Procedures:
Visual Basic application
is made up of small, self-contained segments. It will be made up of small
segments called procedures. Procedures are useful for implementing
repeated tasks, such as frequently used calculations.
The two types of
procedures are subroutines and functions.
1. Subroutines:
A subroutine
is a block of statements that carries out a well-defined task. The block of
statements is placed with a pair of Sub/End Sub statements and can be
invoked by name. The following subroutine displays the current date in a
Message Box and can be called by its name, ShowDate:
Sub ShowDate()
MsgBox Date()
End Sub
2. Functions:
A function is
similar to a subroutine, but a function returns a result. Subroutines perform a
task and don’t report anything to the calling program; functions commonly carry
out calculations and report the result. The statements that make up a function
are placed in a pair of Function/End Function statements. Moreover,
because a function reports a result, it must have a type, for example:
Function NextDay() As Date
NextDay = Date() + 1
End Function
Argument-Passing Mechanisms: One of the most important procedural issues is the
mechanism used to pass arguments. The examples so far have used the default
mechanism: passing arguments by reference. The other mechanism is passing by
value.
1. Passing
Arguments by Reference: Passing arguments by reference gives the procedure
access to the actual variable. The calling procedure passes the address of the
variable in memory so that the procedure can change its value permanently.
Function Add(num1 As Integer, num2 As Integer) As Integer
Add = num1 + num2
num1 = 0
num2 = 0
End Function
2. Passing
Arguments by Value: When you pass an argument by value, the procedure sees
only a copy of the argument. Even if the procedure changes it, the changes
aren’t permanent. The benefit of passing arguments by value is that the
argument values are isolated from the procedure, and only the program in which
they are declared can change their values. Passing arguments by value requires
a bit of extra typing, since this isn’t the default argument-passing mechanism.
Function Degrees(ByVal Celsius as Single) As Single
Degrees = (9 / 5) * Celsius + 32
End Function
