Math & Science ⇒ How do I define this command?

[Yue]

I want to make a command \en so that \en{x} gives \epsilon_{x} and \en gives \epsilon.

\newcommand{\en}[1]{\epsilon_{#1}} does not work because this will always take a argument.

[gmedina]

Hi,

try this:

Code: Select all

\documentclass{book}

\newcommand*{\en}[1][]{\epsilon\sb{#1}}

\begin{document}

$\en[x]$

$\en$

\end{document}

[josephwright]

Code: Select all

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

[Yue]

Thx guys, I will use the short one.

[gmedina]

Code: Select all

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

Thank you, Joseph, for bringing the xparse package to my attention; even with this short example I can tell I like it (the syntax for both the command definition and for the command "in action" is more "natural"). In my free time (if any) I will have a look at the packages from the xpackage bundle.

[josephwright]

Thank you, Joseph, for bringing the xparse package to my attention; even with this short example I can tell I like it (the syntax for both the command definition and for the command "in action" is more "natural"). In my free time (if any) I will have a look at the packages from the xpackage bundle.

Not a problem. I can't claim to have had the ideas for xparse, but I did write a lot the current implementation, so I do hope it is useful.

[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?

Maybe it is best I use \newcommand{\en}[1]{\epsilon_{#1}} and use \epsilon for normal.

[gmedina]

Use curly braces to enclose the bold-faced expresion:

Code: Select all

\( \en{{\mathbf{k}}} \) 

[Yue]

Use curly braces to enclose the bold-faced expresion:

Code: Select all

\( \en{{\mathbf{k}}} \) 

Okay thx for your help, but I have already used \en{\mathbf{k}} like 100 times in my project, hard to change it all:P. In the other posters idea I have to use \en[] instead of \en{}, hard to change as well.

[josephwright]

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!)