numbers as string and so it just joins up the two strings. On the
other hand, running exampled 4.5 will give you the correct result, i.e., 15.
