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Introduction to Procedures and Functions

 

Introduction to Procedures

 

Procedures

A procedure is a set-aside assignment the compiler must take care of to complement a program. A program developer writes the procedure and a user (the user is either you or another programmer) would only see the result. There are two categories of procedures you will use in your programs: those that have already been created thus made available to you, and those you will create yourself.

In the Visual Basic language, like most other languages, there are two types of procedures: functions and sub routines.

Introduction to Sub-Procedures

A sub procedure is an assignment that is carried but does not give back a result. To create a sub procedure, start by typing the Sub keyword followed by a name (like everything else, a procedure must have a name). The name of a procedure is always followed by parentheses. At the end of the sub procedure, you must type End Sub. Therefore, the primary syntax of a sub procedure is:

Sub ProcedureName()

End Sub

The name of a procedure should follow the same rules we learned to name the variables. In addition:

  • If the procedure performs an action that can be represented with a verb, you can use that verb to name it. Here are examples: show, display
  • To make the name of a procedure stand, you should start it in uppercase. Examples are Show, Play, Dispose, Close
  • You should use explicit names that identify the purpose of the procedure. If a procedure would be used as a result of another procedure or a control's event, reflect it on the name of the sub procedure. Examples would be: afterupdate, longbefore.
  • If the name of a procedure is a combination of words, you should start each word in uppercase. Examples are: AfterUpdate, SayItLoud

The section between the Sub and the End Sub lines is referred to as the body of the procedure. Here is an example:

Sub Assign()

End Sub

The body of the procedure is used to define what, and how, the assignment should be carried. For example, if you need to use a variable, you can declare it and specify the kind of variable you need. There is no restriction on the type of variables that can be declared in a procedure. Here is an example in which a string variable is declared in the body of a sub routine:

Sub Assign()
    Dim strFullName As String
End Sub

In the same way, you can declare as many variables as you need inside of a procedure. The actions you perform inside of a procedure depend on what you are trying to accomplish. For example, a procedure can simply be used to create a string. The above procedure can be changed as follows:

Sub Assign()
    Dim strFullName As String
    strFullName = "Paul Bertrand Yamaguchi"
End Sub

Practical Learning: Introducing Procedures

  1. Start Microsoft Visual Basic
  2. To create a new application, on the main menu, click File -> New -> Project or File -> New Project
  3. In the Templates list, click Console Application
  4. In the Name text box, type Squared
  5. In the Solution Name text box, type Geometry1
  6. Click OK
  7. In the Solution Explorer, under the Square node, right-click Module1.vb and click Rename
  8. Type Square.vb and press Enter
  9. To create a procedure, change the document as follows:
      
    Public Module Square
    
        Sub ProcessSquare()
            Dim dblSide As Double
            Dim dblPerimeter As Double
    
            dblSide = InputBox("Enter Side: ")
    
            dblPerimeter = dblSide * 4
    
            MsgBox("=-= Square Characteristics=-=" & vbCrLf & _
                   "Side:      " & dblSide & vbCrLf & _
                   "Perimeter: " & dblPerimeter)
        End Sub
    
        Sub Main()
    
        End Sub
    
    End Module
  10. Save the document

Calling a Sub Procedure

Once you have a procedure, whether you created it or it is part of the Visual Basic language, you can use it. Using a procedure is also referred to as calling it. Before calling a procedure, you should first locate the section of code in which you want to use it. To call a simple procedure, type its name followed by parentheses in the section where you want to use it. Here is an example:

Module Exercise

    Sub Assign()
        Dim strFullName As String
        strFullName = "Paul Bertrand Yamaguchi"

        MsgBox(strFullName)
    End Sub

    Friend Sub Main()
        Assign()
    End Sub

End Module

Besides using the name of a procedure to call it, you can also precede it with the Call keyword. Here is an example:

Module Exercise

    Sub Assign()
        Dim strFullName As String
        strFullName = "Paul Bertrand Yamaguchi"

        MsgBox(strFullName)
    End Sub

    Friend Sub Main()
        Call Assign()
    End Sub

End Module

Practical Learning: Calling a Sub Procedure

  1. To call the procedure, type its name in the Main() procedure as follows:
     
    Module Central
    
        Sub ProcessSquare()
            Dim dblSide As Double
            Dim dblPerimeter As Double
    
            dblSide = InputBox("Enter Side: ")
    
            dblPerimeter = dblSide * 4
    
            MsgBox("=-= Square Characteristics=-=" & vbCrLf & _
                   "Side: " & dblSide & vbCrLf & _
                   "Perimeter: " & dblPerimeter)
        End Sub
    
        Sub Main()
            ProcessSquare()
        End Sub
    
    End Module
  2. Execute the application and enter the side of the square as 48.14
     
    Square
  3. Close the DOS window and return to your programming environment

Procedures and Access Modifiers

Like a variable, a procedure can use access modifiers. A procedure can be made a private procedure, a friendly procedure, or a public procedure, using the Private, the Friend, or the Public keywords respectively:

  • Private: A procedure marked as private can be called only by members of the same module
  • Friend: A friendly procedure can be called by members of the modules of the same application
  • Public: A public procedure can be called from its program and other programs

Practical Learning: Access Modifying a Procedure

  1. To control the access to a procedure, change the file as follows:
     
    Public Module Square
    
        Private Sub ProcessSquare()
            Dim dblSide As Double
            Dim dblPerimeter As Double
    
            dblSide = InputBox("Enter Side: ")
    
            dblPerimeter = dblSide * 4
    
            MsgBox("=-= Square Characteristics =-=" & vbCrLf & _
                   "Side: " & dblSide & vbCrLf & _
                   "Perimeter: " & dblPerimeter)
        End Sub
    
        Public Sub Main()
            ProcessSquare()
        End Sub
    
    End Module
    
  2. Execute the application and enter the side of the square as 105.15
  3. Close the DOS window and return to your programming environment

Introduction to Functions

 

Introduction

Like a sub procedure, a function is used to perform an assignment. The main difference between a sub procedure and a function is that, after carrying its assignment, a function gives back a result. We also say that a function "returns a value". To distinguish both, there is a different syntax you use for a function.

Creating a Function

To create a function, you use the Function keyword followed by a name and parentheses. Unlike a sub procedure, because a function returns a value, you must specify the type of value the function will produce. To give this information, on the right side of the closing parentheses, you can type the As keyword, followed by a data type. To indicate where a function stops, type End Function. Based on this, the minimum syntax used to create a function is:

AccessModifier(s) Function FunctionName() As DataType
    
End Function

As seen for a sub procedure, a function can have an access modifier. The rules are the same as we described for a sub procedure.

The Function keyword is required.

The name of a function follows the same rules and suggestions we reviewed for sub procedures.

The As keyword may be required (in the next sections, we will review the alternatives to the As DataType expression).

The DataType factor indicates the type of value that the function will return. If the function will produce a word or a group of words, you can create it as String. The other data types are also valid in the contexts we reviewed them. Here is an example:

Function GetFullName() As String
        
End Function

As done with the variables, you can also use a type character as the return type of a function and omit the As DataType expression. The type character is typed on the right side of the function name and before the opening parenthesis. An example would be GetFullname$(). As with the variables, you must use the appropriate type character for the function:

Character The function must return
$ a String type
% a Byte, Short, Int16, or In32
& an Int64 or a Long
! a Single type
# a Double
@ a Long integer

Here is an example:

Function GetFullName$()

End Function

As mentioned already, the section between the Function and the End Function lines is the body of the function. It is used to describe what the function does. As done on a sub procedure, one of the actions you can perform in a function is to declare a (local) variable and use it as you see fit. Here is an example:

Function CallMe() As String
    Dim Salute As String
    Salute = "You can call me Al"
End Function

Returning a Value From a Function

After performing an assignment in a function, to indicate the value it returns, somewhere after the assignment and before the End Function line, you can type the name of the function, followed by the = sign, followed by the value the function returns. Here is an example in which a function returns a name:

Function GetFullName$()
        Dim FirstName As String, LastName As String

        FirstName = InputBox("Enter First Name: ")
        LastName = InputBox("Enter Last Name: ")

        GetFullName = LastName & ", " & FirstName
End Function

Alternatively, instead of using the name of the function to indicate the value it returns, you can type Return, followed by the value or the expression that the function returns. Based on this, the above function could also be created as follows:

Function GetFullName$()
    Dim FirstName As String, LastName As String

    FirstName = InputBox("Enter First Name: ")
    LastName = InputBox("Enter Last Name: ")

    Return LastName & ", " & FirstName
End Function

You can also use some local variables in the function to perform an assignment and then assign their result to the name of the function.

