Math & Science ⇒ How do I define this command?

[Yue]

Code: Select all

\documentclass{article}
\usepackage{xparse}
\NewDocumentCommand\en{g}{%
  \IfNoValueTF#1{\epsilon}{\epsilon_{#1}}%
}
\begin{document}
See \( \en \) but \( \en{a} \)!
\end{document}

This works, but when I use \en{\textbf{k}}, it gives me two \epsilons. Is there no easy way to do \en{\textbf{k}} and \en?

Sorry, my error in the definition above. Try

Code: Select all

\documentclass{article}
\usepackage{xparse}
\NewDocumentCommand\en{g}{%
  \IfNoValueTF{#1}{\epsilon}{\epsilon_{#1}}%
}
\begin{document}
See \( \en \) but \( \en{\textbf{k}} \)!
\end{document}

(Note the braces around #1 after \IfNoValueTF: I forgot those!)

Works great! Thanks! I replayed \epsilon with \en 45 times ^^, I hope this wont cause me problems, because then I can't change it back. I mean, this package and command wont cause trouble?

[josephwright]

Works great! Thanks! I replayed \epsilon with \en 45 times ^^, I hope this wont cause me problems, because then I can't change it back. I mean, this package and command wont cause trouble?

Shouldn't do. xparse is part of the development efforts for LaTeX3, and is regarded as "stable" by the LaTeX Team.

[Yue]

Good to hear and so many friendly ppl here^^