修改 eqnarray 以匹配 amsmath 的 align

Question 1

这是一个仅基于激活的强大解决方案&。我希望代码注释得很好：

\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

这是一个仅基于激活的强大解决方案&。我希望代码注释得很好：

\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}

Question 2

将 eqnarray 转换为 align

原始热情：当我看到它起作用时，我简直不敢相信。好吧，在这个非常小的情况下，它确实有效，但我想在现实生活中肯定会失败。

当我提出这个方法时，我犹豫了是使用一种active &方法（tochecz代码似乎非常稳健地实现了这个方法）还是使用分隔参数的更实际的方法。我很惊讶分隔参数方法能够快速工作，所以我在这里发布了它，然后扩展到多行情况。我还浪费了一些时间扩展到多选项卡，因为我不知道那eqnarray 是一种严格的三列格式，有两个选项卡。

但是，如果想要扩展代码以允许\begin{env}...\end{env}这样做，则需要付出相当大的努力。实际上，可能要重新创建 amsmath\collect@body或类似代码（也许可以强制它在这里为我们服务）。这是我在这个阶段的一点想法，因为我当时正忙于其他事情，回来后发现已经tochecz用该技术实现了转化的想法active &，这显然是更好的选择。所以纯粹出于感性，我将在这里留下一些我的初始代码。

\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}

逐字逐句地使用代码

Answer