Practical Learning: Creating a Function

  1. To add a project to the current solution, on the main menu, click File -> Add -> New Project
  2. Set the Name to Rectangular and click OK
  3. In the Solution Explorer, under Rectangular, right-click Module1 and click Rename
  4. Type Rectangle.vb and press Enter
  5. To create a function, change the file as follows:
     
    Public Module Rectangle
    
        Private Function CalculatePerimeter() As Double
            Dim dblLength As Double
            Dim dblWidth As Double
    
            dblLength = InputBox("Enter Rectangle Length: ")
            dblWidth = InputBox("Enter Rectangle Width:  ")
    
            CalculatePerimeter = (dblLength + dblWidth) * 2
        End Function
    
        Public Sub Main()
    
        End Sub
    
    End Module
  6. Save all

Calling a Function

As done for the sub procedure, in order to use a function in your program, you must call it. Like a sub procedure, to call a function, you can simply type its name in the desired section of the program. Here is an example:

Sub Main()
    CallMe
End Sub

Since the primary purpose of a function is to return a value, to better take advantage of such a value, you can assign the name of a function to a variable in the section where you are calling the function. Here is an example:

Module Exercise

    Function GetFullName$()
        Dim FirstName As String, LastName As String

        FirstName = InputBox("Enter First Name: ")
        LastName = InputBox("Enter Last Name: ")

        Return LastName & ", " & FirstName
    End Function

    Friend Sub Main()
        Dim FullName$

        FullName = GetFullName()
        MsgBox(FullName)
    End Sub

End Module

Here is an example of running this program:

Function

Function

Function

Practical Learning: Calling a Function

  1. To call a function, change the Rectangular.vb file as follows:
     
    Public Module Rectangle
    
        Private Function CalculatePerimeter() As Double
            Dim dblLength As Double
            Dim dblWidth As Double
    
            dblLength = InputBox("Enter Rectangle Length: ")
            dblWidth = InputBox("Enter Rectangle Width:  ")
    
            CalculatePerimeter = (dblLength + dblWidth) * 2
        End Function
    
        Public Sub Main()
            Dim Perimeter As Double
    
            Perimeter = CalculatePerimeter()
    
            MsgBox("=-= Square Characteristics=-=" & vbCrLf & _
                   "Perimeter: " & Perimeter)
        End Sub
    
    End Module
  2. Execute the application
  3. Enter the length as 25.55 and the width as 20.05 
  4. Close the DOS window and return to your programming environment

A Function and a Procedure

Depending on an author, in the Visual Basic language, the word "procedure" includes either a sub-procedure created with the Sub keyword, or a function created with the Function keyword. In the same way, for the rest of our lessons, the word procedure will be used to represent both types. Only when we want to be precise will we use the expression "a sub-procedure" to explicitly mean the type of procedure that does not return a value. When the word "function" is used in our lessons, it explicitly refers to the type of procedure that returns a value.

The Main() Procedure

The Visual Basic Language uses a special procedure called Main. The Main procedure is the entry point of a program as we have used it so far. Particularly, Main can be used as a sub procedure or a function. So far, we have used Main only as a sub procedure. To use Main as a function, type the Function keyword required for each function, followed by Main(), followed by As Integer. When declared like this, the Main function must return an integer. The most regularly return value is 0, which indicates that the function ended successfully. Here is an example:

Module Exercise

    Friend Function GetFullName$()
        Dim FirstName As String, LastName As String

        FirstName = InputBox("Enter First Name: ")
        LastName = InputBox("Enter Last Name: ")

