This is in continuation from VB Script and QTP - Part1 on our series of posts on VB Script. Here, we will dwell upon conditional constructs, iterative constructs and arrays.
Conditional Constructs
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 Then
MsgBox value
ElseIf value = 1 Then
MsgBox value
ElseIf value = 2 then
Msgbox value
Else
Msgbox "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
