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In Visual Basic 2013, data are stored either as variable or as constant. Variables are similar to mailboxes in a post office. The contents of the variables change from time to time, just like the mailboxes. In VB2013, variables are areas allocated by the computer memory to store data.
Examples of valid and invalid variable names are displayed in Table 9.1
|Valid Names||Invalid Name|
|Long_Name_Can_beUSE||LongName&Canbe&Use *& is not acceptable|
9.2 Declaring Variables
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,………….
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 of is
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 VB 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 occurred, as shown in Example 9.4:
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 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. Private declares a local variable or a variable that is local to a procedure or module. The Static keyword declares a variable that is being used multiple times, even after a procedure has been terminated. Most variables created inside a procedure are discarded by Visual Basic when the procedure is finished, static keyword preserves the value of a variable even after the procedure is terminated. Public is the keyword that 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
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: