2.9. Functions and Subroutines

2.9.1. Function

A function is a block of code that performs some operations and then returns a value. For example, the following function Area takes in two input parameters, computes the area, and then returns the result:

Public

Function

Area(ByVal length As Single, _

                 ByVal breadth As Single) As Single

         Dim result As Single

         result = length * breadth

Return

result

End Function

To invoke a function, you simply call the function name with the required argument(s):

Dim areaOfRect As Single =

Area(4, 5)

VB 6 Tip : In VB 6, only functions require the mandatory use of parentheses around the parameter list. But in VB 2005, all functions or subroutine calls require parentheses around the parameter list (even if the parameter list is empty).

The value returned by the Area function is then assigned to the areaOfRect variable.

In VB 6, you use the function name to return the value of a function, like this:

Public

Function

Area(ByVal length As Single, _

	            ByVal breadth As Single) As Single

        Dim result As Single

        result = length * breadth

Area = result

End Function

In VB 2005, you can either use the Return keyword or the function name to return the value of a function. Note that when a Return statement is encountered in a function, the execution is immediately transferred back to the statement that called it.

2.9.2. Subroutine

A subroutine is similar to a function, except that it does not return a value. For example, the following subroutine PrintMessage accepts a single input parameter and prints a message box.

Public Sub PrintMessage(ByVal str As String)

        MsgBox(str)

	End Sub

To invoke a subroutine, you simply call the subroutine name and pass it the required argument(s):

PrintMessage("File deletion completed.")

VB 6 Tip : In VB 6, you can call the PrintMessage subroutine without using parentheses to enclose the parameter list: PrintMessage "File deletion completed."

2.9.3. Passing Arguments

There are two ways to pass values to a subroutine or function:

• By value

• By reference

Let's take a closer look at these two methods in the following sections.

2.9.3.1. Passing by value

Consider the following subroutine:

Public Sub ProcessValue(

ByVal

num As Integer)

        num += 1

        MsgBox("In ProcessValue(), num is " & num)

    End Sub

The ProcessValue subroutine takes a single input parameter: num . The parameter declaration is preceded by the ByVal keyword.

By default, Visual Basic 2005 passes an argument via ByVal . In VB 6, the default is ByRef (see "Passing by reference," next).

The following statements call the ProcessValue subroutine and display a value at each stage:

Dim num As Integer = 5

	MsgBox("Before ProcessValue(), num is " & num)

	ProcessValue(num) ' pass by value

	MsgBox("After ProcessValue(), num is " & num)

You will realize that the value of num remains at 5 before and after calling the ProcessValue subroutine.

As you can deduce, even though the variable num is modified within the subroutine, the change is not reflected outside the subroutine. When you pass an argument by value, a copy of the variable is created to be used within the subroutine. When the subroutine exits, the variable is destroyed .

2.9.3.2. Passing by reference

Consider the following subroutine:

Public Sub ProcessValue(

ByRef

num As Integer)

        num += 1

        MsgBox("In ProcessValue(), num is " & num)

    End Sub

The ProcessValue subroutine takes in a single input parameter: num . The parameter declaration is preceded with the ByRef keyword.

The following statements call the ProcessValue subroutine and display the value at every stage:

Dim num As Integer = 5

	MsgBox("Before ProcessValue(), num is " & num)

	ProcessValue(num) ' pass by value

	MsgBox("After ProcessValue(), num is " & num)

In contrast to passing by value, when you pass an argument by reference, the subroutine receives a reference that points to the location where the argument is stored in memory. When the variable is modified within the subroutine, the change will affect the original variable. Hence, the change remains even after the subroutine exits.

2.9.3.3. Optional parameters

Consider the following definition of a modified PrintMessage subroutine:

Public Sub PrintMessage(ByVal str1 As String, _

				ByVal str2 As String, _

Optional

ByVal str3 As String = "rocks!")

       MsgBox(str1 & str2 & str3)

	End Sub

This version of the PrintMessage subroutine takes three input parameters: str1, str2 , and str3 . The first two are required; str3 is an optional parameter, as called out by the Optional keyword. For an optional parameter, a default value is required.

Optional arguments must always be declared last in a subroutine definition. You can specify one or more optional parameters.

VB 6 Tip : In VB 6, optional parameters are not required to have default values, but in VB 2005, optional parameters must have default values.

'--- with and without optional arguments



	' prints out Visual Basic rocks!

	PrintMessage("Visual ", "Basic ")



    ' prints out Visual Basic rocks!

	PrintMessage("Visual ", "Basic ", )



	' prints out Visual Basic really rocks!

	PrintMessage("Visual ", "Basic ", "really rocks!")