Modify eqnarray to match that of amsmath's align

Question

This is a follow-up question to \eqnarray vs \align.

Sadly, some journals (and their accompanying document classes) still require the use of eqnarray to arrange a number of equations in a single block. What would be the easiest (and best) way to modify the eqnarray environment to match the output of align without having to modify the input regime. Something that produces the output of the second set of equations (in align) given the first set (in eqnarray):

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{eqnarray}
  f(x) & = & ax^2 + bx + c \\
  g(x) & = & dx^2 + ex + f
\end{eqnarray}

\setcounter{equation}{0}
\begin{align}
  f(x) & = ax^2 + bx + c \\
  g(x) & = dx^2 + ex + f
\end{align}
\end{document} 

The idea is to provide a very minimal package that could be added to a document when it still uses eqnarray, without having to modify the code used for eqnarray.

My first attempt was to modify the setup within \eqnarray (from latex.ltx). Currently defined as

\def\eqnarray{%
   \stepcounter{equation}%
   \def\@currentlabel{\p@equation\theequation}%
   \global\@eqnswtrue
   \m@th
   \global\@eqcnt\z@
   \tabskip\@centering
   \let\\\@eqncr
   $$\everycr{}\halign to\displaywidth\bgroup
       \hskip\@centering$\displaystyle\tabskip\z@skip{##}$\@eqnsel
      &\global\@eqcnt\@ne\hskip \tw@\arraycolsep \hfil${##}$\hfil
      &\global\@eqcnt\tw@ \hskip \tw@\arraycolsep
         $\displaystyle{##}$\hfil\tabskip\@centering
      &\global\@eqcnt\thr@@ \hb@xt@\z@\bgroup\hss##\egroup
         \tabskip\z@skip
      \cr
}

It's obvious that the three components of an equation are handled separately. Additionally, a horizontal skip of 2\arraycolsep is inserted around the middle component while also making that component equivalent to \mathord (wrapping it within {..}). A possible correction of this is therefore:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\makeatletter
\def\eqnarray{%
   \stepcounter{equation}%
   \def\@currentlabel{\p@equation\theequation}%
   \global\@eqnswtrue
   \m@th
   \global\@eqcnt\z@
   \tabskip\@centering
   \let\\\@eqncr
   $$\everycr{}\halign to\displaywidth\bgroup
       \hskip\@centering$\displaystyle\tabskip\z@skip{##}$\@eqnsel
      &\global\@eqcnt\@ne${}##{}$% Removed horizontal skip + forced mathord
      &\global\@eqcnt\tw@% Removed horizontal skip
         $\displaystyle{##}$\hfil\tabskip\@centering
      &\global\@eqcnt\thr@@ \hb@xt@\z@\bgroup\hss##\egroup
         \tabskip\z@skip
      \cr
}
\makeatother
\begin{document}
\begin{eqnarray}
  f(x) & = & ax^2 + bx + c \\
  g(x) & = & dx^2 + ex + f
\end{eqnarray}

\setcounter{equation}{0}
\begin{align}
  f(x) & = ax^2 + bx + c \\
  g(x) & = dx^2 + ex + f
\end{align}
\end{document}

What else?

Answer 1

This is a robust solution based only on making & active. The code is well commented I hope:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath

\begingroup % we group everything so that our \catcode changes are local, and we use \gdef everywhere
\makeatletter
\gdef\eqna@origamp{&} % `&` stored
\catcode`\&\active % set `&` active in the scope of these hooks
\gdef\eqna@newamp{%
  \ifx\@currenvir\eqna@currenvir % if the inner-most environment is `eqnarray`
    \eqna@onlyfirstamp\let\eqna@onlyfirstamp\@empty % use `&`, but only once for each line
  \else % if there's another environment inside `eqnarray`
    \eqna@origamp % use `&` as is
  \fi
}
\gdef\eqna@hook{% modification of `eqnarray`
  \let\eqna@currenvir\@currenvir % store the current environment, which is `eqnarray` or `eqnarray*`
  \catcode`\&\active % `&` has to be active
  \let&\eqna@newamp % define `&` expansion
  \let\eqna@onlyfirstamp\eqna@origamp % `&` that is used only once on a line
  }
\catcode`\*11 % set `*` letter so that we can redefine `eqnarray*`
\gdef\eqnarray{\eqna@hook\align} % `eqnarray` is our hooks and `align`
\gdef\eqnarray*{\eqna@hook\align*} % `eqnarray*` is our hooks and `align*`
\global\let\endeqnarray\endalign
\global\let\endeqnarray*\endalign*
\endgroup

