Lec 9

Visual Basic 2015  

Trabajar con variables y constantes 

En la lección 8 , hemos comprendido el concepto de datos y aprendió acerca de cómo Visual Basic 2015 clasifica los diferentes tipos de datos . En esta lección , vamos a aprender cómo almacenar los datos y cómo declarar ellos. En Visual Basic 2015 , los datos se pueden almacenar como variables o como constantes . Las variables son como los buzones de correo en la oficina de correos . El contenido de la variable cambia de vez en cuando , al igual que los buzones de correo . En Visual Basic 2015, las variables son las áreas específicas asignadas por la memoria del ordenador para almacenar datos.

9.1 Variable Names

Like the mail boxes , each variable must be given a name. To name a variable in Visual Basic 2015, you need to follow a set of rules.The following are the rules when naming the variables in Visual Basic 2015:

• It must be less than 255 characters
• No spacing is allowed
• It must not begin with a number
• Period is not permitted

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

Table 9.1:

VALID NAMES	INVALID NAME
My_Name	My.Name
VB2015	2015VB
Long_Name_Can_beUSE	LongName&Canbe&Use                  *& is not acceptable

9.2 Declaring Variables

In Visual Basic 2015, you need to declare the variables before using them. To declare a variable, you have to assign a name to the variable and state its data type. If you fail to do so, the program will runs into an error. Variables are usually declared in the general section of the code windows using the Dim statement.

The syntax to declare a variable in Visual Basic 2015  is as follows:

Dim VariableName As Data Type

If you want to declare more variables, you can declare them in separate lines or you may also combine more 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.LoadDim password As String
Dim MyName As String
Dim Num1 As Integer
Dim Num2 As Single
Dim Sum As Integer
Dim StartDate 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, MyName As String, Num1 As Integer, Num2 as Single. Sum as Integer, StartDate as Date

Example 9.2

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

Dim YourName as String=”George”
Dim MyMsg As String

MyMsg = “Happy Birthday!”
MsgBox(MyMsg&”,”&YourName)

End Sub

Notice that you can assign a value to the string in one line using the = sign instead of declaring the variable and then give it a value in another line.

Dim YourName as String=”George”

is same as

Dim YourName as String
YourName=”George”

When you run the program, a message box that shows the text “Happy Birthday, George” will appear in a message box, as shown in Figure 9.1.

                                                           Figure 9.1

9.3 Assigning Values to Variables

After declaring various variables using the Dim statements, we can assign values to those variables. The syntax of an assignment in Visual Basic 2015 is

Variable=Expression

*You may also declare a variable by assigning an initial value to it, as shown in the following examples:

Dim VarName as String=”ABC”

Dim VarNum as Interger=100

The variable can be a declared variable or a control property value. The expression could be 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

An error occurs when you try to assign a value to a variable of 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 9.3:

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

 Figure 9.2 

9.4 Scope of Declaration

In Visual Basic 2015, we usually use the Dim keyword to declare the data. However, you can also use other keywords to declare the data. Three other keywords areprivate ,static and public. The forms are as shown below:

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

The above keywords indicate the scope of declaration. Private declares a local variable, or a variable that is local to a procedure or module. However, Private is rarely used, we normally use Dim to declare a local variable. 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 terminated. Static keyword preserve 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 Visual Basic 2015 program.

 9.5 Declaring Constants

Constants are different from variables in the sense that their values do not change during the running of the program.The syntax to declare a constant in Visual Basic 2015 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 will produce the following message:

    Figure 9.3