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<head>
<title>UK TeX FAQ -- question label csname</title>
</head><body>
<h3>Defining a macro from an argument</h3>
<p/>It’s common to want a command to create another command: often one
wants the new command’s name to derive from an argument. LaTeX
does this all the time: for example, <code>\</code><code>newenvironment</code> creates
start- and end-environment commands whose names are derived from the
name of the environment command.
<p/>The (seemingly) obvious approach:
<blockquote>
<pre>
\def\relay#1#2{\def\#1{#2}}
</pre>
</blockquote><p>
doesn’t work (the TeX engine interprets it
as a rather strange redefinition of <code>#</code>). The trick is to use
<code>\</code><code>csname</code>, which is a TeX primitive for generating command names
from random text, together with <code>\</code><code>expandafter</code>. The definition
above should read:
<blockquote>
<pre>
\def\relay#1#2{%
\expandafter\def\csname #1\endcsname{#2}%
}
</pre>
</blockquote><p>
With this definition, <code>\</code><code>relay{blah}{bleah}</code> is equivalent to
<code>\</code><code>def</code><code>\</code><code>blah{bleah}</code>.
<p/>Note that the definition of <code>\</code><code>relay</code> omits the braces round the
‘command name’ in the <code>\</code><code>newcommand</code> it executes. This is
because they’re not necessary (in fact they seldom are), and in this
circumstance they make the macro code slightly more tedious.
<p/>The name created need not (of course) be <em>just</em> the argument:
<blockquote>
<pre>
\def\newrace#1#2#3{%
\expandafter\def\csname start#1\endcsname{%
#2%
}%
\expandafter\def\csname finish#1\endcsname{%
#3%
}%
}
</pre>
</blockquote><p>
With commands
<blockquote>
<pre>
\def\start#1{\csname start#1\endcsname}
\def\finish#1{\csname finish#1\endcsname}
</pre>
</blockquote><p>
these ‘races’ could behave a bit like LaTeX environments.
<p/><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=csname">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=csname</a>
</body>
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