SELECT CASE


Conditional statement block

Syntax:
Select Case expression
[ Case expressionlist]
[statements]
[ Case Else ]
[statements]
End Select
or
Select Case As Const integer_expression
[ Case constant | enumeration ]
[ statements ]
[ Case Else ]
[ statements ]
End Select

Description:
Select Case executes specific code depending on the value of an expression. The expression is evaluated once, and compared against each Case, in order, until a matching expression is found. The code inside the matching Case branch is executed, and the program skips down to the end of the Select Case block. Case Else matches any case not already matched, so if there is a Case Else, at least one Case is guaranteed to be executed. If no Cases match, the whole Select Case block will be skipped.

End Select is used to close the Select Case...End Select block.

Note for C users: In FreeBASIC, Select Case works like a switch block where all cases have a break at the end. As there is no fall-through, multiple options must be put in an expression list in a single Case.

Besides integer types, floating point and string expressions are also supported with the first syntax.

Syntax of an expression list:
{ expression | expression To expression | Is relational operator expression }[, ...]


Multiple checks can be made in each Case, by separating them by a comma (,). Once a match is found, the program finishes its checks, and goes on to execute the code statements for that Case block. No further expressions are evaluated or checked.

example of expression lists:
Case 1 constant
Case 5.4 To 10.1 range
Case Is > 3 bigger than-smaller than
Case 1, 3, 5, 7 to 9 match against a set of values
Case x value of a variable


If As Const is used, only integer constants (all numeric constants excluding the two floating-point constants: single and double) can be evaluated and the expression list supports simple constants and enumerations only. "To" ranges are supported, but "Is" relational operators are not.

With As Const, a jump table is created to contain the full range of integer Cases handled. This allows Select Case As Const to be faster than Select Case. However, the size of the range of values is limited, and the largest value in the range may be no higher than the smallest value + 8191.

Examples:
Dim choice As Integer

Input "Choose a number between 1 and 10: "; choice

Select Case As Const choice
Case 1
    Print "number is 1"
Case 2
    Print "number is 2"
Case 3, 4
    Print "number is 3 or 4"
Case 5 To 10
    Print "number is in the range of 5 to 10"
Case Else
    Print "number is outside the 1-10 range"
End Select


'' SELECT CASE vs. SELECT CASE AS CONST speed test

Const N = 50000000

Dim As Integer dummy = 0
Dim As Double t = Timer()

For i As Integer = 1 To N
    Select Case i
    Case 1, 3, 5, 7, 9
        dummy += 1
    Case 2, 4, 6, 8, 10
        dummy += 1
    Case 11 To 20
        dummy += 1
    Case 21 To 30
        dummy += 1
    Case 31
        dummy += 1
    Case 32
        dummy += 1
    Case 33
        dummy += 1
    Case Is >= 34
        dummy += 1
    Case Else
        Print "can't happen"
    End Select
Next

Print Using "SELECT CASE: ##.### seconds"; Timer() - t
t = Timer()

For i As Integer = 1 To N
    Select Case As Const i
    Case 1, 3, 5, 7, 9
        dummy += 1
    Case 2, 4, 6, 8, 10
        dummy += 1
    Case 11 To 20
        dummy += 1
    Case 21 To 30
        dummy += 1
    Case 31
        dummy += 1
    Case 32
        dummy += 1
    Case 33
        dummy += 1
    Case Else
        If( i >= 34 ) Then
            dummy += 1
        Else
            Print "can't happen"
        End If
    End Select
Next

Print Using "SELECT CASE AS CONST: ##.### seconds"; Timer() - t
Sleep


Differences from QB:
See also:
Back to Control Flow
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