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Week 2 – Visual Basic Tutorial 

Data types in Visual Basic 6

By default Visual Basic variables are of variant data types. The variant data type can store numeric, date/time or string data. When a variable is declared, a data type is supplied for it that determines the kind of data they can store. The fundamental data types in Visual Basic including variant are integer, long, single, double, string, currency, byte and boolean. Visual Basic supports a vast array of data types. Each data type has limits to the kind of information and the minimum and maximum values it can hold. In addition, some types can interchange with some other types. A list of Visual Basic's simple data types are given below. 

1. Numeric

2. String

Use to store alphanumeric values. A variable length string can store approximately 4 billion characters 

3. Date

Use to store date and time values. A variable declared as date type can store both date and time values and it can store date values 01/01/0100 up to 12/31/9999 

4. Boolean

Boolean data types hold either a true or false value. These are not stored as numeric values and cannot be used as such. Values are internally stored as -1 (True) and 0 (False) and any non-zero value is considered as true. 

5. Variant

Stores any type of data and is the default Visual Basic data type. In Visual Basic if we declare a variable without any data type by default the data type is assigned as default. 

Operators in Visual Basic 

Arithmetical Operators

Relational Operators

Logical Operators

Variables in Visual Basic 6 

Variables are the memory locations which are used to store values temporarily. A defined naming strategy has to be followed while naming a variable. A variable name must begin with an alphabet letter and should not exceed 255 characters. It must be unique within the same scope. It should not contain any special character like %, &, !, #, @ or $.  

There are many ways of declaring variables in Visual Basic. Depending on where the variables are declared and how they are declared, we can determine how they can be used by our application. The different ways of declaring variables in Visual Basic are listed below and elucidated in this section. 

·                  Explicit Declaration 

·                  Using Option Explicit statement 

·                  Scope of Variables 

Explicit Declaration 

Declaring a variable tells Visual Basic to reserve space in memory. It is not must that a variable should be declared before using it. Automatically whenever Visual Basic encounters a new variable, it assigns the default variable type and value. This is called implicit declaration. Though this type of declaration is easier for the user, to have more control over the variables, it is advisable to declare them explicitly. The variables are declared with a Dim statement to name the variable and its type. The As type clause in the Dim statement allows to define the data type or object type of the variable. This is called explicit declaration. 

Syntax 

Dim variable [As Type] 

For example, 

Dim strName As String
Dim intCounter As Integer 

Using Option Explicit statement 

It may be convenient to declare variables implicitly, but it can lead to errors that may not be recognized at run time. Say, for example a variable by name intcount is used implicitly and is assigned to a value. In the next step, this field is incremented by 1 by the following statement 

Intcount = intcont + 1 

This calculation will result in intcount yielding a value of 1 as intcount would have been initialized to zero. This is because the intcount variable has been mityped as incont in the right hand side of the second variable. But Visual Basic does not see this as a mistake and considers it to be new variable and therefore gives a wrong result. 

In Visual Basic, to prevent errors of this nature, we can declare a variable by adding the following statement to the general declaration section of the Form. 

Option Explicit 

This forces the user to declare all the variables. The Option Explicit statement checks in the module for usage of any undeclared variables and reports an error to the user. The user can thus rectify the error on seeing this error message. 

The Option Explicit statement can be explicitly placed in the general declaration section of each module using the following steps. 

·        Click Options item in the Tools menu 

·        Click the Editor tab in the Options dialog box 

·        Check Require Variable Declaration option and then click the OK button 

Scope of variables 

A variable is scoped to a procedure-level (local) or module-level variable depending on how it is declared. The scope of a variable, procedure or object determines which part of the code in our application are aware of the variable's existence. A variable is declared in general declaration section of e Form, and hence is available to all the procedures. Local variables are recognized only in the procedure in which they are declared. They can be declared with Dim and Static keywords. If we want a variable to be available to all of the procedures within the same module, or to all the procedures in an application, a variable is declared with broader scope. 

Local Variables 

A local variable is one that is declared inside a procedure. This variable is only available to the code inside the procedure and can be declared using the Dim statements as given below. 

Dim sum As Integer 

The local variables exist as long as the procedure in which they are declared, is executing. Once a procedure is executed, the values of its local variables are lost and the memory used by these variables is freed and can be reclaimed. Variables that are declared with keyword Dim exist only as long as the procedure is being executed. 

