Aligning `\dot` for a custom command

Question

For convenience, I'll often create custom commands for variables I'm using often in a document, for example

\newcommand{\cartPos}{\ensuremath{x}}

This works fine, and I can now do all the usual things like $\dot{\cartPos}$. But if I have something with an underscore, such as

\newcommand{\vectorPos}{\ensuremath{r_x}}

if I now do \dot{\vectorPos}, the dot is applied to the entire object, instead of being aligned above the r only, and on in between the variable and the underscore. Is there a way to deal with this elegantly?

Basically, I would like to get instead of

@Werner that worked, thanks! Feel free to turn it into a proper answer (a little more explanation will also help for posterity) — Steve Heim, Jun 25 '23 at 16:54

score 0 · Accepted Answer · answered Jun 29 '23 at 18:35

If you're defining macros for your vectors, you can redefine \dot to condition on whether you're placing it on a vector macro, or something else.

\documentclass{article}
\NewCommandCopy{\olddot}{\dot}
\RenewDocumentCommand{\dot}{ m }{%
  \ifx\vectorPos#1
    \olddot{r}_x
  \else
    \olddot{#1}
  \fi
}
\newcommand{\cartPos}{x}
\newcommand{\vectorPos}{r_x}
\begin{document}
$\cartPos ~ \vectorPos$
$\dot{\cartPos} ~ \dot{\vectorPos}$
$\dot{r_x} ~ \dot{r}_x ~ \dot{\vectorPos}$
\end{document}

The above option works, but doesn't translate to any general setup easily.

Aligning `\dot` for a custom command

1 Answers1