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From: jeffrey@cs.chalmers.se (Alan Jeffrey)
Newsgroups: comp.text.tex
Subject: Re: Testing for null arguments
Keywords: latex
Message-ID: <4409@undis.cs.chalmers.se>
Date: 15 Mar 91 13:55:44 GMT
References: <5396@vela.acs.oakland.edu>
Sender: news@cs.chalmers.se
Organization: Dept. of CS, Chalmers, Sweden
Lines: 25

In article <5396@vela.acs.oakland.edu> rjohnson@vela.acs.oakland.edu (R o d Johnson) writes:
[...]
>I just can't figure out how to come up with an \ifnull or \ifnotnull.
>Any suggestions?  Thanks in advance.

One simple solution is to test

   \def\unlikely{This is rather unlikely}
   \def\ifnull#1{\ifx\unlikely#1\unlikely}

Then 

   \ifnull{}    A \else B \fi = \ifx\unlikely\unlikely A \else B \fi = A
   \ifnull{xyz} A \else B \fi = \ifx\unlikely xyz\unlikely A \else B \fi = B

Actually, this is true if #1 is empty, or #1 begins with \unlikely.
So as long as \unlikely is an unlikely control sequence to appear,
you're OK.

What a hack...

Alan.

Alan Jeffrey         Tel: +46 31 72 10 98         jeffrey@cs.chalmers.se
Department of Computer Sciences, Chalmers University, Gothenburg, Sweden