ASP.NET in Visual Basic - Lesson 05: Introduction to Procedures and Functions

	Introduction to Procedures and Functions

Introduction to Procedures

Overview

A procedure is a section of instructions that solve a relatively small problem. There are two categories of procedures you will use in your programs: those that have already been created and are available to you, and those you will create yourself.

Creating a Procedure

To create a procedure, start by typing the Sub keyword followed by a name (like everything else, a procedure must have a name). At the end of the procedure, you must type End Sub. Therefore, the primary formula of a procedure is:

Sub procedure-name()

End Sub

A procedure is created in a script. Therefore, its code can be included in a script element. This would be done as follows:

<script runat="server">
Sub procedure-name()

End Sub
</script>

The name of a procedure follows the rules of variables in the Visual Basic language. The section between the Subprocedure-name() and the End Sub lines is referred to as the body of the procedure. The body of the procedure is used to define what, and how, the assignment should be carried. Here is an example:

<script runat="server">
Sub Describe()
    Response.Write("Helium is a chemical element found, or used, in solids, in liquids, and in gases.")
End Sub
</script>

Iff you need to use a variable inside the procude, you can declare it and use it.

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. To call a simple procedure, type its name followed by parentheses in the section where you want to use it. To call a procedure in the body section of a webpage, you can do it side the <% and %> delimiters. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<html>
<head>
<script runat="server">
Sub Describe()
    Response.Write("Helium is a chemical element found, or used, in solids, in liquids, and in gases.")
End Sub
</script>
<title>Chemistry - Helium</title>

</head>
<body>

<%
Describe()
%>

</body>
</html>

This would produce:

Calling a Sub Procedure

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

<%@ Page Language="VB" %>

<!DOCTYPE html>

<html>
<head>
<script runat="server">
Sub Display()
    Response.Write("Welcome to our website.")
End Sub
</script>
<title>Exercises</title>

</head>
<body>

<%
Call Display()
%>

</body>
</html>

Introduction to Functions

Introduction

Like a procedure, a function is a sub-program used to perform an assignment. The main difference between a 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".

Creating a Function

To create a function, you use the Function keyword followed by a name and parentheses. Unlike a 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 parenthesis, 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 formula to create a function is:

AccessModifier(s) Function function-name() As data-type
    
End Function

As seen for procedures, a function can be created in a script section. Other than that, a function can start as follows:

<script runat="server">
Function GetFullName() As String
        
End Function
</script>

As done with 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:

<script runat="server">
Function GetFullName$()

End Function
</script>

As mentioned already, the section between the Function function-name() 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:

<script runat="server">
Function CalculateNetPay() As Double
    Dim HourlySalary = 48.26
    Dim TimeWorked = 36.50

    Dim Multiplication As Double
    Multiplication = HourlySalary * TimeWorked
End Function
</script>

Returning a Value From a Function

After performing an assignment in a function, to indicate the value it returns, 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:

<script runat="server">
Function CalculateNetPay() As Double
    Dim HourlySalary = 48.26
    Dim TimeWorked = 36.50

    Dim Multiplication As Double
    Multiplication = HourlySalary * TimeWorked

    CalculateNetPay = Multiplication
End Function
</script>

As an alternative, in place of the name of the function and =, type the Return keyword followed by the value or the expression that the function returns. Here is an example:

<script runat="server">
Function CalculateNetPay() As Double
    Dim HourlySalary = 48.26
    Dim TimeWorked = 36.50

    Dim Multiplication As Double
    Multiplication = HourlySalary * TimeWorked

    Return Multiplication
End Function
</script>

As you can see from the above example, you can use a local variable that would carry the returned value. As an alternative, you can return an expression from the function. This can be done as follows:

<script runat="server">
Function CalculateNetPay() As Double
    Dim HourlySalary = 48.26
    Dim TimeWorked = 36.50

    Dim Multiplication As Double
    Multiplication = HourlySalary * TimeWorked

    ' This
    CalculateNetPay = HourlySalary * TimeWorked
    ' or this
    ' Return HourlySalary * TimeWorked
End Function
</script>

New Convention:

From now on, in our lessons, to refer to a procedure or function, we will use the name of a procedure followed by parentheses. For example, we may write "the Convert() procedure" or "the Convert() function".

