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I want to define a macro, call it \func, so that \func will output\phi, and \func[i] will output \phi(i).

what is the easiest way to achieve this?

I know there are other questions that address this issue - I guess one of them is exactly my question and I just don't get it :)

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2 Answers 2

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You can define \func to take an optional argument:

\documentclass{article}

\newcommand\func[1][]{\phi\ifx\\#1\\\else(#1)\fi}

\begin{document}

$\func\quad\func[i]$

\end{document}

enter image description here

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  • Thanks! But this just exploits the fact that \phi_{} looks the same as \phi, right? I need a slightly more general solution. I will edit my question to prevent this "loop hole" :)
    – olamundo
    Nov 3, 2012 at 15:37
  • @Qrrbrbirlbel right; will do so.
    – Gonzalo Medina
    Nov 3, 2012 at 15:38
  • @noam please see my updated answer. Is this what you want?
    – Gonzalo Medina
    Nov 3, 2012 at 15:45
  • 1
    @GonzaloMedina A subscript (even if it's empty) adds a small space after the symbol; its size is in the parameter \scriptspace, default value 0.5pt. I've also put a better test in the conditional.
    – egreg
    Nov 3, 2012 at 15:48
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xparse-based solution

\documentclass{article}
\usepackage{xparse}
\NewDocumentCommand{\func}{o}{%
  \phi
  \IfNoValueF{#1}{(#1)}%
}

\begin{document}

$\func\quad\func[i]$

\end{document}

where you want a single optional ('o-type') argument.

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