\begin{document}\thispagestyle{empty}

\begin{eqnarray}
  f(x) & = & \begin{cases} 1 & x\geq0 \\ 0 & x<0 \end{cases}\\
  \begin{pmatrix}1&0\\0&1\end{pmatrix} & = & dx^2 + ex + f \nonumber\\
  kkk(x) & = & gx^2 + kx + m
\end{eqnarray}

\begin{align}
  f(x) & = \begin{cases} 1 & x\geq0 \\ 0 & x<0 \end{cases}\\
  \begin{pmatrix}1&0\\0&1\end{pmatrix} & = dx^2 + ex + f \notag\\
  kkk(x) & = gx^2 + kx + m
\end{align}

\end{document}

Answer 2

Transmutation of eqnarray into align

original enthusiasm: I just didn't believe it when I saw it worked. Well it works in this very minimal case, but will certainly break I guess in real life.

When I proposed the method I had hesitated between a an active & approach (exactly what tochecz's code seems to do very robustly) and a more down-to-earth approach using delimited arguments. I was very surprised that I got the delimited argument method to work quickly so I posted it here and then extended to multi-line situations. I also wasted some time on extending to many-tabs as I didn't know that eqnarray was a strictly three column format with two tabs.

However if one wanted to extend the code to allow \begin{env}...\end{env} that would require quite an effort. Actually possibly recreating the amsmath \collect@body or similar (perhaps one could just coerce it to serve us here). That's my two pence thought at this stage as I was busy with other things in-between and I come back to see that tochecz has implemented the idea of transmutation with the active & technique, obviously the much better choice. So purely by sentimentalism I will leave here some of my initial code.

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath

\newtoks\arraytoaligntoks

%% Initial version for two lines
%%\long\def\eqnarray #1&#2&#3\\#4&#5&#6\end{%
%%    \arraytoaligntoks={#1&#2#3\\#4&#5#6}%
%%    \edef\endeqnarray{\noexpand\begin{align}\the\arraytoaligntoks
%%                     \noexpand\end{align}}\end}

%% version for arbitrarily many lines*
%% But this will fail completely if some \begin..\end is not within braces


\def\transmute #1&#2&#3\\{\arraytoaligntoks\expandafter
                     {\the\arraytoaligntoks #1&#2#3}}

\def\jfbu{\jfbu}

\long\def\eqnarray #1\end{\arraytoaligntoks{}%
    \def\\##1\\{\transmute ##1\\%
                \def\\####1\\{\ifx\jfbu####1\jfbu\else
                \arraytoaligntoks\expandafter{\the\arraytoaligntoks \\}%
                \transmute ####1\\\expandafter\\\fi}\\}%
    \\#1\\\\%
    \edef\endeqnarray{\noexpand\begin{align}\the\arraytoaligntoks
                      \noexpand\end{align}}\end}



\begin{document}\thispagestyle{empty}

\begin{eqnarray}
  f(x) & = & ax^2 + bx + c \\
  g(x) & = & dx^2 + ex + f \nonumber\\
  k(x) & = & gx^2 + kx + m
\end{eqnarray}

\begin{align}
  f(x) & = ax^2 + bx + c \\
  g(x) & = dx^2 + ex + f \notag\\
  k(x) & = gx^2 + kx + m 
\end{align}

\end{document}