Difference between revisions of "Documentation/BASIC Guide/Branching"

Revision as of 01:13, 23 January 2009

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Use branching statements to restrict the execution of a code block until a particular condition is satisfied.

If...Then...Else

The most common branching statement is the If statement as shown in the following example:

If A > 3 Then
  B = 2
End If

The B = 2 assignment only occurs when value of variable A is greater than three. A variation of the If statement is the If/Else clause:

If A > 3 Then
  B = 2
Else
  B = 0
End If

In this example, the variable B is assigned the value of 2 when A is greater than 3, otherwise B is assigned the value of 0.

For more complex statements, you can cascade the If statement, for example:

If A = 0 Then
  B = 0
ElseIf A < 3 Then
  B = 1
Else 
  B = 2
End If

If the value of variable A equals zero, B is assigned the value 0. If A is less than 3 (but not equal to zero), then B is assigned the value 1. In all other instances (that is, if A is greater than or equal to 3), B is assigned the value 2.

Select...Case

The Select...Case instruction is an alternative to the cascaded If statement and is used when you need to check a value against various conditions:

Select Case DayOfWeek
  Case 1:
    NameOfWeekday = "Sunday"
  Case 2: 
    NameOfWeekday = "Monday"
  Case 3: 
    NameOfWeekday = "Tuesday"
  Case 4:
    NameOfWeekday = "Wednesday"
  Case 5:
    NameOfWeekday = "Thursday"
  Case 6:
    NameOfWeekday = "Friday"
  Case 7:
    NameOfWeekday = "Saturday"
End Select

In this example, the name of a weekday corresponds to a number, so that the DayOfWeek variable is assigned the value of 1 for Sunday, 2 for Monday value, and so on.

The Select command is not restricted to simple 1:1 assignments — you can also specify comparison operators or lists of expressions in a Case branch. The following example lists the most important syntax variants:

Select Case Var
  Case 1 To 5                   
    ' ... Var is between the numbers 1 and 5 (including the values 1 and 5).
  Case > 100
    ' ... Var is greater than 100
  Case 6, 7, 8                  
    ' ... Var is 6, 7 or 8
  Case 6, 7, 8, > 15, < 0
    ' ... Var is 6, 7, 8, greater than 15, or less than 0
  Case Else
    ' ... all other instances
End Select

Now consider a misleading (advanced) example, and a common error:

Select Case Var
  Case Var = 8
    ' ... Var is 0
  Case Else
    ' ... all other instances
End Select

The statement (Var = 8) evaluates to TRUE if Var is 8, and FALSE otherwise. TRUE is -1 and FALSE is 0. The Select Case statement evaluates the expression, which is TRUE or FALSE, and then compares that value to Var. When Var is 0, there is a match. If you understand the last example, then you also know why this example does not do what it appears

Select Case Var
  Case Var > 8 And Var < 11
    ' ... Var is 0
  Case Else
    ' ... all other instances
End Select