ITSYLLABUS

Controlling Program Execution

You can control the flow of your script with conditional statements and looping statements. Using conditional statement, you can write VBScript code that makes decisions and repeats actions. The following conditional statements are available in VBScript:

• If...Then...Else statement
• Select Case statement

Making Decisions Using If...Then...Else

The If...Then...Else statement is used to evaluate whether a condition is True or False and, depending on the result, to specify one or more statements to run. Usually the condition is an expression that uses a comparison operator to compare one value or variable with another. For information about comparison operators, see Comparison Operators. If...Then...Else statements can be nested to as many levels as you need.

 

Running Statements if a Condition is True

To run only one statement when a condition is True, use the single-line syntax for the If...Then...Else statement. The following example shows the single-line syntax. Notice that this example omits the Else keyword.

Sub FixDate()

     Dim myDate

     myDate = #2/13/95#

     If myDate < Now Then myDate = Now

 End Sub

To run more than one line of code, you must use the multiple-line (or block) syntax. This syntax includes the End If statement, as shown in the following example:

 Sub AlertUser(value)

     If value = 0 Then

         AlertLabel.ForeColor = vbRed

         AlertLabel.Font.Bold = True

         AlertLabel.Font.Italic = True

     End If

 End Sub

Running Certain Statements if a Condition is True and Running Others if a Condition is False

You can use an If...Then...Else statement to define two blocks of executable statements: one block to run if the condition is True, the other block to run if the condition is False.

Sub AlertUser(value)

     If value = 0 Then

         AlertLabel.ForeColor = vbRed

         AlertLabel.Font.Bold = True

         AlertLabel.Font.Italic = True

     Else

         AlertLabel.Forecolor = vbBlack

         AlertLabel.Font.Bold = False

         AlertLabel.Font.Italic = False

     End If

 End Sub

Deciding Between Several Alternatives

A variation on the If...Then...Else statement allows you to choose from several alternatives. Adding ElseIf clauses expands the functionality of the If...Then...Else statement so you can control program flow based on different possibilities. 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.

Making Decisions with Select Case

The Select Case structure provides an alternative to If...Then...ElseIf for selectively executing one block of statements from among multiple blocks of statements. A Select Case statement provides capability similar to the If...Then...Else statement, but it 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:

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

Notice that the Select Case structure evaluates an expression once at the top of the structure. In contrast, the If...Then...ElseIf structure can evaluate a different expression for each ElseIf statement. You can replace an If...Then...ElseIf structure with a Select Case structure only if each ElseIf statement evaluates the same expression.