A macro that detects the subscript of the argument

Question

I'm not good at TeX programming. I've looked for the method that detects the subscript or superscript of the argument in macro. Existing relevant answers using the xifthen package (Detecting subscript in command argument) are somewhat verbose for me to understand. Specifically, I want to make a macro such that if the argument has a subscript, \dot{} is applied only to the base character. It would have the form

\newcommand{\rdot}[1]{'code body'}

For example, \rdot{a_{bcd}} will give instead of that \dot{a_{bcd}} gives. \rdot should apply to superscripts in the same way.

Edit: I noticed that some macros in the answers yield a different result for boldface letters. For example, consider \rdot{\bf{v}_{abc}}. I didn't know it would make such a difference.

Welcome to TeX.SX! So you want \dot{a}_{bcd} as result? Why not just use it like this? — dexteritas, Oct 02 '21 at 13:19
are you sure you need a test, and not simply \newcommand\rdot[1]{\dot #1} ? — David Carlisle, Oct 02 '21 at 13:20
@dexteritas I want to use \dot for macro characters. For example, I'd like to define \text = v_{abc} and apply \dot to \text. — Hermis14, Oct 02 '21 at 18:29
I most certainly wouldn’t advice you to define a command called \text, as this command already exists and is pretty important! — Gaussler, Oct 03 '21 at 13:21

score 5 · Accepted Answer · answered Oct 02 '21 at 13:48

5

Testing for _ is fragile and likely to fail if the subscript is hidden in a macro or made with \sb rather than _ or perhaps if it follows ^ so the base isn't "everything before _" .

I don't think it is needed as the accent will naturally apply to the first token or brace group.

\documentclass{article}
\newcommand\rdot[1]{\dot #1}
\begin{document}
$\rdot{a_{bcd}}$ and $\rdot{a}$ or even $\rdot{a^{2}_b}$
\end{document}

answered Oct 02 '21 at 13:48

David Carlisle

757,742

Thank you! But it gives a different result for a character inside another macro; for example, $ \bf{v}_{a} $. What would be a correction for that? – Hermis14 Oct 02 '21 at 18:16
@Hermis14 well of course \bf should never be used in lates, and if you do use it (when it os defined, which it is not defined by default) the syntax would be {\bf a} not \bf{a} then it would work with \rdot as would teh standard latex \rdot{\mathbf{a}_{bcd}} in general you can use {} as in \rdot{{abcdef}_{bcd}} – David Carlisle Oct 02 '21 at 21:29
https://tex.stackexchange.com/a/617514/1090 – David Carlisle Oct 02 '21 at 22:01
Wow, I didn't know there is a difference between \bf{} and \mathbf{}. It works great! – Hermis14 Oct 02 '21 at 22:23
@Hermis14 \bf would make the rest of the expression be bold (or be undefined, it is not a standard latex command) compare $\mathbf{a}bc$ and $\bf{a}bc$ – David Carlisle Oct 02 '21 at 22:31
I have a problem. When I define \newcommand{\vect}{{\mathbf{v}}_{a}} and apply \rdot{\vect}, the trick doesn't work. – Hermis14 Oct 03 '21 at 00:49
@Hermis14 why questions should always have a full example.... If you add your \vect code to the example here you get this output ie it works fine. I would guess you are using amsmath version of \dot which gives an error in that case, you could use \newcommand\rdot[1]{\expandafter\dot #1} – David Carlisle Oct 03 '21 at 09:47
It works robustly. I really appreciate your persistent advice! – Hermis14 Oct 03 '21 at 13:26

Gaussler · Answer 2 · 2021-10-03T13:20:29.753

This is a problem that has been bothering me, too. For instance, if you define a command \abcd that expands to a_{bcd}, then \dot{\abcd} expands to \dot{a_{bcd}} rather than \dot{a}_{bcd}. You can hack your way around this, but it’s never going to work robustly enough to cover all the cases you might run into. That was part of my motivation for developing the package semantex which allows things like this to work, but with a different syntax:

\documentclass{article}
\usepackage{semantex}
\NewVariableClass\MyVar[
    output=\MyVar,
    define keys={
        {dot}{ command=\dot },
    },
]
\NewObject\MyVar\va{a}
\NewObject\MyVar\vb{b}
\NewObject\MyVar\vc{c}
\NewObject\MyVar\vd{d}
\NewObject\MyVar\vect{\mathbf{v}}[sep i=\va]
\begin{document}
( \va[\vb\vc\vd, dot] )
( \va[\vb\vc\vd][dot] )
\NewObject\MyVar\abcd[copy=\va,sep i=\vb\vc\vd]
( \abcd[dot] )
( \vect[dot] )
\end{document}

score 2 · Answer 3 · answered Oct 02 '21 at 13:38

2

You can use the command \IfSubStr (of package xstring) to check if the parameter contains _ and then use \StrBefore and \StrBehind. This places the dot also centered over the variable, if it consists of multiple characters.

Code:

\documentclass{article}
\usepackage{xstring}
\makeatletter
\newcommand{\rdot}[1]{%
    \IfSubStr{#1}{}{%
        \StrBefore{#1}{}[@tempa]%
        \StrBehind{#1}{}[@tempb]%
        \dot{@tempa}{@tempb}%
    }{%
        #1%
    }%
}
\makeatother
\begin{document}
$\rdot{a}, \rdot{a_{bcd}}, \rdot{asdf_{bcd}}$
\end{document}

Result:

answered Oct 02 '21 at 13:38

dexteritas

9,161

It seems your method is simple and practical. Would you give some comment on how to apply it to a boldface base character? Your method removes the base character for \rdot{\bf{v}_{abc}}. – Hermis14 Oct 02 '21 at 18:39
I don't know a quick fix for this, but e.g. egreg's solution works for this. – dexteritas Oct 04 '21 at 10:19

egreg · Answer 4 · 2021-10-02T14:01:20.553

2

I believe it's more natural to type \dot{a}_{x}, but if you really want to, you can do it.

\documentclass{article}
\ExplSyntaxOn
\NewDocumentCommand{\rdot}{m}
 {
  \hermis_check_subsup:nn { \dot } { #1 }
 }
% define analogously \rddot` and so on
\cs_new_protected:Nn \hermis_check_subsup:nn
 {
  \regex_match:nnTF { (^|_) } { #2 }
   {% there is a _ or ^ in the argument
    \tl_set:Nn \l_tmpa_tl { #2 }
    \regex_replace_once:nnN { (.?)(^|_)(.) } { \c{dot}\cB{\1\cE}\2\3 } \l_tmpa_tl
    \tl_use:N \l_tmpa_tl
   }
   {% no _ or ^ in the argument
    #1{#2}
   }
 }
\ExplSyntaxOff
\begin{document}
$\rdot{a}+\rdot{a_{x}}+\rdot{a_{x_1}}+\rdot{a^2_x}+\rdot{a^2}+\rdot{a_x^2}$
$\rdot{\mathit{abc}_x}+\rdot{\mathit{ab}^2_x}$
\end{document}

edited Oct 02 '21 at 14:01

answered Oct 02 '21 at 13:39

egreg

1,121,712

I had to add: \usepackage{xparse} – dexteritas Oct 04 '21 at 10:19
@daxteritas Then you might want to update your TeX distribution. :-) – Gaussler Oct 04 '21 at 11:18
@Gaussler Ah, ok. Yes I have an older version on this machine. – dexteritas Oct 04 '21 at 11:57

A macro that detects the subscript of the argument

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