Re: programming: doifdefined & friends

[Aditya Mahajan <adityam@umich.edu>]

On Thu, 5 Oct 2006, Henning Hraban Ramm wrote:

> While I can't cope with counters, I'm trying other stuff that I don't
> understand:
> How must I use \doifdefined & friends?
>
> Consider this snippet:
>
> -----
> \starttext
>
> \def\myvar{JAJAJA}
>
> \input tufte
> \doifdefined{\myvar}{\par bla \myvar blub\par}
> \input tufte
>
> \stoptext
> -----
>
> I'd guess it would print "bla JAJAJAblub" between the two Tufte texts
> and don't if I comment \def\myvar.
>
> But I guess wrongly: It does nothing in this case, and complains
> about undefined \myvar in the latter.

That is because \doifdefined is for strings. It adds a \csname ... 
\endcsname around its first argument.

> But if I really want to check if something is defined before I try to
> use it, how must I write it?

The plain tex way

  \ifx\myvar\undefined
    %Nothing here
  \else
    \par bla \myvar blub\par
  \fi

or the context way (for strings)

  \doifdefined{myvar}{\par bla \myvar blub\par}


Aditya