equation with auto numbering

Question

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

\begin{equation}
    x=y \label{eq:myeq}
\end{equation}

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 — , Jun 05 '17 at 13:06
https://tex.stackexchange.com/questions/4728/how-do-i-number-equations-only-if-they-are-referred-to-in-the-text — Torbjørn T., Jun 05 '17 at 13:10
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. — God bless, Jun 05 '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 05 '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. — , Jun 05 '17 at 13:24
@David, that's perfect. Along with Christian's suggestion of using \let\[\equation and \let\]\endequation I get the desired effect even with amsmath. — Tohiko, Jun 05 '17 at 15:32

score 2 · Accepted Answer · answered Jun 05 '17 at 13:45

I don't recommend this really, but here's a way for \[...\] being the same like \begin{equation}...\end{equation} (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 05 '17 at 13:59

equation with auto numbering

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