new table environment based on ltablex's tabularx

Question

I defined a new environment for some custom tables (to not have to repeat the header and column definitions etc. again and again).

\newenvironment{mytable}{
  \noindent\tabularx{\textwidth}{|c|c|X|} 
    \hline 
    \bf Col 1 & \bf Col 2 & \bf Col 3 \\ 
    \hline
}{%
  \endtabularx
}

It's based on tabularx (for the variable size X column) and should be able to span multiple pages. There's the package ltablex which can do this for a tabularx environment. (See the tabularx in my example below, you can comment out the mytable).

However, I am receiving the error:

! LaTeX Error: \begin{mytable} on input line 26 ended by \end{tabularx}.

I assume this comes from the redefinition of \TX@endtabularx in ltablex.sty where it has the following code snippet:

  \ifnum0=`{\fi}%
   \end{tabularx}

Is it possible to use a "longtable tabularx" as a new environment?

Example to try out:

\documentclass{article}
% filling table with for, relentlessy copied from 
% http://tex.stackexchange.com/a/183756/51583
% extremely shortened, see address for more explanations
\def\addto#1#2{\expandafter\def\expandafter#1\expandafter{#1#2}}\def\tabledata{}
\newcount\n\n=1\loop\advance\n by1\addto\tabledata{Lorem&Ipsum&XCell\\}
  \ifnum\n<100\repeat

% package and new environment
\usepackage{ltablex}
\newenvironment{mytable}{%
  \noindent\tabularx{\textwidth}{|c|c|X|} 
    \hline 
    \bf Col 1 & \bf Col 2 & \bf Col 3 \\ 
    \hline
}{%
  \endtabularx
}

\begin{document}

% working tabularx
\begin{tabularx}{\textwidth}{|c|c|X|}
  \tabledata
\end{tabularx}

% not working mytable
\begin{mytable}
  \tabledata
  \hline
\end{mytable}

\end{document}

Answer 1

It is to do with the way tabularx deals with its argument. The package environ provides a work around for such situations. Instead of \newenvironment you say

\NewEnviron{myenv}{start code
   \BODY
   end code}

Then

\begin{myenv}
  mybody
\end{myenv}

will run the contents of myenv with \BODY replaced by mybody. In your case the relevant definition is

\usepackage{environ}
\NewEnviron{mytable}{%
  \noindent\begin{tabularx}{\textwidth}{|c|c|X|} 
    \hline 
    \bf Col 1 & \bf Col 2 & \bf Col 3 \\ 
    \hline
  \BODY
\end{tabularx}
}

And a complete file is

\documentclass{article}
% filling table with for, relentlessy copied from 
% http://tex.stackexchange.com/a/183756/51583
% extremely shortened, see address for more explanations
\def\addto#1#2{\expandafter\def\expandafter#1\expandafter{#1#2}}\def\tabledata{}
\newcount\n\n=1\loop\advance\n by1\addto\tabledata{Lorem&Ipsum&XCell\\}
  \ifnum\n<100\repeat

% package and new environment
\usepackage{ltablex}
\usepackage{environ}
\NewEnviron{mytable}{%
  \noindent\begin{tabularx}{\textwidth}{|c|c|X|} 
    \hline 
    \bf Col 1 & \bf Col 2 & \bf Col 3 \\ 
    \hline
  \BODY
\end{tabularx}
}

\begin{document}

% working tabularx
\begin{tabularx}{\textwidth}{|c|c|X|}
  \tabledata
\end{tabularx}

% working mytable
\begin{mytable}
  \tabledata
  \hline
\end{mytable}

\end{document}