[texhax] How to test for existence of a macro?

[Vladimir Lomov]

2009/8/29 Neal H. Walfield <neal at walfield.org>:
> At Fri, 28 Aug 2009 18:14:48 -0600,
> D. R. Evans wrote:
>> I couldn't find this in the TeXbook, but perhaps I was just looking in the
>> wrong place.
>>
>> How should I test cleanly for the existence of a particular macro?
>
> The secret is how \csname ... \endcsname acts when the specified
> control sequence is not defined: if the resulting control sequence
> token was not previously defined, the resulting control sequence acts
> like \relax.  We can combine this with \ifx, which compares the value
> of two marcos, as follows:
>
>  % Prints `not defined'.
>  \expandafter\ifx\csname foo\endcsname \relax not defined \else defined \fi
>
>  \def\foo{}
>  % Prints `defined'.
>  \expandafter\ifx\csname foo\endcsname \relax not defined \else defined \fi
>
> We need to use \expandafter as \ifx does not expand its arguments and
> we want the expansion of the \csname ...\endcsname sequence.
>
> Neal
> _______________________________________________
> TeX FAQ: http://www.tex.ac.uk/faq
> Mailing list archives: http://tug.org/pipermail/texhax/
> More links: http://tug.org/begin.html
As I understand question from first post this should help:
http://www.tex.ac.uk/cgi-bin/texfaq2html?label=isdef

---
WBR, Vladimir Lomov