# equation with auto numbering

In LaTeX, when I need to create an equation without a number, I write

$x=y$


(or using equation*) and when I need an equation with a number/label I do

$$x=y \label{eq:myeq}$$


My question is: is there a way to somehow combine both modes, so that only the label is required to have an equation number?

In other words, I want to somehow make it so that writing something like this

 $x=y \label{eq:myeq}$


produces an equation with a number and a label, but writing $x=y$ still produces just an equation. This seems to me to be a more elegant way of doing things.

• Laziness isn't the best advisor ;-) I think it is possible but it will bite you in the end – user31729 Jun 5 '17 at 13:06
• I don't particularly advise you to change anything. When you are writing a thesis and you want to rehearse at a certain point with a hypertext reference the equation environment very helpful. If you mess around with any re-definitions chances are that you will end up with some error. This will be particularly annoying if you are writing a long thesis. You don't want to correct a source code for 30+ pages, do you. Stick with equation. – Gregor Perčič Jun 5 '17 at 13:13
• The point of this method is two fold: the first is laziness, or as I like to call it "code brevity". The second is more significant. Most of the time when writing manuscripts, I would come across an equation that I need to reference and which I used  to typeset. Making it reference-able requires not only adding the label, but also changing the environment (the same is true if I had been using equation*). Which is both annoying and wasteful. I guess just don't the reason behind the original design requiring two modification to the script to have a label, instead of an obvious one. – Tohiko Jun 5 '17 at 13:21
• @Tohiko: $...$ is the better LaTeX form of $$...$$, which is plain TeX, which does not not know of labels as you want to have them. – user31729 Jun 5 '17 at 13:24

I don't recommend this really, but here's a way for $...$ being the same like $$...$$ (which basically is true anyway, since both environments use $$...$$ in the end again)

It does not work with amsmath, however!

\documentclass{article}

\usepackage{xpatch}
\let$\equation \let$\endequation

\newif\iflabelused

\makeatletter
\xpretocmd{$}{\global\labelusedfalse}{}{} \xpretocmd{\label}{\global\labelusedtrue}{}{} \xpretocmd{$}{%
\iflabelused
\global\labelusedfalse%
\else
\let\eqno\relax
\let\@eqnnum\relax
\global\labelusedfalse
\addtocounter{equation}{-1}% Correct the value of the equation counter!
\fi
}{}{}
\makeatother

\begin{document}

$E = mc^{2} \label{foo}$

$E^{2} = (mc^{2})^{2} + (pc)^{2}$

$E = mc^{2} \label{fooagain}$

$E^{2} = (mc^{2})^{2} + (pc)^{2}$

\end{document}

• Shame that it doesn't work with amsmath, but this looks very interesting. I thought using \notag instead of playing with counters might fix it, but it doesn't it. I will try to play around with this code. Thanks! – Tohiko Jun 5 '17 at 13:59
• @Tohiko: I'll try to fix it for amsmath... – user31729 Jun 5 '17 at 14:14