[t152@vmars.tuwien.ac.at: Re: [texhax] Macro Problem]

[Michael Doob]

> That's exactly the point; I have extended your example:
> 
> INPUT:
> 
> \def\truefalseX#1{%
>   \ifx#1\empty\typeout{X:true}\else\typeout{X:false}\fi}
> 
> \def\truefalseY#1{%
>   \ifx#1\empty\else\typeout{Y:false}\fi}
> 
> \truefalseX{}
> \truefalseX{\empty}
> \truefalseX{hello}
> 
> \truefalseY{}
> \truefalseY{\empty}
> \truefalseY{hello}
> 
> OUTPUT:
> 
> X:false
> X:true
> X:false
> % empty string
> % empty string
> Y:false
> 
> 
> -> it works properly if I remove the true-expression.
> 

Take a look at the TeXBook, page 209 and 210  where it discusses the expansion
of tokens when using \if and \ifx. For the latter the comparison must be between
macros or between characters. Sometimes measuring with width of expanded value
of the macro parametercan be a quick, easily understandable,  but 
rather inelegant solution.

Cheers,
Michael


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Michael Doob                    Telephone: (204) 474-9796
Department of Mathematics       Fax: (204) 474-7606
University of Manitoba          email: Michael_Doob at umanitoba.ca
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