Aligning equations in a list

Question

1. I would like a list of equations (ideally using enumitem's enumerate environment) aligned (e.g.) by the equal sign. Is there a way to achieve that?

   The following produces a list of equations that are not aligned:

   \begin{enumerate}
     \item $a=b$
     \item $c+d=e$
   \end{enumerate}
   

   I want them aligned as in

   \begin{align*}
       a &= b \\
     c+d &= e
   \end{align*}
   

   but numbered with the same spacing of labels that would be produced by enumerate.

2. If possible, I would also like to know how to have them as much to the left as possible (as opposed to centering produced by align* environment). In the examples above this would mean that the second equation (i.e. "c+d=e") would typeset exactly as it does using enumerate and the first one would be aligned to it.

3. And finally, at the risk of being unmodest, I wish to be able to choose between inline and display style.

Answer 1

I would recommend using the functionality provided by eqparbox. Specifically, \eqparbox[<tag>][<align>]{<stuff>} will set <stuff> in a box that has maximum width across the same <tag>, with an additional <align>ment specification (left, center/default or right). With a slight modification that accommodates for math mode, \eqmathbox defined below does the same:

\documentclass{article}
\usepackage{amsmath,eqparbox}
% https://tex.stackexchange.com/a/34412/5764
\makeatletter
\NewDocumentCommand{\eqmathbox}{o O{c} m}{%
  \IfValueTF{#1}
    {\def\eqmathbox@##1##2{\eqmakebox[#1][#2]{$##1##2$}}}
    {\def\eqmathbox@##1##2{\eqmakebox{$##1##2$}}}
  \mathpalette\eqmathbox@{#3}
}
\makeatother
\begin{document}
\begin{enumerate}
  \item
  $\eqmathbox[LHS][r]{a} = b$
\item
  $\eqmathbox[LHS]{c + d} = e$
\end{enumerate}
\end{document}

The principle behind eqparbox is to store the width of the largest element in the .aux file, after processing the entire document. This .aux is then read during a subsequent compilation, setting elements within the appropriately-sized (maximal) boxes. Since it uses the .aux for processing, it requires at least two compilations with every change in the widest element for a <tag>.

Answer 2

This solution (which is inspired by code from appendix D of The TeXbook) first typesets the formulas using \halign and then unpacks the \hboxes thus produced, placing each \hbox content after an \item:

\def\itemize{\unskip\setbox0=\lastbox
  \ifhbox0{\itemize}\item\unhbox0 \fi}
\begin{enumerate}
\vbox{\halign{\hfil$#$&${}#$\hfil\cr
  a&=b\cr
  c+d&=e\cr
}\itemize}
\end{enumerate}

The \vbox is necessary because \lastbox cannot be used on the main vertical list.

To achieve displaystyle, replace the \halign template in the fourth row with \hfil$\displaystyle#$&$\displaystyle{}#$\hfil