Static Variables 

Static variables are not reinitialized each time Visual Invokes a procedure and therefore retains or preserves value even when a procedure ends. In case we need to keep track of the number of times a command button in an application is clicked, a static counter variable has to be declared. These static variables are also ideal for making controls alternately visible or invisible. A static variable is declared as given below. 

Static intPermanent As Integer

Variables have a lifetime in addition to scope. The values in a module-level and public variables are preserved for the lifetime of an application whereas local variables declared with Dim exist only while the procedure in which they are declared is still being executed. The value of a local variable can be preserved using the Static keyword. The follwoing procedure calculates the running total by adding new values to the previous values stored in the static variable value. 

Function RunningTotal ( )
Static Accumulate
Accumulate = Accumulate + num
RunningTotal = Accumulate
End Function 

If the variable Accumulate was declared with Dim instead of static, the previously accumulated values would not be preserved accross calls to the procedure, and the procedure would return the same value with which it was called. To make all variables in a procedure static, the Static keyword is placed at the beginning of the procedure heading as given in the below statement. 

Static Function RunningTotal ( ) 

Example 

The following is an example of an event procedure for a CommandButton that counts and displays the number of clicks made. 

Private Sub Command1_Click ( )
Static Counter As Integer
Counter = Counter = 1
Print Counter
End Sub 

The first time we click the CommandButton, the Counter starts with its default value of zero. Visual Basic then adds 1 to it and prints the result. 

Module Level Variables

A module level variable is available to all the procedures in the module. They are declared using the Public or the Private keyword. If you declare a variable using a Private or a Dim statement in the declaration section of a module—a standard BAS module, a form module, a class module, and so on—you're creating a private module-level variable. Such variables are visible only from within the module they belong to and can't be accessed from the outside. In general, these variables are useful for sharing data among procedures in the same module: 

' In the declarative section of any module
Private LoginTime As Date ' A private module-level variable
Dim LoginPassword As String ' Another private module-level variable 

You can also use the Public attribute for module-level variables, for all module types except BAS modules. (Public variables in BAS modules are global variables.) In this case, you're creating a strange beast: a Public module-level variable that can be accessed by all procedures in the module to share data and that also can be accessed from outside the module. In this case, however, it's more appropriate to describe such a variable as a property: 

' In the declarative section of Form1 module
Public CustomerName As String ' A Public property 

You can access a module property as a regular variable from inside the module and as a custom property from the outside: 

' From outside Form1 module...
Form1.CustomerName = "John Smith" 

The lifetime of a module-level variable coincides with the lifetime of the module itself. Private variables in standard BAS modules live for the entire life of the application, even if they can be accessed only while Visual Basic is executing code in that module. Variables in form and class modules exist only when that module is loaded in memory. In other words, while a form is active (but not necessarily visible to the user) all its variables take some memory, and this memory is released only when the form is completely unloaded from memory. The next time the form is re-created, Visual Basic reallocates memory for all variables and resets them to their default values (0 for numeric values, "" for strings, Nothing for object variables).  

Public vs Local Variables 

A variable can have the same name and different scope. For example, we can have a public variable named R and within a procedure we can declare a local variable R. References to the name R within the procedure would access the local variable and references to R outside the procedure would access the public variable. 

Procedures in Visual Basic 6 

Visual Basic offers different types of procedures to execute small sections of coding in applications. The various procedures are elucidated in details in this section. Visual Basic programs can be broken into smaller logical components called Procedures. Procedures are useful for condensing repeated operations such as the frequently used calculations, text and control manipulation etc. The benefits of using procedures in programming are:  

It is easier to debug a program a program with procedures, which breaks a program into discrete logical limits. 

Procedures used in one program can act as building blocks for other programs with slight modifications. 

A Procedure can be Sub, Function or Property Procedure. 

Sub Procedures 

A sub procedure can be placed in standard, class and form modules. Each time the procedure is called, the statements between Sub and End Sub are executed. The syntax for a sub procedure is as follows: 

[Private | Public] [Static] Sub Procedurename [( arglist)]
[ statements]
End Sub 

arglist is a list of argument names separated by commas. Each argument acts like a variable in the procedure. There are two types of Sub Procedures namely general procedures and event procedures. 

Event Procedures 

An event procedure is a procedure block that contains the control's actual name, an underscore(_), and the event name. The following syntax represents the event procedure for a Form_Load event. 

Private Sub Form_Load()
....statement block..
End Sub 

Event Procedures acquire the declarations as Private by default. 

General Procedures 

A general procedure is declared when several event procedures perform the same actions. It is a good programming practice to write common statements in a separate procedure (general procedure) and then call them in the event procedure. 

