Giving an equation between \[ \]'s a number

Question

I usually use \[ ... \] to typeset equations in latex and sometimes I come back and realize that I want to have an equation number and label. I have been coming back and editing it to \begin{equation} ... \end{equation} but this is annoying and time consuming. Is there a command to do something like \[ \addeqnumber{} \] which will work the same way as the equation environment? I am aware this probably isn't the best practice, but it would save me some annoyance.

Answer 1

\documentclass{article}
\usepackage{amsmath}
\newcommand\addtag{\refstepcounter{equation}\tag{\theequation}}

\begin{document}

\[
  1+1=2  \addtag
\]

\end{document}

See also: Equation numbering question

Answer 2

Not answering your question but maybe solving your problem: mathtools has a mechanism that suppresses equation numbers for unreferenced equations. (So you'd always use equation and friends and only get numbers if you ref the equation.)

Answer 3

If you find yourself doing this for all equations I would use search & replace but if you’re reluctant you can do the following:

\let\[\equation
\let\]\endequation

After that, \[ … \] will behave identical to \begin{equation} … \end{equation}.

You can also wrap this into a macro, and create another one to switch back so that you can switch between the two styles via two macros.

Answer 4

As you're aware that this isn't best practice, I don't mind answering! The following seems to work:

\newcommand{\addlabel}[1]{%
\refstepcounter{equation}%
\label{#1}%
\let\]\endequation}

Use it as:

\[
\addlabel{pythagoras}
x^2 + y^2 = z^2
\]

The start of \equation simply steps the counter, so we do that and don't have to worry about doing anything else at the begining. Then we assign the label (since there's no point in having one command to add the number and then another to assign a label to it). To place the label, we use the \endequation command. As we're doing this inside the mathematics, the assignment \let\]\endequation only holds until the end of the mathematics and so it doesn't affect future mathematics (which is a bit lucky for future mathematics).