Meaning of square brackets, optional arguments

macrosoptional arguments

I'm a bit confused as to how LaTeX parses square brackets/optional macro arguments.

I'll try to explain what I understood by means of the macro \section which has one optional and one mandatory argument. Please correct me if you think I got anything wrong.

This

\documentclass{article}

\begin{document}
\section ab
\end{document}

finds no optional argument, and a mandatory argument a. b is just text then. With {}, one can group things, so

\section {ab}

takes ab as mandatory argument. The space doesn't matter.

Square brackets [] usually get rendered as square brackets. Unless, it seems, if it could potentially contain optional arguments. This

\section[a]{b}

takes a as optional, b as mandatory argument. Are optional arguments always indicated by square brackets?

This

\section[a][b]{c}

takes a as optional argument, [ as mandatory argument, and b]{c} is just text.

This, too, seems to indicate that square brackets are square brackets if and only if they aren't in their function of optional argument. Is this correct?

Best Answer

Depends on the command. The way arguments are parsed is command specific, not TeX/LaTeX imposed.

Some commands expect optional arguments and then parse the square brackets as the start of optional arguments. Other commands expect no optional arguments and then a square bracket is just an argument.

See these examples:

\documentclass{article}
\usepackage{xparse}


\NewDocumentCommand{\mycmd}{o o m}{\IfValueTF{#1}{first #1}{nothing on one},
  \IfValueTF{#2}{second #2}{nothing on two},
  mandatory #3}
\newcommand\anothercmd[1]{My argument is /#1/}

\begin{document}
\mycmd{Hello}

\mycmd[1]{Hello}

\mycmd[1][2]{Hello}

\anothercmd{test}

\anothercmd[2]{Test}
\end{document}

Related Solutions

[Tex/LaTex] Optional arguments: xparse vs. xargs

With xargs you're limited to pass a default value to optional arguments, so it's not easy to distinguish when the argument is missing. Conversely, xparse provides \IFNoValueTF just for this case.

For instance, one can say

\NewDocumentCommand{\foo}{o}{%
  \IfNoValueTF{#1}
    {foo without optional argument}
    {foo with optional argument #1}%
}

With xparse you can also define *-variants of commands and also commands taking arguments delimited in various ways. Arguments can be “long” on an individual basis, while with xargs either all are “long” or none is. An example of *-variant is the following command that has the same syntax as \section:

\NewDocumentCommand{\foo}{ s o m }
 {
  \IfBooleanTF{#1}
    {*-variant called, do something with #3}
    {%
     \IfNoValueTF{#2}
      {normal foo without optional argument, do something with #3}
      {normal foo with optional argument, do something with #2 and #3}%
    }%
 }

So \foo*{Xyz}, \foo{Xyz} or \foo[Abc]{Xyz} are the possible calls. Actually also \foo*[Abc]{Xyz} is allowed, but with the code above, argument #2 would be ignored.

[Tex/LaTex] Use of optional argument stretch in \parbox

With

\parbox[c][3cm][s]{5cm}{I want to stretch this text inside this box}

there is no stretchable glue in the box and indeed you get the warning

Underfull \vbox (badness 10000) detected

If you want that the first line is at the top and the second line at the bottom, you need to make the interline glue stretchable:

\documentclass{article}

\begin{document}
\fbox{\parbox[c][3cm][s]{5cm}{
  \setlength{\baselineskip}{1\baselineskip plus 1fil}
  I want to stretch this text inside this box
}}
\end{document}

enter image description here

In order to specify the second (and third) optional argument, you must specify the first. So

\parbox[3cm]{5cm}{...}

gives the same result as \parbox{5cm}{...}; unfortunately LaTeX doesn't do a check for the contents of the first optional argument, which should only be c (default), t or b. If it's different from any of these, it's silently ignored.

If you don't specify the third optional argument, it defaults to be the same as the first.

Best Answer

Related Solutions

[Tex/LaTex] Optional arguments: xparse vs. xargs

[Tex/LaTex] Use of optional argument stretch in \parbox

Related Question