1. Introduction to the Visual Basic Language and Environment
Variable Declaration
There are three ways for a variable to be typed (declared):
Default
Implicit
Explicit
If variables are not implicitly or explicitly typed, they are
assigned the variant type by default. The variant
data type is a special type used by Visual Basic that can contain
numeric, string, or date data.
To implicitly type a variable, use the corresponding
suffix shown above in the data type table. For example,
TextValue$ = "This is a string"
creates a string variable, while
Amount% = 300
creates an integer variable.
There are many advantages to explicitly typing variables.
Primarily, we insure all computations are properly done, mistyped
variable names are easily spotted, and Visual Basic will take care
of insuring consistency in upper and lower case letters used in
variable names. Because of these advantages, and because it is good
programming practice, we will explicitly type all variables.
To explicitly type a variable, you must first determine
its scope. There are four levels of scope:
Procedure level
Procedure level, static
Form and module level
Global level
Within a procedure, variables are declared using the Dim
statement:
Dim MyInt as Integer
Dim MyDouble as Double
Dim MyString, YourString as String
Procedure level variables declared in this manner do not retain their
value once a procedure terminates.
To make a procedure level variable retain its value upon exiting
the procedure, replace the Dim keyword with Static:
Static MyInt as Integer
Static MyDouble as Double
Form (module) level variables retain their value and are available
to all procedures within that form (module). Form (module) level
variables are declared in the declarations part of the
general object in the form's (module's) code window.
The Dim keyword is used:
Dim MyInt as Integer
Dim MyDate as Date
Global level variables retain their value and are available to
all procedures within an application. Module level variables are
declared in the declarations part of the general object
of a module's code window. (It is advisable to keep all global
variables in one module.) Use the Global keyword:
Global MyInt as Integer
Global MyDate as Date
What happens if you declare a variable with the same name in two
or more places? More local variables shadow (are accessed in
preference to) less local variables. For example, if a variable MyInt
is defined as Global in a module and declared local in a routine
MyRoutine, while in MyRoutine, the local value of MyInt is accessed.
Outside MyRoutine, the global value of MyInt is accessed.
Example of Variable Scope:
Module1
Global X as Integer
Form1
Dim Y as Integer
Sub Routine1()
Dim A as Double
.
.
End Sub
Sub Routine2()
Dim B as Double
.
.
End Sub
Form2
Dim Z as Integer
Sub Routine3()
Dim C as String
.
.
End Sub
Procedure Routine1 has access to X, Y, and A (loses value upon termination)
Procedure Routine2 has access to X, Y, and B (retains value)
Procedure Routine3 has access to X, Z, and C (loses value)
