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<head>
<title>UK TeX FAQ -- question label oarglikesect</title>
</head><body>
<h3>Optional arguments like <code>\</code><code>section</code></h3>
<p/>Optional arguments, in macros defined using <code>\</code><code>newcommand</code>, don’t
quite work like the optional argument to <code>\</code><code>section</code>. The default
value of <code>\</code><code>section</code>’s optional argument is the value of the
mandatory argument, but <code>\</code><code>newcommand</code> requires that you ‘know’ the
value of the default beforehand.
<p/>The requisite trick is to use a macro in the optional argument:
<blockquote>
<pre>
\documentclass{article}
\newcommand\thing[2][\DefaultOpt]{%
\def\DefaultOpt{#2}%
optional arg: #1, mandatory arg: #2%
}
\begin{document}
\thing{manda}% #1=#2
\thing[opti]{manda}% #1="opti"
\end{document}
</pre>
</blockquote><p>
LaTeX itself has a trickier (but less readily understandable)
method, using a macro <code>\</code><code>@dblarg</code>; inside LaTeX, the example
above would have been programmed:
<blockquote>
<pre>
\newcommand\thing{\@dblarg\@thing}
\newcommand\@thing[2][\@error]{%
optional arg: #1, mandatory arg: #2%
}
</pre>
</blockquote><p>
In that code, <code>\</code><code>@thing</code> is only ever called with an optional and a
mandatory argument; if the default from the <code>\</code><code>newcommand</code> is
invoked, a bug in user code has bitten...
<p/><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=oarglikesect">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=oarglikesect</a>
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