Lesson 5 : Managing VB Data

5.2.2 Declaring Variables Explicitly

In Visual Basic, it is a good practice to declare the variables before using them by assigning names and data types. They are normally declared in the general section of the codes' windows using the Dim statement. You can use any variable to hold any data , but different types of variables are designed to work efficiently with different data types .
The syantax is as follows:

Dim VariableName As DataType

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 5.1

Dim password As String 
Dim yourName As String 
Dim firstnum As Integer 
Dim secondnum As Integer 
Dim total As Integer
Dim doDate As Date 
Dim password As String, yourName As String, firstnum As Integer

Unlike other programming languages, Visual Basic actually doesn't require you to specifically declare a variable before it's used. If a variable isn't declared, VB willautomatically declare the variable as a Variant. A variant is data type that can hold any type of data.

For string declaration, there are two possible types, one for the variable-length string and another for the fixed-length string. For the variable-length string, just use the same format as example 5.1 above. However, for the fixed-length string, you have to use the syntax as shown below:

Dim VariableName as String * n

where n defines the number of characters the string can hold.

For example,

Dim yourName as String * 10
  

*yourName can holds no more than 10 Characters.

5.2.2 Scope of Declaration

Other than using the Dim keyword to declare the data, you can also use other keywords to declare the data. Three other keywords are private ,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 the 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 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.

5.3 Constants

Constants are different from variables in the sense that their values do not change during the running of the program.

5.3.1 Declaring a Constant

The syntax to declare a constant is

Constant Name As Data Type = Value

Example 5.3

In this example, we insert a Shape control and two command buttons. Set the shape value of the Shape control to 3 so that it becomes a circle. Rename one of the command buttons to CmdResize for changing the size of the circle. Rename the other command button as CmdArea for calculation of the area of the circle. In this program, we declare four variables and a constant in the General section. The varaible h is to store the value of height of the circle and the variable r is to store the value of the radius which is half of the height. In addtion, the variable a is to store the value of area in twip using the formula area of circle=πr². Besides that, the constant Pi represents π which we fixed at 3.142. Finally, the variable area is to store the value in cm by multiplying a with 0.001763889. (1 twip =0.001763889 cm)

The Code

Dim h, r, a, rad, area As Single
Const Pi As Single = 3.142

Private Sub CmdArea_Click()
r = h / 2
rad = r * 0.001763889
a = Pi * rad ^ 2
area = Round(a, 2)
MsgBox ("The Area of the circle is " & area)
End Sub

Private Sub CmdResize_Click()
h = InputBox("Enter the value of height")
MyShape.Height = h

End Sub

The Output