Code between \begin{environment} and \end{environment} stored in an array

Question

Is there a way to store the text between the \begin and \end commands of a generic environment in an array?

A code that obviously doesn't work but shows what I mean is the following

\documentclass{memoir}
\usepackage{arrayjobx}
\newarray\entry
\newnvironment{mycenter}{\begin{center}\entry{0}={}{}\end{center}} % \entry{0} should be the text between \begin{mycenter} and \end{mycenter}
\begin{document}
\begin{mycenter}
This line is centered.
\end{mycenter}
\entry{0} % \entry{0} should be 'This line is centered'
\end{document}

Here I used the environment mycenter based on center, but the specific environment doesn't really matter, I just used it as an example.

Edit: I tried using the environ package as suggested by David Carlisle but I think I'm missing something here.

The edited code is

\documentclass{memoir}
\usepackage{arrayjobx, environ}
\newarray\entry
\NewEnviron{mycenter}{\entry(0)={\BODY}\begin{center}\BODY\end{center}}
\begin{document}
\begin{mycenter}
This line is centered.
\end{mycenter}
\entry(0)
\end{document}

which still doesn't compile. What am I doing wrong?

Answer 1

Definitely not a simple task.

\documentclass{article}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{environ}
\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand{\newtheoremx}{momo}
 {
  % * Let's duplicate the working of \newtheorem; \newtheoremx should
  %   be used for theorems that need to be listed
  % * \newtheoremx{theorem}{Theorem} will actually do
  %   \newtheorem{theorem@inner}{Theorem} (honoring the usual optional arguments)
  % * We also need a property list to store along with the theorem
  %   the one which it is subordinate to
  \IfValueTF{#2}
   {
    \newtheorem{#1@inner}[#2@inner]{#3}
    % #1 is subordinate to #2
    \prop_gput:Nnn \g_riccardo_theorems_prop { #1 } { #2 }
   }
   {
    \IfValueTF{#4}
     {
      \newtheorem{#1@inner}{#3}[#4]
     }
     {
      \newtheorem{#1@inner}{#3}
     }
    % #1 is not subordinate, store the name itself
    \prop_gput:Nnn \g_riccardo_theorems_prop { #1 } { #1 }
   }
  % define a "grabbing" environment #1 with the usual features 
  \NewEnviron{#1}[1][]
   {
    % start the inner environment (without or with optional argument)
    \tl_if_empty:nTF { ##1 }
     { \begin{#1@inner} }
     { \begin{#1@inner}[##1] }
    % save the statement number
    \tl_gset:Nx \g__riccardo_theorems_number_tl { \use:c {@currentlabel} }
    % typeset the statement
    \BODY
    % end the inner environment
    \end{#1@inner}
   % store the statement in a sequence variable, actually as
   % four arguments as shown
   \seq_gput_right:Nx \g_riccardo_theorems_seq
     {
      { #1 } % name
      { \g__riccardo_theorems_number_tl } % number
      { \exp_not:n { ##1 } } % attribution
      { \exp_not:V \BODY } } % body
    }
 }

% allocate the needed variables
\prop_new:N \g_riccardo_theorems_prop
\seq_new:N \g_riccardo_theorems_seq
\tl_new:N \g__riccardo_theorems_number_tl

% print the stored theorems
\NewDocumentCommand{\printtheorems}{}
 {
  % we need a group where nullifying the action of \label
  \group_begin:
  \cs_set_eq:NN \label \use_none:n
  % map the sequence, passing each item to the function that prints a theorem
  \seq_map_function:NN \g_riccardo_theorems_seq \riccardo_printtheorems:n
  % end the group
  \group_end:
 }

\cs_new_protected:Nn \riccardo_printtheorems:n
 {
  % just pass the argument in the form {name}{number}{attribution}{text}
  % to a four argument function
  \__riccardo_printtheorems:nnnn #1
 }
\cs_new_protected:Nn \__riccardo_printtheorems:nnnn
 {
  % redefine \the<statement>@inner to yield the stored number
  % we use the property list to use the correct counter
  % (for instance, in case of "lemma", <statement> will be "theorem"
  \cs_set:cpn { the \prop_item:Nn \g_riccardo_theorems_prop {#1} @inner } { #2 }
  \tl_if_empty:nTF { #3 }
   {
    % no attribution
    \begin{#1@inner} #4 \end{#1@inner}
   }
   {
    % attribution
    \begin{#1@inner}[#3] #4 \end{#1@inner}
   }
 }

\ExplSyntaxOff

\newtheoremx{theorem}{Theorem}[section]
\newtheoremx{lemma}[theorem]{Lemma}

\begin{document}

\section{First test}

\begin{theorem}[Important]\label{thm:important}
This is a theorem about $\log_a x$.
\end{theorem}

\begin{lemma}\label{lem:whatever}
This is a lemma.
\end{lemma}

\begin{theorem}\label{thm:unimportant}
This is another theorem
\end{theorem}

\section{Second test}

\begin{theorem}\label{thm:soandso}
Again a theorem.
\end{theorem}

\section{Theorems}

\printtheorems

\end{document}