lesson 9 Working with Variables and Constants

In Visual Basic 2013, data are stored either as a variable or as a constant.  A variable is similar to a mailbox in the post office because of its contents changes from time to time, just like the mailbox. In  VB2013, variables are areas allocated by the computer memory to store data.

9.1 Variable Names

In Visual Basic 2013, we have to assign a name to every variable. In order to name a variable, you have to follow a set of rules, as follows:

• It must be less than 255 characters
• No spacing is allowed
• It should not begin with a number
• A period is not allowed.

Some examples of valid and invalid variable names are illustrated in Table 9.1

Table 9.1 Valid and Invalid Variables Names

Valid Names	Invalid Name
My_Mobilephone	My.Mobilephone
Apple123	123Apple
Long_Name_Can_beUSE	LongName&Canbe&Use                  *& is not acceptable

9.2 Declaring Variables

In VB2013, we declare a variable using the Dim statement. The syntax is as follows:

Dim VariableName As DataType.

Notice that you need to assign a name to the variable and also specify its data type.  In addition, if you intend to declare more variables, you can declare them in separate lines or you can combine them in one line, separating each variable with a comma, as follows:

Dim VariableName1 As DataType1, VariableName2 As DataType2, VariableName3 As DataType3

Example 9.1

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim password As String
Dim yourName As String
Dim firstnum As Integer
Dim secondnum As Integer
Dim total As Integer
Dim doDate As Date
End Sub

You may also combine the above statements in one line, separating each variable with a comma, as follows:

Dim password As String, yourName As String, firstnum As Integer,………….

Example 9.2

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim YourMessage As String
 YourMessage = "Happy Birthday!"
 MsgBox(YourMessage)
End Sub

When you run the program, a message box that shows the text "Happy Birthday!" will appear, as shown in Figure 9.1

 

 

9.3 Assigning Values to Variables

After declaring variables using the Dim statements, we can then assign values to these variables. The syntax  is

Variable=Expression

The variable can be a declared variable or a control property value. The expressions include a mathematical expression, a number, a string, a Boolean value (true or false) and more, as illustrated in the following examples:

firstNumber=100
secondNumber=firstNumber-99
userName=”John Lyan”
userpass.Text = password
Label1.Visible = True
Command1.Visible = false
Label4.text = textbox1.Text
ThirdNumber = Val(usernum1.Text)
total = firstNumber + secondNumber+ThirdNumber
MeanScore% = SumScores% / NumSubjects%
X=sqr (16)
TrimString= Ltrim (“ Visual Basic”, 4)
Num=Int(Rnd*6)+1

In Visual Basic 2013, an error occurs when you try to assign a value to a variable of the incompatible data type. For example, if you have declared a variable as an integer but you assigned a string value to it, an error will occur, as shown in Example 9.4:

Example 9.3

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim YourMessage As Integer
 YourMessage = "Happy Birthday!"
 MsgBox(YourMessage)
End Sub

When you run the program, the following error messages will appear in a dialog box, as shown in Figure 9.2

You can either break the program and continue to run the program.

9.4 The Scope of Declaration

Besides using the Dim keyword to declare the data, we may also use other keywords to declare the data. Three other keywords are private , static and public. The syntaxes are as shown below:

Private VariableName as Datatype
Static VariableName as Datatype
Public VariableName as Datatype

The aforementioned keywords indicate the scope of the declaration. First of all, the Private keyword declares a local variable which means it is local to a procedure or module. On the other hand, the Static keyword declares a variable that can be used multiple times, even after a procedure has been terminated. As a matter of fact, most variables created inside a procedure are discarded by Visual Basic when the procedure is finished. However, the Static keyword preserves the value of a variable even after the procedure is terminated. Lastly, the Public keyword declares a global variable, which means it can be used by all the procedures and modules of the whole program.

 9.5 Declaring Constants

A Constant's value does not change during the running of the program.The syntax to declare a constant is

Const Constant Name As Data Type = Value

Example 9.4

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Const Pi As Single = 3.142
Dim R As Single = 10
Dim AreaCircle As Single
 AreaCircle = Pi * R ^ 2
 MsgBox("Area of circle with " & "radius" & R & "=" & AreaCircle)
End Sub

Press F5 to run the program and clicking the button produces the following message: