Using Conditional Statements

项目
10/24/2011

You can control the flow of your script with conditional statements and looping statements.

Controlling Program Execution

Using conditional statements, 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, as in the following 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

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.

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