LaTeX \newcommand default argument: is empty?

Question

I'm trying to write a simple example command that prints nothing without an argument, but with an argument it surrounds it with something.

I've read that the default value should be \@empty and the simple \ifx\@empty#1 condition should do the job:

\newcommand{\optarg}[1][\@empty]{%
\ifx\@empty#1  {}  \else  {(((#1)))}  \fi
}

\optarg % (((empty)))
\optarg{} % (((empty)))
\optarg{test} % (((empty))) test

The latter three commands all print the empty word for some reason, and I want the first two to print nothing and the last to print (((test))).

I'm using TeXLive/Ubuntu. An ideas?

Answer 1

Try the following test:

\documentclass{article}

\usepackage{xifthen}% provides \isempty test

\newcommand{\optarg}[1][]{%
  \ifthenelse{\isempty{#1}}%
    {}% if #1 is empty
    {(((#1)))}% if #1 is not empty
}

\begin{document}

Testing \verb|\optarg|: \optarg% prints nothing

Testing \verb|\optarg[]|: \optarg[]% prints nothing

Testing \verb|\optarg[test]|: \optarg[test]% prints (((test)))

\end{document}

The xifthen package provides the \ifthenelse construct and the \isempty test.

Another option is to use the ifmtarg package (see the ifmtarg.sty file for the documentation).