Calling a Function

In order to use a function, you must call it. Like a sub procedure, to call a function, you can simply type its name in the desired section of your code. 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 the function to a variable in the section where you are calling the function. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<html>
<head>
<script runat="server">
Function CalculateNetPay() As Double
    Dim HourlySalary = 48.26
    Dim TimeWorked = 36.50

    Dim Multiplication As Double
    Multiplication = HourlySalary * TimeWorked

    CalculateNetPay = Multiplication
End Function
</script>
<title>Exercises</title>

</head>
<body>

<%
Dim Result As Double

Result = CalculateNetPay()

Response.Write("Net Pay: ")
Response.Write(Result)
%>

</body>
</html>

This would produce:

Calling a Function

The Scope and Lifetime of a Variable

Local Variables

The scope of a variable determines the areas of code where the variable is available. A variable declared inside of a procedure or function is referred to as a local variable. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<html>
<head>
<script runat="server">

Sub PresentRectangle()
    ' A local variable
    Dim Height = 27.19

    Response.Write("Geometry: Rectangle")
    Response.Write("<br>Height: " & CStr(Height))
End Sub
</script>
<title>Exercises</title>

</head>
<body>

<%
PresentRectangle()
%>

</body>
</html>

This would produce:

Local Variables

Such as variable cannot be accessed by another function or another procedure created in the webpage.

Global Variables

A variable that is declared outside of any procedure or function is referred to as global. To declare a global variable, use the same formula as we have done so far. You can type Dim, followed by the name of the variable, the As keyword, its type and an optional initialization. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<html>
<head>
<script runat="server">
' A global variable
Dim Width = 48.16

Sub PresentRectangle()
    ' A local variable
    Dim Height = 27.19

    Response.Write("Geometry: Rectangle")
    Response.Write("<br>Width: " & Width)
    Response.Write("<br>Height: " & Height)
End Sub
</script>
<title>Exercises</title>

</head>
<body>

<%
PresentRectangle()
%>

</body>
</html>

This would produce:

Global Variables

You can initialize the global variable when or after declaring it.

Access Modifiers

Friendly Members

A global variable declared in the code of a webpage is referred to as a friend. To declare a friendly variable, instead of Dim, use the Friend keyword. Here is an example:

<script runat="server">
Friend Length = 117.09

Sub Present()

End Sub
</script>

You can also declare many friendly variables with one Friend keyword. Here are examples:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<html>
<head>
<script runat="server">
Friend AreaOfStudy = "Social Science", Major = "Psychology", Degree = "Bachelor of Art (B.A.)"

Sub Display()
    Response.Write("Area of Study: " & AreaOfStudy)
    Response.Write("<br>Major: " & Major)
    Response.Write("<br>Degree: " & Degree)
End Sub
</script>
<title>Exercises</title>

</head>
<body>

<%
Display()
%>

</body>
</html>

This would produce:

Friendly Members

After declaring a friendly variable, if your website has many files with code, the variable can be accessed from other files.

Private Members

You can declare a variable that can be accessed only within the file that contains it. Such a variable is referred to as private.

To declare a private variable, in place of Dim or Friend, use the Private keyword. Here is an example:

<script runat="server">
Private Side = 34.85

Sub Present()

End Sub
</script>

Public Members

A variable can be made publicly available. To declare such a variable, use the Public keyword. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<html>
<head>
<script runat="server">
Friend FullName As String
Private DateHired As String
Public HourlySalary As Double

Sub Display()
    FullName = "Abigail Ammos"
    DateHired = "4-8-2016"
    HourlySalary = 36.75

    Dim Information As String
    Information = "Full Name: " & FullName & 
                         "<br>Date Hired: " & DateHired &
                         "<br>Hourly Salary: " & HourlySalary

    Response.Write(Information)
End Sub
</script>
<title>Exercises</title>

</head>
<body>

<%
Display()
%>

</body>
</html>

This would produce:

Public Members

Access Modifiers Applied to Procedures and Functions

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

Private: A procedure or function marked as private can be called only by members of the same code section
Friend: A friendly procedure or function can be called by members of the code section and other files of the same project
Public: A public procedure can be called from its code section and other code sections


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