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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,

    That's expected. The target values (the first entry on each branch in the case function) are not evaluated and have to be literal values. So they can be a literal integer, floating point number (not a good idea though, because this invokes floating point equality which is dangerous due to potential rounding errors), strings and symbols. You can also specify multiple values using:

    case(testString
      (("one" "two") printf("1 or 2\n"))
      (("three" "four") printf("3 or 4\n"))
      (t printf("something else\n"))
    )

    So actually when you used the nthelem, a branch will look like this: ((nthelem 1 testOptions) printf("...")) - note that the SKILL parser converts the C-like syntax into LISP-like form internally. So that means it would match 'nthelem, 1 or 'testOptions. You can try out the second form by using testString='nthelem or testString=1 or testString=2 and see what happens!

    If you want the target values to be an evaluated expression, you should use cold() instead.

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    Sjoerd,

    That's expected. The target values (the first entry on each branch in the case function) are not evaluated and have to be literal values. So they can be a literal integer, floating point number (not a good idea though, because this invokes floating point equality which is dangerous due to potential rounding errors), strings and symbols. You can also specify multiple values using:

    case(testString
      (("one" "two") printf("1 or 2\n"))
      (("three" "four") printf("3 or 4\n"))
      (t printf("something else\n"))
    )

    So actually when you used the nthelem, a branch will look like this: ((nthelem 1 testOptions) printf("...")) - note that the SKILL parser converts the C-like syntax into LISP-like form internally. So that means it would match 'nthelem, 1 or 'testOptions. You can try out the second form by using testString='nthelem or testString=1 or testString=2 and see what happens!

    If you want the target values to be an evaluated expression, you should use cold() instead.

    Regards,

    Andrew.

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