In order to add General procedure: 

·                  The Code window is opened for the module to which the procedure is to be added. 

·                  The Add Procedure option is chosen from the Tools menu, which opens an Add Procedure dialog box as shown in the figure given below. 

·                  The name of the procedure is typed in the Name textbox 

·                  Under Type, Sub is selected to create a Sub procedure, Function to create a Function procedure or Property to create a Property procedure. 

·                  Under Scope, Public is selected to create a procedure that can be invoked outside the module, or Private to create a procedure that can be invoked only from within the module. 

We can also create a new procedure in the current module by typing Sub ProcedureName, Function ProcedureName, or Property ProcedureName in the Code window. A Function procedure returns a value and a Sub Procedure does not return a value.

Function Procedures 

Functions are like sub procedures, except they return a value to the calling procedure. They are especially useful for taking one or more pieces of data, called arguments and performing some tasks with them. Then the functions returns a value that indicates the results of the tasks complete within the function. 

The following function procedure calculates the third side or hypotenuse of a right triangle, where A and B are the other two sides. It takes two arguments A and B (of data type Double) and finally returns the results. 

Function Hypotenuse (A As Double, B As Double) As Double
Hypotenuse = sqr (A^2 + B^2)
End Function 

The above function procedure is written in the general declarations section of the Code window. A function can also be written by selecting the Add Procedure dialog box from the Tools menu and by choosing the required scope and type. 

Property Procedures 

A property procedure is used to create and manipulate custom properties. It is used to create read only properties for Forms, Standard modules and Class modules. Visual Basic provides three kind of property procedures-Property Let procedure that sets the value of a property, Property Get procedure that returns the value of a property, and Property Set procedure that sets the references to an object. 

Control Structures in Visual Basic 6.0 

Control Statements are used to control the flow of program's execution. Visual Basic supports control structures such as if... Then, if...Then ...Else, Select...Case, and Loop structures such as Do While...Loop, While...Wend, For...Next etc method.  

If...Then selection structure 

The If...Then selection structure performs an indicated action only when the condition is True; otherwise the action is skipped. 

Syntax of the If...Then selection 

If <condition> Then
statement
End If 

e.g.: If average>75 Then
txtGrade.Text = "A"
End If 

If...Then...Else selection structure 

The If...Then...Else selection structure allows the programmer to specify that a different action is to be performed when the condition is True than when the condition is False.  

Syntax of the If...Then...Else selection 

If <condition > Then
statements
Else
statements
End If 

e.g.: If average>50 Then
txtGrade.Text = "Pass"
Else
txtGrade.Text = "Fail"
End If 

Nested If...Then...Else selection structure 

Nested If...Then...Else selection structures test for multiple cases by placing If...Then...Else selection structures inside If...Then...Else structures. 

Syntax of the Nested If...Then...Else selection structure 

You can use Nested If either of the methods as shown above 

Method 1

If < condition 1 > Then
statements
ElseIf < condition 2 > Then
statements
ElseIf < condition 3 > Then
statements
Else
Statements
End If 

Method 2

If < condition 1 > Then
statements
Else
If < condition 2 > Then
statements
Else
If < condition 3 > Then
statements
Else
Statements
End If
End If
EndIf 

e.g.: Assume you have to find the grade using nested if and display in a text box 

If average > 75 Then
txtGrade.Text = "A"
ElseIf average > 65 Then
txtGrade.Text = "B"
ElseIf average > 55 Then
txtGrade.text = "C"
ElseIf average > 45 Then
txtGrade.Text = "S"
Else
txtGrade.Text = "F"
End If 

Select...Case selection structure 

Select...Case structure is an alternative to If...Then...ElseIf for selectively executing a single block of statements from among multiple block of statements. Select...case is more convenient to use than the If...Else...End If. The following program block illustrate the working of Select...Case. 

Syntax of the Select...Case selection structure 

Select Case Index
Case 0
Statements
Case 1
Statements
End Select

  e.g.: Assume you have to find the grade using select...case and display in the text box 

Dim average as Integer

average = txtAverage.Text
Select Case average
Case 100 To 75
txtGrade.Text ="A"
Case 74 To 65
txtGrade.Text ="B"
Case 64 To 55
txtGrade.Text ="C"
Case 54 To 45
txtGrade.Text ="S"
Case 44 To 0
txtGrade.Text ="F"
Case Else
MsgBox "Invalid average marks"
End Select 

Note: In this example I have used a message box function. In later lessons you will learn how to use message box functions