Conditional Constructs execute statements or repeat certain set of statements based on conditions.
The following conditional constructs are available in VBScript
· If – Then –Else
· Select Case
If – Then – Else Construct
The If – Then- Else Construct is used to evaluate whether a condition is true or false and depending on the result, to specify one or more statements to execute. Usually the condition is an expression that uses a comparison operator to compare one value or variable with another. The If- Then – Else statements can be nested to as many levels as needed.For example:
Sub ReportValue(value)If value = 0 ThenMsgBox valueElseIf value = 1 ThenMsgBox valueElseIf value = 2 thenMsgbox valueElseMsgbox "Value out of range!"End If
You can add as many ElseIf clauses as you need to provide alternative choices. Extensive use of the ElseIf clauses often becomes cumbersome. A better way to choose between several alternatives is the Select Case statement.
Select Case Construct
The Select-Case structure is an alternative to If Then Else for selectively executing one block of statements from among multiple blocks of statements. The Select Case Construct makes code more efficient and readable.
A Select Case structure works with a single test expression that is evaluated once, at the top of the structure. The result of the expression is then compared with the values for each Case in the structure. If there is a match, the block of statements associated with that Case is executed.
For example:
Select Case Document.Form1.CardType.Options(SelectedIndex).Text
Case "MasterCard"
DisplayMCLogo
ValidateMCAccount
Case "Visa"
DisplayVisaLogo
ValidateVisaAccount
Case "American Express"
DisplayAMEXCOLogo
ValidateAMEXCOAccount
Case Else DisplayUnknownImage PromptAgain
End Select
Iterative Constructs
Looping allows to run a group of statements repeatedly. The loop is repeated based on a condition. The loop runs as long as the condition is true. The following looping constructs are available in VBScript.
· Do – Loop
· While – Wend
· For – Next
Do – Loop
Do – Loop statements are used to execute a block of statements based on a condition. The statements are repeated either while a condition is true or until a condition becomes true. While Keyword can be used to check a condition in a Do – Loop construct. The condition can be checked before entering into the loop or after the loop has run at least once.The basic difference between a “Do while – Loop” and “Do - Loop while” is that the previous one gets executed only when the condition in the while statement holds true where as a “Do – Loop while” gets executed atleast once, because the condition in the while statement gets checked at the end of the first iteration.
While – Wend
The While...Wend statement is provided in VBScript for those who are familiar with its usage. However, because of the lack of flexibility in while...wend, it is recommended that you use Do...Loop instead.
For..Next
The For-Next loop can be used to run a block of statements a specific number of times. For loops use a counter variable whose value is increased or decreased with each repetition of the loop. The Step Keyword is used to increase or decrease the counter variable by the value that is specified along with it. The For-Next statement can be terminated before the counter reaches its end value by using the Exit For statement.For example:
Dim j, total
For j = 2 To 10 Step 2
total = total + j
Next
MsgBox "The total is " & total
Arrays
An array is a contiguous area in the memory referred to by a common name. It is a series of variables having the same data type. Arrays are used to store related data values. VBScript allows you to store a group of common values together in the same location. These values can be accessed with their reference numbers.
An array is made up of two parts, the array name and the array subscript. The subscript indicates the highest index value for the elements within the array. Each element of an array has a unique identifying index number by which it can be referenced. VBScript creates zero based arrays where the first element of the array has an index value of zero.
Declaring Arrays
An array must be declared before it can be used. Depending upon the accessibility, arrays are of two types:· Local Arrays
A local array is available only within the function or procedure, where it is declared.
· Global Arrays
A global array is an array that can be used by all functions and procedures. It is declared at the beginning of the VBScript Code.
The Dim statement is used to declare arrays. The syntax for declaring an array is as follows:
Dim ArrayName(subscriptvalue)
Where, ArrayName is the unique name for the array and SubscriptValue is a numeric value that indicates the number of elements in the array dimension within the array.
Example:
Dim No_Passengers(3)
The No_Passengers can store 4 values.
Assigning values to the array
No_Passengers(0) = 1No_Passengers(1) = 2
No_Passengers(2) = 3
No_Passengers(3) = 4
Static and Dynamic Arrays:
VBScript provides flexibility for declaring arrays as static or dynamic.
A static array has a specific number of elements. The size of a static array cannot be altered at run time.
A dynamic array can be resized at any time. Dynamic arrays are useful when size of the array cannot be determined. The array size can be changed at run time.
Next we will deal with user defined procedures, functions and subroutines.
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, VBScript will be very familiar. Even if you do not know Visual Basic, once you learn VBScript, you are on your way to programming with the whole family of Visual Basic languages.)