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Weird "case" statement behavior (at least to me...), can someone please explain this to me?

Sheppy

Hello all,

Today I ran into a problem which toke me a long time to find the cause. It is with the "case" statement. First I show you something that is very simple and that does work:

testString = "one"
case( testString
    ( "one"
        printf("it is 1!\n")
    )
    ( "two"
        printf("it is 2!\n")
    )
    ( t
        printf("it is something else!\n")
    )
)

testString = "two"
case( testString
    ( "one"
        printf("it is 1!\n")
    )
    ( "two"
        printf("it is 2!\n")
    )
    ( t
        printf("it is something else!\n")
    )
)

testString = "bla"
case( testString
    ( "one"
        printf("it is 1!\n")
    )
    ( "two"
        printf("it is 2!\n")
    )
    ( t
        printf("it is something else!\n")
    )
)

When you run this code (just copy-past into CIW...) the output of the case statements is as you would expect:

it is 1!
it is 2!
it is something else!

However, if you specify a variable like so:

testOptions = list( "one" "two" )

And replace the "case" statement with this:

case( testString
    ( nthelem( 1 testOptions )
        printf("it is 1!\n")
    )
    ( nthelem( 2 testOptions )
        printf("it is 2!\n")
    )
    ( t
        printf("it is something else!\n")
    )
)

The output is not what I expected:

it is something else!
it is something else!
it is something else!

Testing the "nthelem" part shows no problem, it perfectly outputs the right string (or whatever you put in the list).

If you do the same with a "cond" statement (using the "nthelem" statement), it works perfectly fine.

My question: what is happening here? Why is the result what it is, and not what I was expecting?

Thanks in advance.

With kind regards,

Sjoerd

Parents
  • Andrew Beckett

    Sjoerd,

    Actually I'd rather hope that  you don't use the code! My intention was to explain and illustrate rather than have you use the macro - and to show that lots of things are possible in SKILL if you want.

    There's a great quotation from Peter Norvig (now at Google) in his book Paradigms of Artificial Intelligence Programming: Case Studies in Common Lisp:

    "The first step in writing a macro is to recognize that every time you write one, you are defining a new language that is just like Lisp except for your new macro. The programmer who thinks that way will rightfully be extremely frugal in defining macros. Introducing a macro puts much more memory strain on the reader of your program than does introducing a function, variable or data type, so it should not be taken lightly. Introduce macros only when there is a clear need, and when the macro fits in well with your existing system."

    To me this is an example of where using cond() is much more expressive than hiding it via the CCFcaseEval - and is much clearer to the reader what is going on.

    Anyway, feel free find something else to tease me with!

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    Sjoerd,

    Actually I'd rather hope that  you don't use the code! My intention was to explain and illustrate rather than have you use the macro - and to show that lots of things are possible in SKILL if you want.

    There's a great quotation from Peter Norvig (now at Google) in his book Paradigms of Artificial Intelligence Programming: Case Studies in Common Lisp:

    "The first step in writing a macro is to recognize that every time you write one, you are defining a new language that is just like Lisp except for your new macro. The programmer who thinks that way will rightfully be extremely frugal in defining macros. Introducing a macro puts much more memory strain on the reader of your program than does introducing a function, variable or data type, so it should not be taken lightly. Introduce macros only when there is a clear need, and when the macro fits in well with your existing system."

    To me this is an example of where using cond() is much more expressive than hiding it via the CCFcaseEval - and is much clearer to the reader what is going on.

    Anyway, feel free find something else to tease me with!

    Regards,

    Andrew.

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