Special behavior if optional argument is not passed

Question

I am looking to define a command (actually, an environment, but that shouldn't matter for the purposes of this question) that does some default behavior, and if an optional argument is passed, does something else.

For example,

\newcommand{mycommand}[1]{%
if (#1 != NULL){%
The argument #1 was passed}
else {%
No argument was passed.}}

Obviously this is not valid LaTeX, but hopefully this makes it clear what I am trying to do. Is there a way to do this with ordinary LaTeX? Is it worth switching to LuaLaTeX to accomplish behavior like this?

Remark, in your "MWE" you missed a backslash before the command name, see this https://tex.stackexchange.com/q/29463/250119 — user202729, Oct 26 '22 at 05:25

score 25 · Accepted Answer · answered Dec 12 '14 at 22:11

You can do it easily with xparse:

\documentclass{article}

\usepackage{xparse}

\NewDocumentCommand{\mycommand}{o}{%
  % <code>
  \IfNoValueTF{#1}
    {code when no optional argument is passed}
    {code when the optional argument #1 is present}%
  % <code>
}

\begin{document}

\mycommand

\mycommand[HERE]

\end{document}

This will print

code when no optional argument is passed
code when the optional argument HERE is present

For an environment it's similar

\NewDocumentEnvironment{myenv}{o}
  {\IfNoValueTF{#1}{start code no opt arg}{start code with #1}}
  {\IfNoValueTF{#1}{end code no opt arg}{end code with #1}}

Other code common to the two cases can be added at will. As you see, xparse also allows (but it's not mandatory) to use the argument specifiers also in the end part.

score 22 · Answer 2 · edited Apr 05 '18 at 19:09

22

\makeatletter
\newcommand{\mycommand}[1][\@nil]{%
  \def\tmp{#1}%
   \ifx\tmp\@nnil
       no argument
    \else
         argument #1
    \fi}
\makeatother



\mycommand zzzz \mycommand[hello]  zzz

edited Apr 05 '18 at 19:09

Jan

5,293

answered Dec 12 '14 at 22:09

David Carlisle

757,742

5

What is the command @nnil ? – sergiokapone Dec 12 '14 at 22:29
1

@sergiokapone: That was a minor typo on David's side :) The default optional argument should just match whatever you use against the test; \relax would have worked just as well. – Werner Dec 12 '14 at 22:33
4

@Werner no sorry it was not a typo, and \relax would not work as well, I reverted the edit. \@nnil is defined in the latex format by \def\@nnil{\@nil} for exactly this test – David Carlisle Dec 12 '14 at 22:46
@sergiokapone \@nnil is \def\@nnil{\@nil} so it is equal to \tmp if the default argument of \@nil is used in the case the command has no explicit argument. – David Carlisle Dec 12 '14 at 22:48
2

@DavidCarlisle: Sorry, I was too eager to add some mortar to your brick. – Werner Dec 12 '14 at 22:50
Please could you explain /why/ you use \@nnil instead of \@nil? I can't find this online. – Richard Birkett Sep 21 '19 at 16:35
With the tufte-book class, this gives me \@nil undefined control sequence. – Dominique Nov 20 '19 at 17:36

score 2 · Answer 3 · edited Apr 13 '17 at 12:35

2

Here's a solution using the LaTeX internal \@ifnextchar:

\documentclass{minimal}
\makeatletter
\def\foo{\@ifnextchar[\foo@BT\foo@BF}
\def\foo@BT[#1]{Bracket true. Optional argument was: #1.}
\def\foo@BF{Bracket false. No optional argument.}
\makeatother

\begin{document}
\foo[Hello, world]

\foo
\end{document}

edited Apr 13 '17 at 12:35

Community

1

answered Dec 12 '14 at 22:31

kahen

2,165

1

Please do not use minimal for examples as it is not suitable for this purpose. – cfr Dec 12 '14 at 23:02

Special behavior if optional argument is not passed

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