        Return LastName & ", " & FirstName
    End Function

    Friend Function Main() As Integer
        Dim FullName$

        FullName = GetFullName()
        MsgBox(FullName)

        Return 0
    End Function

End Module

Practical Learning: Using the Main Function

  1. To use Main() as a function, change it as follows:
     
    Module Central
    
        Private Function CalculatePerimeter() As Double
            Dim dblLength As Double
            Dim dblWidth As Double
    
            dblLength = InputBox("Enter Rectangle Length: ")
            dblWidth = InputBox("Enter Rectangle Width:  ")
    
            CalculatePerimeter = (dblLength + dblWidth) * 2
        End Function
    
        Public Function Main() As Integer
            Dim Perimeter As Double
    
            Perimeter = CalculatePerimeter()
    
            MsgBox("=-= Square Characteristics=-=" & vbCrLf & _
                   "Perimeter: " & Perimeter)
    
    	Return 0
        End Function
    
    End Module
  2. Execute the application
  3. Enter the length as 88.16 and the width as 44.14 
  4. Close the DOS window and return to your programming environment
  1.  

Arguments and Parameters

 

Using Global Variables

In the previous lesson, we saw that you could declare a global variable outside of any procedure. When using various procedures in a code file, one of the characteristics of a global variable is that it is automatically accessible to other procedures:

  • Private: A private global variable can be accessed  by any procedure of the same module. No procedure of another module, even of the same program, can access it
     
    Global Variables
  • Friend: A friendly global variable can be accessed by any procedure of any module of the same project. A procedures of another program cannot access that variable
     
    Friend Access Modifier
  • Public: A public global variable can be accessed by any procedure of its project and procedures of other projects
     
    Access Levels

Based on this characteristic of the procedures of a module having access to global variables of the same program, you can declare such variables and initialize or modify them in any procedure of the same code file.

Practical Learning: Using Global Variables

  1. To use global variables, change the document as follows:
     
    Public Module Rectangle
        Private Length As Double
        Private Width As Double
    
        Private Sub GetLength()
            Length = InputBox("Enter Rectangle Length:")
        End Sub
    
        Private Sub GetWidth()
            Width = InputBox("Enter Rectangle Width:")
        End Sub
    
        Private Function CalculatePerimeter() As Double
            CalculatePerimeter = (Length + Width) * 2
        End Function
    
        Public Function Main() As Integer
            Dim Perimeter As Double
            GetLength()
            GetWidth()
            Perimeter = CalculatePerimeter()
    
            MsgBox("=-= Square Characteristics=-=" & vbCrLf & _
                   "Length: " & vbTab & Length & vbCrLf & _
                   "Width: " & vbTab & Width & vbCrLf & _
                   "Perimeter: " & Perimeter)
    	Return 0
        End Function
    
    End Module
  2. Execute the application
  3. Enter the length as 32.08 and the with as 24.84
  4. Close the message box and the DOS window

Introduction to Arguments

So far, to use a value in a procedure, we had to declare it. In some cases, a procedure may need an external value in order to carry its assignment. A value that is supplied to a procedure is called an argument.

When creating a procedure that will use an external value, declare the argument that represents that value between the parentheses of the procedure. For a sub procedure, the syntax you use would be:

Sub ProcedureName(Argument)
      
End Sub

If you are creating a function, the syntax would be:

Function ProcedureName(Argument) As DataType
      
Function Sub

The argument must be declared as a normal variable, omitting the Dim keyword. Here is an example that creates a function that takes a string as argument:

Function CalculatePayroll(strName As String) As Double
      
Function Sub

A certain procedure can take more than one argument. In this case, in the parentheses of the procedure, separate the arguments with a comma. Here is an example of a sub procedure that takes two arguments:

Sub EvaluateInvoice(EmplName As String, HourlySalary As Currency)
      
End Sub

In the body of a procedure that takes one or more arguments, use the argument(s) as you see fit as if they were locally declared variables. For example, you can involve them with values inside of the procedure. You can also exclusively use the values of the arguments to perform the assignment.

