Different command definitions with and without optional argument

Question

I want to define a command that expands to two completely different things depending on whether I supply, or not, an optional argument. For example,

\mycmd{normal}            ->  something(normal)
\mycmd[optional]{normal}  ->  anotherthing(normal, optional)

How can I do this?

Answer 1

If you read deep in the LaTeX code you can find lots of examples where this is done. Any command that has optional arguments actually does this already: there are actually two different commands and which one is called depends on whether or not it is called with an optional command. The TeX trick is to use \@ifnextchar[. For example,

\def\mycmd{\@ifnextchar[{\@with}{\@without}}
\def\@with[#1]#2{hello #1, have you met #2?}
\def\@without#1{goodbye #1}

What happens there is that the TeX interpreter meets the \mycmd macro and expands it (swallowing following white-space, of course). The first thing it then encounters is a test on the next character (which, incidentally, is non-destructive - the next character is simply observed, not processed). If it is a square bracket, then it puts \@with in the stream and starts again. \@with is defined to take two arguments, the first of which is surrounded by square brackets. It therefore matches the first optional argument and the next thingy in the TeX stream. If the next character wasn't a square bracket then \@without is put into the stream. This takes one (normal) parameter. But since \@with and \@without are two completely separate commands, they can do whatever they like with the input.

(Note: For commands defined with \newcommand which take an optional parameter then sort-of what happens is that the two commands \@with and \@without expand to the same command, but the optional parameter of \@with gets passed to it as the first parameter whereas in \@without the default value is passed. It's not quite like that, but the difference is more in conciseness of programming than anything else.)

Here's a fully compilable example, including the mysterious and mystical duo \makeatletter and \makeatother:

\documentclass{article}
%\url{https://tex.stackexchange.com/a/314/86}

\makeatletter
\def\mycmd{\@ifnextchar[{\@with}{\@without}}
\def\@with[#1]#2{Hello #1, have you met #2?}
\def\@without#1{Goodbye #1.}
\makeatother

\begin{document}
\mycmd[Polyphemus]{Odysseus}
\mycmd{Circe}
\end{document}

Produces:

Hello Polyphemus, have you met Odysseus? Goodbye Circe.

Answer 2

The LaTeX3 xparse package is designed to aid the construction of macros that take complex optional arguments. In this case, you'd write

\usepackage{xparse}
....
\DeclareDocumentCommand \foo { o m } {%
  \IfNoValueTF {#1} {%
    \something {#2}%
  }{%
    \someotherthing {#1} {#2}%
  }%
}

Internally, this does the same thing as Andrew's answer, but the coding is a little more straightforward/readable and it is also more easily extended. Want another optional argument? Just add another o in the { o m } argument. (o is ‘optional argument’ and m is ‘mandatory argument’; see the xparse documentation for more.)

Answer 3

I want to thank for all of the other answers received. But lately I've turned into a huge fan of the etoolbox package, which provides a great abstraction to do exactly what I wanted at the LaTeX level (with no low level TeX trickery):

\usepackage{etoolbox}

\newcommand\mycmd[2][]{%
  \ifstrempty{#1}{%
    % something with #2
  }{%
    % some other thing with #1 and #2
  }%
}

Answer 4

I'd prefer Will Robertson's solution, that's more manageable. For a "pure" LaTeX solution:

\makeatletter
\def\ifemptyarg#1{%
  \if\relax\detokenize{#1}\relax % H. Oberdiek
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi}
\makeatother

\newcommand{\mycmd}[1][]{%
   \ifemptyarg{#1}
     {<code for empty argument>}
     {<code for non empty argument>}%
}

Answer 5

I usually just use the ifthen package to solve this, by testing whether the optional argument is empty.

\newcommand\mycmd@noargs{foo}
\newcommand\mycmd@withargs[1]{bar #1}
\newcommand\mycmd[1][]{
  \ifthenelse{\equal{#1}{}}{
    \mycmd@noargs{}}{
    \mycmd@withargs{#1}}}

(Notice that since it uses @ as a letter, you need to use \makeatletter before this definition if it’s in your main LaTeX file (instead of a package), and \makeatother after it.)

Answer 6

From 2020 LaTeX

With modern LaTeX (after 2020), there are new built-in commands to deal with optional arguments. There is no need to load the xparse package any more as in Will's answer. Here is a self-explanatory example taken from pg. II-639 of The LaTeX Companion, 3rd ed., published in 2023.

\NewDocumentCommand\foo{o m}
    {\IfNoValueTF {#1}%
        {\DoSomethingWithJustTheMandatoryArgument{#2}}%
        {\DoSomethingWithBothArguments{#1}{#2}}}