Access argument passed to environment with etoolbox \AtBeginEnvironment

Question

I would like to use etoolbox to run some code at the beginning of the tabular environment. I would like to use the column specification in this code. Can this be done easily?

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{etoolbox}
\AtBeginEnvironment{tabular}{...I want to do something with 'cc' (or whatever the column spec is)...}
%\AtBeginEnvironment{tabular}{something with #1, not surprisingly this fails}
\begin{document}
\begin{tabular}{cc}
     hello & world  \\
\end{tabular}
\end{document}

Answer 1

I did a little poking around trying to figure out what tokens are executed in what order with such a hook, and diagnostically print the first 2 (that follow my \addmeafter argument) out. The second diagnostic token is key: \csname. That means what follows is the environment name and is of variable length. Thus, what I do is expand the \csname and pass the same sequence of tokens to \zzz. By expanding it once, the 2nd token following my argument (aka #3) is no longer \csname, but \zz or \tabular. Thus, by placing my desired #1 argument following #3, it gets placed as if it were the first thing in the environment.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{etoolbox}
\newcommand\addmeafter[3]{(\detokenize{#2!#3!})\par% <--FOR DIAGNOSIS
  \def\tmp{\zzz{#1}}\expandafter\tmp\expandafter#2#3}
\newcommand\zzz[3]{#2#3#1}
\newenvironment{zz}{[}{]}
\AtBeginEnvironment{zz}{\addmeafter{Q}}
\AtBeginEnvironment{tabular}{\addmeafter{{|c|c|}}}
\begin{document}
\begin{zz}
     hello  world
\end{zz}

\bigskip
\begin{tabular}
\hline
a & b\\ 
\hline
c&d\\
\hline
\end{tabular}
\end{document}

If I comment out my diagnostics, and implement the hook as a macro \tabcols

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{etoolbox}
\newcommand\addmeafter[3]{%(\detokenize{#2!#3!})\par% <--FOR DIAGNOSIS
  \def\tmp{\zzz{#1}}\expandafter\tmp\expandafter#2#3}
\newcommand\zzz[3]{#2#3#1}
\newenvironment{zz}{[}{]}
\AtBeginEnvironment{zz}{\addmeafter{Q}}
\AtBeginEnvironment{tabular}{\addmeafter{{\tabcols}}}
\begin{document}
\begin{zz}
     hello  world
\end{zz}

\bigskip
\def\tabcols{|c|c|}
\begin{tabular}
\hline
a & b\\ 
\hline
c&d\\
\hline
\end{tabular}

\bigskip
\def\tabcols{|c|c|c|}
\begin{tabular}
\hline
a & b & xxx\\ 
\hline
c&d& y\\
\hline
\end{tabular}
\end{document}

I get the desired result, allowing me to pre-specify the tabular column format in the macro \tabcols, and not specify it at the time of execution.

---

SUPPLEMENT

On the possibility, as Don suggests, that I misread the question..

If it is not desired to stick in a column specification, but merely to read it in advance, the change is straightforward. Here I read the column specification, #4 in \zzz, and pass it as an argument to #1 (which is the macro \tabcols), in advance of executing the tabular #3.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{etoolbox}
\newcommand\readafter[3]{%
  \def\tmp{\zzz{#1}}\expandafter\tmp\expandafter#2#3}
\newcommand\zzz[4]{#1{#4}#2#3{#4}}
\newcommand\tabcols[1]{%
  The spec for the next tabular will be \detokenize{#1}\par}
\AtBeginEnvironment{tabular}{\readafter\tabcols}
\begin{document}

\begin{tabular}{|c|c|}
\hline
a & b\\ 
\hline
c&d\\
\hline
\end{tabular}


\bigskip
\begin{tabular}{|c|c|c|}
\hline
a & b & xxx\\ 
\hline
c&d& y\\
\hline
\end{tabular}
\end{document}

DOUBLE SUPPLEMENT

The OP asks in a comment about tabular* and tabularx. I will show how to address the two mandatory arguments of tabular*. It is the same logic as the above solutions, but taken to absorb one further argument. For example, compare \readafter to \readtwoafter, then compare \zzz to \zzzz and finally compare \tabcols to \tabstarinputs.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{etoolbox}
\newcommand\readafter[3]{%
  \def\tmp{\zzz{#1}}\expandafter\tmp\expandafter#2#3}
\newcommand\readtwoafter[3]{%
  \def\tmp{\zzzz{#1}}\expandafter\tmp\expandafter#2#3}
\newcommand\zzz[4]{#1{#4}#2#3{#4}}
\newcommand\zzzz[5]{#1{#4}{#5}#2#3{#4}{#5}}
\newcommand\tabcols[1]{%
  The spec for the next tabular will be \detokenize{#1}\par}
\newcommand\tabstarinputs[2]{%
  The width for the next tabular* will be \detokenize{#1}\par
  The spec for the next tabular* will be \detokenize{#2}\par}
\AtBeginEnvironment{tabular}{\readafter\tabcols}
\AtBeginEnvironment{tabular*}{\readtwoafter\tabstarinputs}
\begin{document}

\begin{tabular}{|c|c|}
\hline
a & b\\ 
\hline
c&d\\
\hline
\end{tabular}


\bigskip
\begin{tabular}{|c|c|c|}
\hline
a & b & xxx\\ 
\hline
c&d& y\\
\hline
\end{tabular}

\bigskip
\begin{tabular*}{2in}{|c|c||}
\hline
a & big list of stuff here\\ 
\hline
c&d\\
\hline
\end{tabular*}

\end{document}

Answer 2

The mandatory argument passed to tabular is really absorbed by another macro internally, so it's difficult to grab it naturally. You can, however, redefine tabular to grab that argument using environ, or just intervene by redefining the initial \tabular:

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage{tabularx}

\let\oldtabular\tabular
\expandafter\let\expandafter\oldtabulars\csname tabular*\endcsname
\expandafter\let\expandafter\endoldtabulars\csname endtabular*\endcsname

\RenewDocumentCommand{\tabular}{ O{c} m }{%
  Do something with \detokenize{#2} \par
  \oldtabular[#1]{#2}}
\RenewDocumentEnvironment{tabular*}{ m O{c} m }{%
  Do something with \detokenize{#3} \par
  \begin{oldtabulars}{#1}[#2]{#3}
}{%
  \end{oldtabulars}%
}

\begin{document}

\begin{tabular}{c|c}
  hello & world
\end{tabular}

\hrulefill

\begin{tabular*}{.5\linewidth}{c|c|c}
  hello & world & star
\end{tabular*}

\hrulefill

\begin{tabularx}{.5\linewidth}{c|c|X}
  hello & world & tabularx
\end{tabularx}

\end{document}

The above examples provides the option to manage the column specification for tabular, tabular* and tabularx. tabularx uses tabular* internally, so no need to do anything else.