Calling a Procedure With Argument

To call a procedure that takes an argument, type its name and a space, followed by a value for each argument between parentheses. The value provided for an argument is also called a parameter. If there is more than one argument, separate them with a comma. Here is an example:

Module Exercise

    Private Function GetFullName$(strFirst As String, _
                                  strLast As String)
        Dim FName As String

        FName = strFirst & " " & strLast
        GetFullName = FName
    End Function

    Public Function Main() As Integer
        Dim FirstName, LastName As String
        Dim FullName As String
        Dim ComputerLanguage As String = "Visual Basic"

        FirstName = inputbox("Enter First Name: ")
        LastName = inputbox("Enter Last Name: ")

        FullName = GetFullName(FirstName, LastName)

        msgbox("Hello, " & FullName)
        Welcome(ComputerLanguage)
	Return 0
    End Function

    Sub Welcome(ByVal strLanguage As String)
        msgbox("Welcome to the wonderful world of " & strLanguage)
    End Sub

End Module

As mentioned previously, you can also use the Call keyword to call a procedure.

When you call a procedure that takes more than one argument, you must provide the values of the arguments in the exact order they are listed inside of the parentheses of the function. Fortunately, you do not have to. If you know the names of the arguments, you can type them in any order and provide a value for each. To do that, on the right side of each argument, type the := operator followed by the desired value for the argument. Here is an example:

Public Module Exercise

    Private Function GetFullName$(MI As String, _
				  LastName As String, _
				  FirstName As String)
        GetFullName = FirstName & " " & MI & " " & LastName
    End Function

    Public Function Main() As Integer
        Dim FullName As String
        Dim ComputerLanguage As String = "VBasic"

        FullName = GetFullName(LastName:="Roberts", FirstName:="Alan", MI:="R.")

        MsgBox("Hello " & FullName)
        Call Welcome(ComputerLanguage)
	Return 0
    End Function

    Private Sub Welcome(ByVal strLanguage As String)
        MsgBox("Welcome to the wonderful world of " & strLanguage)
    End Sub

End Module

Practical Learning: Passing Arguments to a Procedure

  1. To pass arguments to a function, change the file as follows (when you type the argument, Microsoft Visual Studio, actually the Visual Basic language parser, will add the ByVal keywords; in the next sections, we will learn what that keyword means; for now, keep but ignore it):  
    Public Module Rectangle
    
        Private Function GetValue(TypeOfValue As String) As Double
            Dim Value As Double
    
            Value = InputBox("Enter the " & TypeOfValue & ":")
            Return Value
        End Function
    
        Private Function CalculatePerimeter(Length As Double, _
                                            Width As Double) As Double
            CalculatePerimeter = (Length + Width) * 2
        End Function
    
        Public Function Main() As Integer
            Dim L As Double, W As Double
            Dim Perimeter As Double
    
            L = GetValue("Length")
            W = GetValue("Width")
            Perimeter = CalculatePerimeter(L, W)
    
            MsgBox("=-= Square Characteristics=-=" & vbCrLf & _
                   "Length: " & L & vbCrLf & _
                   "Width: " & W & vbCrLf & _
                   "Perimeter: " & Perimeter)
    	Return 0
        End Function
    
    End Module
  2. Execute the application
  3. Enter the length as 44.14 and the with as 30.76
  4. Close the message box and the DOS window

Techniques of Passing Arguments

 

Passing Arguments (By Value)

When calling a procedure that takes an argument, we were supplying a value for that argument. When this is done, the procedure that is called makes a copy of the value of the argument and makes that copy available to the calling procedure. That way, the argument itself is not accessed. This is referred to as passing an argument by value. This can be reinforced by typing the ByVal keyword on the left side of the argument. Here is an example:

Private Sub Welcome(ByVal strLanguage As String)
    MsgBox("Welcome to the wonderful world of " & strLanguage)
End Sub

If you create a procedure that takes an argument by value and you have used the ByVal keyword on the argument, when calling the procedure, you do not need to use the ByVal keyword; just the name of the argument is enough, as done in the examples on arguments so far. Here is an example:

Public Module Exercise

    Public Function Main() As Integer
        Dim ComputerLanguage As String = "Visual Basic"

