formatting equations - numbered equations using $$

Question

I was trying to find how to number the equations in LaTeX. In this Peter Grill suggested to use

\begin{equation}
%equation...
\end{equation}

However, I've almost written my entire document using

$$
%equation...
$$

Is there any simple way that makes all or some equations numbered without changing all the $$...$$ to equation environment?

Answer 1

Is there any simple way that makes all ... equations numbered without changing all the $$...$$?

In case you're free to use LuaLaTeX to compile your document, the following solution -- which is a variation on this answer -- should be of interest to you. It sets up a Lua function that acts as pre-processor, automatically replacing all instances of $$ with either \begin{equation} or \end{equation}, as appropriate, before LaTeX starts its usual processing. The Lua function also knows what to do if it comes across pairs of $$ symbols on a single line, e.g., $$a^2+b^2=c^2$$ or $$ e^{i\pi}-1=0 $$.

The answer shown below also sets up two utility macros: \ReplaceDoubleDollarsOn and \ReplaceDoubleDollarsOff. The former switches the Lua function on, and the latter switches it back off. Having the ability to switch off the Lua function should be useful, e.g., if your document contains URL strings that feature instances of $$, or if it contains verbatim material that contains instances of $$.

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{luacode}    % for 'luacode' environment
\begin{luacode}

in_display_math = false -- initialize a Boolean variable
function replace_ddollar ( line )
   line = line:gsub ( "%$%$(.-)%$%$" , -- pairs of "$$" on 1 line
              "\\begin{equation} %1 \\end{equation}" )
   line = line:gsub ( "%$%$" , function (x) -- found single instance of "$$"
                    if not in_display_math then
                       in_display_math = true
                       return "\\begin{equation}" 
                    else
                       in_display_math = false
                       return "\\end{equation}"
                    end
                  end )
   return line
end

\end{luacode}
%% Set up two LaTeX utility macros:
\newcommand\ReplaceDoubleDollarsOn{%
    \directlua{ luatexbase.add_to_callback( 
    "process_input_buffer", replace_ddollar, "replace_ddollar" )}}
\newcommand\ReplaceDoubleDollarsOff{%
    \directlua{ luatexbase.remove_from_callback( 
    "process_input_buffer", "replace_ddollar" )}}
\ReplaceDoubleDollarsOn % Switch Lua function _on_ by default

\usepackage{url}  % just for this example

\begin{document}
$$
E = mc^2
$$ 

$$a^2+b^2=c^2$$ $$d^2+e^2=f^2$$ % Aside: I do not endorse this coding style!

$$
x = 3\alpha^2 + \beta = \int f\, d\mu.
$$

% Switch the Lua function off
\ReplaceDoubleDollarsOff  

\url{A_URL_string_with_a_$$_and_$$$$_and_another_$$}

% Switch the Lua function back on
\ReplaceDoubleDollarsOn 

$$ e^{i\pi}-1=0 $$

$$
1+1=2
$$ 
\end{document}

Answer 2

Or you use a simple perl script:

cat yourfile | perl -p0e 's/\$\$(.*?)\$\$/\\begin{equation}\1\\end{equation}/gs'

This will change all your $$ "environments" to equations.

You could use the additional arguments -i and yourfile (as last one) for perl to replace a file in place instead. g makes this substitution global, s makes the . match also new lines.