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case statement to catch multiple symbols?

caver456

How should the list clause of a case statement clause be structured so that it matches on any one of a set of symbols?

Example:

case(a
   ('a println("A"))
   ('b println("B"))
   (('c 'd) println("C or D"))
)

a='a  --> the above code prints "A"

a='b  --> the above code prints "B"

a='c'  --> nothing is printed

a='d'  --> nothing is printed

Changing it to match on strings instead of symbols:

case(a
   ("a" println("A"))
   ("b" println("B"))
   (("c" "d") println("C or D"))
)

a="a"  --> the above code prints "A"

a="b"  --> the above code prints "B"

a="c"  --> the above code prints "C or D"

a="d"  --> the above code prints "C or D"

Basically I'd like to mimic the string behavior above, but the variable being tested is a symbol.  What's the proper way to structure the ('c or 'd) clause in this case?

  • Hi,

    Your mistake, I think, is in including the ' quote before the items, it's not needed as the case statement compares a value/variable against fixed literal target values. From the docs: "Keys are always treated as constants and are never evaluated."  Here's a cut down example based on yours:

    a = 'c => c
case(a (a "A") (b "B") ((c d) "C or D") (t "SomethingElse")) => "C or D"
a ='a => a
case(a (a "A") (b "B") ((c d) "C or D") (t "SomethingElse")) => "A"
a='f => f
case(a (a "A") (b "B") ((c d) "C or D") (t "SomethingElse")) => "SomethingElse"
a='d => d
caseq(a (a "A") (b "B") ((c d) "C or D") (t "SomethingElse")) => "C or D"

    In the symbol situation (I was going to say 'case' but ...) you can use 'caseq' as it can use 'eq' for the equality test rather than 'equal' - 'eq' is a little more efficient if you are working with symbols (just beware that 'eq' can return nil when comparing values that you think are the same but they are not 'pointing to the same object').

    Hopefully this helps you.

    Best regards,

    Lawrence.

  • Bingo, that's it. Thanks. To sum up, just using 'case' and not 'caseq':


    ; workey for strings
    ;case(a
    ; ("a" println("A"))
    ; ("b" println("B"))
    ; (("c" "d") println("C or D"))
    ;)

    ; no workey for symbols
    ;case(a
    ; ('a println("A"))
    ; ('b println("B"))
    ; (('c 'd) println("C or D"))
    ;)

    ; workey for symbols
    case(a
    (a println("A"))
    (b println("B"))
    ((c d) println("C or D"))
    )

  • I should also point out that there's really little point in using caseq over case. Since the target values (the candidate values in each branch) can only be symbols, strings, and numbers (not lists), there's not really any benefit in using an eq test. Also, eq should never be used with strings because it can lead to misleading results - sometimes two identical strings are eq, but sometimes they are not - you're at the mercy of how the string hashing works internally (the documentation warns about this).

    The key point is that the target values are always literal values rather than evaluated. If  you need them to be evaluated you'd have to use the cond() construct or similar instead.

    Regards,

    Andrew.

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