Re: variables

[Taco Hoekwater <taco@elvenkind.com>]

On Tue, 15 Jul 2003 14:59:44 +0200, Peter wrote:

> I want to test, if a given variable is empty or not. I tried the following,
> but the test is always false. Is this a bug or a feature ;)

It is a  feature/bug of TeX itself that "\if" tests perform a conditionality test 
that is not even remotely intuitive ;-) 

The '\if' looks ahead to see if the next two non-expandable tokens have 
the same 'character code' *after* expansion. It will actually compare 's' 
to 'o' in the "second" example (false). The input "second=oops" would have
given a 'true' result, but would also have typeset the extra letters ('ps') as a 
side-effect.

TeX's \if test can be quite confusing and should normally not
be used in a document. It is much better to use one of ConTeXt's
predefined commands, like this one:

	\doifelse 
            {\TestVAR{first}}
            {}
            {The variable test:first is empty.}
            {The variable test:first contains \TestVAR{first}.}

this compares argument #1 to argument #2, executing #3 or #4 depending
on the outcome of the test.

Greetings, Taco

> Any help is welcome.
> 
> 
> \starttext
> 
> \setvariables[test]
>   [first=,
>    second=something,]
> 
> \def\TestVAR#1{\getvariable{test}{#1}}
> 
> \if\TestVAR{first}\empty %
>   The variable test:first is empty.\else
>   The variable test:first contains \TestVAR{first}.\fi
> 
> \if\TestVAR{second}\empty %
>   The variable test:second is empty.\else
>   The variable test:second contains \TestVAR{second}.\fi
> 
> \stoptext
> 
> 
> Regards,
> 
>    Peter
> 
> 
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-- 
groeten,

Taco