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From: gvr@cs.brown.edu (George V. Reilly)
Newsgroups: comp.text.tex
Subject: Re: Testing for null arguments
Keywords: latex
Message-ID: <68473@brunix.UUCP>
Date: 14 Mar 91 00:23:46 GMT
References: <5396@vela.acs.oakland.edu>
Sender: news@brunix.UUCP
Reply-To: gvr@cs.brown.edu (George V. Reilly)
Organization: Brown University, Dept. of Computer Science
Lines: 61

In article <5396@vela.acs.oakland.edu> rjohnson@vela.acs.oakland.edu (R o d Johnson) writes:
| 
| This is undoubtedly a question with a simple answer that I just can't
| seem to get. I want to have a macro (for use in LaTeX documents) that
| can have three arguments, two of which can be null.  It's invoked like
| this:
| 
|   \prt{arg1}{arg2}{arg3}
| 
| but could also look like any of the following:
| 
|   \prt{}{arg2}{arg3}
|   \prt{arg1}{arg2}{}
|   \prt{}{arg2}{}
| 
| (arg2 will always be present).  I want to define an new \if (I guess)
| that would allow me to define a macro like the following:
| 
|   \def\part#1#2#3{%           % or should there be braces around
|    [some code]                % the arguments in the definition?
|   %
|   \ifnull{#1} \relax \else    % I guess it would be simpler to have
|    [some more code]           % an \ifnotnull to do this
|   \fi
|   %
|   \ifnull{#3} \relax \else
|    [some more code]
|   \fi
|   }
| 
| I just can't figure out how to come up with an \ifnull or \ifnotnull.
| Any suggestions?  Thanks in advance.

Try this:

\def\part#1#2#3{%
    \def\first{#1}	% Or something with more mnemonic value
    \def\third{#1}
    [some code]
    %
    \ifx\first\empty
	% do nothing
    \else
	[some more code]
    \fi
    [etc.]
}

You might find it more convenient to define \part using
strings to delimit the three arguments, e.g.:
	\def\part #1:#2:#3!{%...
so you can say
	\part :arg2:!		or
	\part :arg2:arg3!	or
	\part arg1:arg2:!	or
	\part arg1:arg2:arg3!	or
you can use multicharacter strings instead of the `:'s and the `!',
of course.
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