Problem with a conditional in a tabular

Question

I want to use conditionals to fill a tabular, but it fails. My MWE:

\documentclass{article}
\usepackage{environ}

\newcounter{mycounter}

\NewEnviron{montab}{%
    \setcounter{mycounter}{0}
    \newcommand{\mcItem}[1]{%
    \stepcounter{mycounter}%

        \ifcase \arabic{mycounter} % If counter = 0
            \relax
        \or % If counter = 1 
      ##1 & % 
        \or % If counter = 2
            ##1 \\%
        \or  % If counter = 3
            ##1 &
        \or  % If counter = 4
            ##1
        \fi
            }
\begin{tabular}{cc}
        \BODY
\end{tabular}
}

\begin{document}

\begin{montab}
\mcItem{premier}
\mcItem{deuxieme}
\mcItem{troisieme}
\mcItem{quatrieme}
\end{montab}

\end{document}

I hoped with this code to obtain the output :

premier    deuxieme
troisieme  quatrieme

I think the problem comes from the fact for example that a part of the conditional (the \ifcase) is before a & and the rest is after it. Since any cell of a tabular forms a group, the different elements of the conditional lie in different groups.

1. Did I understand the problem correctly?
2. How to do it better? :)

Answer 1

You can't begin a conditional in one cell and end it in another one:

\usepackage{environ}

\newcounter{mycounter}

\NewEnviron{montab}{%
    \setcounter{mycounter}{0}
    \newcommand{\mcItem}[1]{%
    \stepcounter{mycounter}%
        \ifcase \arabic{mycounter} % If counter = 0
        \or
    \expandafter\firstoffour\or % If counter = 1 
        \expandafter\secondoffour\or % If counter = 2
        \expandafter\thirdoffour\or  % If counter = 3
        \expandafter\fourthoffour  % If counter = 4
        \fi
      {##1 &}{##1 \\}{##1 &}{##1}
  }
\begin{tabular}{cc}
        \BODY
\end{tabular}
}
\long\def\firstoffour#1#2#3#4{#1}
\long\def\secondoffour#1#2#3#4{#2}
\long\def\thirdoffour#1#2#3#4{#3}
\long\def\fourthoffour#1#2#3#4{#4}

Actually you don't need four cases, but only two, as a final \\ is good in a tabular.

\makeatletter
\newif\iffirstcol
\NewEnviron{montab}{%
  \global\firstcoltrue
  \newcommand\mcItem[1]{%
    \iffirstcol
      \global\firstcolfalse
      \expandafter\@firstoftwo
    \else
      \global\firstcoltrue
      \expandafter\@secondoftwo
    \fi{##1 &}{##1 \\}}%
  \begin{tabular}{cc}
  \BODY
  \end{tabular}}
\makeatother