        Welcome(ComputerLanguage)
	Return 0
    End Function

    Private Sub Welcome(ByVal strLanguage As String)
        MsgBox("Welcome to the wonderful world of " & strLanguage)
    End Sub

End Module

This would produce:

Arguments

Passing Arguments By Reference

An alternative to passing arguments as done so far is to pass the address of the argument to the called procedure. When this is done, the called procedure does not receive a simple copy of the value of the argument: the argument is accessed by its address. That is, at its memory address. With this technique, any action carried on the argument will be kept. If the value of the argument is modified, the argument would now have the new value, dismissing or losing the original value it had. This technique is referred to as passing an argument by reference. Consider the following program:

Public Module Exercise

    Private Function Addition#(ByVal Value1 As Double, ByVal Value2 As Double)
        Value1 = InputBox("Enter First Number:  ")
        Value2 = InputBox("Enter Second Number: ")

        Addition = Value1 + Value2
    End Function

    Public Function Main() As Integer
        Dim Result As String
        Dim Number1, Number2 As Double

        Result = Addition(Number1, Number2)
        MsgBox(Number1 & " + " & Number2 & " = " & Result)
	Return 0
    End Function

End Module

Here is an example of running the program:

Notice that, although the values of the arguments were changed in the Addition() procedure, at the end of the procedure, they lose the value they got in the function. If you want a procedure to change the value of an argument, you can pass the argument by reference.

To pass an argument by reference, on its left, type the ByRef keyword. This is done only when creating the procedure. When the called procedure finishes with the argument, the argument would keep whatever modification was made on its value. Now consider the same program as above but with arguments passed by reference:

Public Module Exercise

    Private Function Addition#(ByRef Value1 As Double, ByRef Value2 As Double)
        Value1 = InputBox("Enter First Number:  ")
        Value2 = InputBox("Enter Second Number: ")

        Addition = Value1 + Value2
    End Function

    Public Function Main() As Integer
        Dim Result As String
        Dim Number1, Number2 As Double

        Result = Addition(Number1, Number2)
        MsgBox(Number1 & " + " & Number2 & " = " & Result)
	Return 0
    End Function

End Module

Here is an example of running the program:

Using this technique, you can pass as many arguments by reference and as many arguments by value as you want. As you may guess already, this technique is also used to make a sub procedure return a value, which a regular sub routine cannot do. Furthermore, passing arguments by reference allows a procedure to return as many values as possible while a regular function can return only one value.

Practical Learning: Passing Arguments by Reference

  1. To pass an argument by reference, change the file as follows:
     
    Public Module Rectangle
    
        Private Sub GetValues(ByRef Length As Double, ByRef Width As Double)
            Length = InputBox("Enter the length:")
            Width = InputBox("Enter the width:")
        End Sub
    
        Private Function CalculatePerimeter(ByVal Length As Double, _
                                            ByVal Width As Double) As Double
            CalculatePerimeter = (Length + Width) * 2
        End Function
    
        Private Function CalculateArea(ByVal Length As Double, _
                                            ByVal Width As Double) As Double
            CalculateArea = Length * Width
        End Function
    
        Private Sub ShowCharacteristics(ByVal Length As Double, _
                                            ByVal Width As Double)
            Dim Result As String
    
            Result = "=-= Rectangle Characteristics =-=" & vbCrLf & _
                           "Length: " & vbTab & vbTab & CStr(Length) & vbCrLf & _
                           "Width: " & vbTab & vbTab & CStr(Width) & vbCrLf & _
                           "Perimeter: " & vbTab & _
                           CalculatePerimeter(Length, Width) & vbCrLf & _
                           "Area: " & vbTab & vbTab & _
    			CalculateArea(Length, Width)
            MsgBox(Result)
        End Sub
    
        Public Function Main() As Integer
            Dim L As Double, W As Double
    
            GetValues(L, W)
            ShowCharacteristics(L, W)
    	Return 0
        End Function
    
    End Module
  2. Execute the application
  3. Enter the length as 24.55 and the width as 22.85
     
  4. Close the message box and the DOS window
 
 

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