Understanding Subroutines and Functions in VB6

Subroutine and Functions

1. Understanding Subroutines and Functions in VB

2. What is/are Subroutines ? A subroutine has a name attributed with it, much like a variable does. Unlike a variable, a subroutine doesn't hold data. Instead, it holds code. For example, the Form_Load subroutine is automatically called when a form loads. This is where you have been inserting your test code before. What you were actually doing was adding code to this subroutine.

3. What is/are Functions ? Functions are basically the exact same as subroutines, except that they return a value. That means that the function itself has a type, and the function will return a value to the calling subroutine based on the code that it contains. A function is declared the exact same way as a subroutine, except using the "Function" keyword instead of "Sub".

4. The difference between a Subs and a Functions Sub does not produce a return value (one that can be assigned directly to a variable) whereas a Function does produce a return value. Both Subroutines and Functions can be called with or without parameters.

5. Calling a Sub Without Parameters To call a Sub, you use the Call statement . Example, the statement below calls a Sub named " AddNum ". Call AddNum This would cause VB to execute the statements in the Sub named " AddEm " and then return to the statement following the Call. Private Sub AddNum () [statements] End Sub The keyword "Call" is optional , thus a call can be made simply by coding the name of the Sub: AddNum ' same as "Call AddNum "

6. Example of Calling a Sub Without Parameters Option Explicit ‘ which requires that you declare every variable before you use it . Dim mintNum1 As Integer Dim mintNum2 As Integer Dim mintSum As Integer Private Sub cmdTryIt_Click () mintNum1 = Val( InputBox ("Enter first number:", "Add Program")) mintNum2 = Val( InputBox ("Enter second number:", "Add Program")) AddNum ' Could also be coded as "Call AddNum ()" with or without the () Print "The sum is "; mintSum End Sub Private Sub AddNum () mintSum = mintNum1 + mintNum2 End Sub

7. Calling a Sub With Parameters A Sub Procedure will be expecting one or more parameters. This enhances the reusability and flexibility of the Sub procedure. The full syntax for a Sub procedure is: [Private | Public] Sub SubName [(parameter list)] [statements] End Sub where "parameter list " is a comma-separated list of the parameter variables with their data types (ex. var1 As datatype , var2 As datatype , etc.). Example: Private Sub AddNum (pintNum1 As Integer, pintNum2 As Integer, pintSum As Integer) The CALL can be coded in one of two ways . Call AddNum (intNum1, intNum2, intSum ) or AddEm intNum1, intNum2, intSum With the naming conventions use to start the variable names of the parameters in a Sub or Function header with the letter "p" for "parameter

8. Example of Calling a Sub With Parameters Private Sub cmdTryIt_Click ()   ' The variables can be declared at the local level Dim intNum1 As Integer Dim intNum2 As Integer Dim intSum As Integer intNum1 = Val( InputBox ("Enter first number:", "Add Program")) intNum2 = Val( InputBox ("Enter second number:", "Add Program ")) AddNum intNum1, intNum2, intSum ' Could also be coded as :' Call AddNum (intNum1, intNum2, intSum ) Print "The sum is "; intSum End Sub Private Sub AddNum(pintNum1 As Integer, pintNum2 As Integer, pintSum As Integer) pintSum = pintNum1 + pintNum2 End Sub

9. Calling a Function To call a Function, the syntax is identical to an assignment statement that uses a VB built-in function: VariableName = FunctionName [(argument list )] For example, if you made a function called " AddNum " that returned a value (like the sum, for example), you could invoke the function as follows: intSum = AddEm (intNum1, intNum2) The above statement would cause the following to happen : The procedure AddNum would be called, passing intNum1 and intNum2 The AddNum procedure would add the values of intNum1 and intNum2 to produce its return value, the sum. When the AddNum procedure exits, its return value would be stored in the variable intSum

10. The format of the Function Procedure [Private | Public] Function FunctionName [( parameter list)] [As datatype ] [ statements] FunctionName = value [statements] End Function

11. Example of Calling a Function Option Explicit . . . Private Sub cmdTryIt_Click () ' The variables can be declared at the local level Dim intNum1 As Integer Dim intNum2 As Integer Dim intSum As Integer intNum1 = Val( InputBox ("Enter first number:", "Add Program")) intNum2 = Val( InputBox ("Enter second number:", "Add Program")) intSum = AddNum (intNum1 , intNum2) Print "The sum is "; intSum End Sub Private Function AddNum(pintNum1 As Integer,pintNum2 As Integer) As Integer AddNum = pintNum1 + pintNum2 End Function

12. Laboratory Hands-on Exercise : Apply the Subroutines and Functions in a Simple Math Operation (that can add, subtract, multiply and divide) Filenames: UsingSubs.vbp and UsingFunctions.vbp In the last line of the program/code answer the question and put as remarks to avoid errors. What do you think the best way to determine which is better?