How can I generate token list of given length and use it in tabular environment?

I am trying to build a LaTeX environment such that for every integer N,

\begin{mytab}{N}
% foo bar
\end{mytab}

would expand to

\begin{tabular}{lr.....lr}
% foo bar
\end{tabular}

with N token pairs "lr".

Here is my poor attempt at doing it. I feel I am on the right track开发者_开发技巧, as \mytokens is the token list I want. If only it would work when I replace the \junk by \mytokens on the next to last line...

\newcounter{ct}
\newenvironment{mytab}[1]{%
  \setcounter{ct}{#1}
  \newcommand{\mytokens}{%
     \ifthenelse{\thect>0}{\addtocounter{ct}{-1}lr\mytokens}{}
  }
  \def\junk{lrlrlrlrlrlr}
  \expandafter\tabular\expandafter{\junk}%
}{
  \endtabular
}

You can repeat column definitions with *:

\begin{tabular}{*{6}{lr}}
 %...
\end{tabular}