David Z David Z - 28 days ago 14
LaTeX Question

LaTeX: how to use a required arg as the default for an optional one?

I'm trying to create a LaTeX command with two arguments, one of them optional. Normally I'd do this as

\newcommand{\whatever}[2][default]{first #1 second #2}


where
default
is the default value for the first argument. But for this command I want the value of the second argument to be used as the default value for the first argument - that is, I want

\whatever{blah}
\whatever{foo}
\whatever[lol]{rofl}


to be equivalent to

\whatever[blah]{blah}
\whatever[foo]{foo}
\whatever[lol]{rofl}


Does anyone know how to do this? I can drop down to plain TeX if necessary.

Answer

The LaTeX kernel has an inbuilt method of doing this, although it's not widely used. Here's an example:

\makeatletter
\newcommand\@foo[2][]{[1: #1, 2: #2.] }
\newcommand\foo{\@dblarg\@foo}
\makeatother

\foo{same}
\foo[hello]{world}

Obviously, the \makeatletter commands can be dropped if you're doing this inside a .sty or .cls file.

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