SELECT CASE
Conditional statement block
Syntax:
Select Case expression
[ Case expressionlist]
or[ Case expressionlist]
[statements]
[ Case Else ]
[statements]
End Select
Select Case As Const integer_expression
[ Case constant | enumeration ]
[ Case constant | enumeration ]
[ statements ]
[ Case Else ]
[ statements ]
End SelectDescription:
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.
Exit Select can be used to escape the Select Case block, preventing further code in the Case block from being executed.
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 }[, ...]
Exit Select can be used to escape the Select Case block, preventing further code in the Case block from being executed.
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 }[, ...]
- expr: evaluates expr, and compares for equality with the original expression. If they are equal, then a match has been found. This could be considered as a shorthand for "Is = expr" (see below).
- expr1 To expr2: evaluates expr1 and checks to see if it is less than or equal to the original expression. If so, it evaluates expr2, and checks to see if it is greater than or equal to the original expression. If so, then a match has been found.
- Is relational_operator expr: evaluates expr, and compares the original operation against it, using the supplied relational_operator (=, >, <, <>, <=, >=). If the comparison is true, then a match has been found.
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 after converting the values to the uinteger type, the largest value in the range may be no higher than the smallest value + 8191 (current implementation).
Note: No statement can be placed between the Select Case statement and the first Case statement.
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
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
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:
- Select Case As Const did not exist in QB.
- in an "expr1 TO expr2" case, QB would always evaluate both expressions, even if expr1 was higher than the original expression.
- In the -lang qb and -lang fblite dialects, variables declared inside a Select..End Select block have a function-wide scope as in QB.
- In the -lang fb and -lang deprecated dialects, variables declared inside a Select..End Select block are visible only inside the block, and can't be accessed outside it. To access duplicated symbols defined as global outside this block, add one or preferably two dot(s) as prefix: .SomeSymbol or preferably ..SomeSymbol (or only ..SomeSymbol if inside a With..End With block).
See also:
Back to Control Flow
