How can I check in LaTeX (or plain TeX) whether a command exists, by name?

Question

I'm looking for a LaTeX way to control macro expansion, contingent on some macro being defined or not. I'm looking for something like this (but then actually working):

\newcommand[1]{\checkfor}{
  \if\isdef\csname{#1}
  ... expand this if command exists ...
  \else
  ... expand this if command does not exist ...
  \fi
}

which can then be called using

\checkfor{CommandName}

(this example is, of course, useless. The actual code I want to use this kind of expansion in is a package that dynamically creates a large number of macros from an even larger set of possible macros, with default behaviour for "all macros". Since not "all" macros may exist, I need some way to test whether a macro was declared, before I can expand based on its value).

Answer 1

Is this what you are looking for:

\documentclass[12pt]{article}

\newcommand{\checkfor}[1]{%
  \ifcsname#1\endcsname%
    ... command '#1' exists ...%
  \else%
    ... command '#1' does not exist ...%
  \fi%
}

\begin{document}
\checkfor{CommandName}\par
\checkfor{section}
\end{document}

Answer 2

The etoolbox provides two macros for this:

\ifdef{<control sequence>}{<true>}{<false>}
Expands to <true> if the <control sequence> is defined, and to <false> otherwise.
Note that control sequences will be considered as defined even if their meaning
is \relax. This command is a LaTeX wrapper for the e-TeX primitive \ifdefined.

\ifcsdef{<csname>}{<true>}{<false>}
Similar to \ifdef except that it takes a control sequence name as its first argument.
This command is a LaTeX wrapper for the e-TeX primitive \ifcsname.

Answer 3

The LaTeX kernel standard macro here is \@ifundefined, used as

\@ifundefined{foo}
  {%
    % \foo not defined
  }
  {%
    % \foo defined
  }%

---

In earlier LaTeX2e releases, this test was not 'expandable' and would cause \foo to be equal to \relax if it had not previously been defined. However, the kernel now uses essentially the same code as in Peter's answer, which means that these issues are no longer present. (The change is possible as the LaTeX2e kernel now requires and uses e-TeX.)

Answer 4

The package xifthen provides \isundefined which apparently has slightly different semantics:

This test differs from \@ifundefined in that it takes a real command—and not a command name—as argument, and also in that command which is let equal to \relax is not considered undefined.

So what you are looking for can be implemented as:

\newcommand[1]{\checkfor}{
  \ifthenelse{\isundefined{#1}}{%
    % expand undefined
  }{%
    % expand defined
  }%
}

Answer 5

A non-expandable variant of the test which does without whatsoever extensions:

\documentclass{article}
\makeatletter
\DeclareRobustCommand\checkfor[1]{%
  \begingroup
  \expandafter\ifx\csname#1\endcsname\relax\expandafter\@firstoftwo\else\expandafter\@secondoftwo\fi
  {%
    \expandafter\endgroup\expandafter\ifx\csname#1\endcsname\relax\expandafter\@firstoftwo\else\expandafter\@secondoftwo\fi
  }{\endgroup\@firstoftwo}%
}%
\makeatother

\begin{document}

\verb|\LaTeX| is \checkfor{LaTeX}{defined}{undefined}.

\verb|\relax| is \checkfor{relax}{defined}{undefined}.

\verb|\UndefiNED| is \checkfor{UndefiNED}{defined}{undefined}.

\verb|\UndefiNED| is \checkfor{UndefiNED}{defined}{undefined}.

\verb|\UndefiNED| is \checkfor{UndefiNED}{defined}{undefined}.

\end{document}