General ⇒ New Command with one or two Arguments

[gizzmo]

Hi, I am trying to define a command that would take one or two arguments. One should be mandatory and the other one not. This is what I have right now

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\newcommand\braket[2][]{\langle\ifx\@empty#2#1\else#1\lvert#2\fi\rangle}

The desired behaviour is that:

• \braket{a} gives me \langle a \rangle
• \braket{a}{b} gives me \langle a | b \rangle

What am I doing wrong?

[localghost]

There are two small details.

1. The optional argument is always #1. You are testing argument #2 for \empty.
2. The optional argument is always given in brackets to the command instead of braces.

This translated to your example gives the below line.

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\newcommand*{\braket}[2][]{\langle\ifx &#1& #2\else #2\lvert #1\fi\rangle}

Some useful explanations by cgnieder about the \ifx test can be found in another topic [1]. Application of the command is like this.

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$\braket[b]{a}$

It will result in ‹a|b›. For others it should be noted that this needs the amsmath package for the \lvert command.

Perhaps you are interested in the braket package. It offers similar structures to typeset such expressions.

[1] View topic: ifx and ifthenelse


Best regards and welcome to the board
Thorsten

[cgnieder]

If you want to stick with your original syntax you can use the xparse package:

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\documentclass{article}
\usepackage{xparse}

\NewDocumentCommand\braket{mg}{%
  \langle #1%
    \IfNoValueF{#2}{\vert#2}%
  \rangle}

\begin{document}
 $\braket{a}$  $\braket{a}{b}$
\end{document}

Regards