problem with using loop inside the tabular environment

Question

I am having trouble with using a loop inside a tabular environment:

\newcounter{it}
\setcounter{it}{0}

\begin{tabular}{ll}
  \loop\ifnum\theit<4
    \addtocounter{it}{1}
    Q & A\\
  \repeat
\end{tabular}

I receive an error about argument of loop having an extra } and the compilation stops at &. I don't get an error when it is a single column table, but even in that case the result is not as expected: I get a single execution of the loop in place of 4.

Can someone tell me what I am missing? Is there special considerations when using loops inside tabular?

Answer 1

Yep, don't do that. tabulars are really special and macro-unfriendly (for instance every & closes a group).

Try

\def\mylines{}%
\loop\ifnum\theit<4
  \addtocounter{it}{1}
  \expandafter\def\expandafter\mylines\expandafter{%
    \mylines
    Q & A\\
  }%
\repeat

outside the tabular and then

\begin{tabular}{ll}
  \mylines
\end{tabular}

Answer 2

collect all lines with a token register and then print the lines:

\documentclass{article}
\newcounter{it}  \setcounter{it}{0}
\newtoks\tabtoks
\newcommand\addtabtoks[1]{\tabtoks\expandafter{\the\tabtoks#1}}
\newcommand*\resettabtoks{\tabtoks{}}
\newcommand*\printtabtoks{\the\tabtoks}
\begin{document}

\resettabtoks
\loop\ifnum\theit<4
  \stepcounter{it} \addtabtoks{Q & A\\}
\repeat

\begin{tabular}{ll}
\printtabtoks
\end{tabular}

\end{document}

Answer 3

I solved my problem by using a while loop instead:

\def\tand{&}

\newcounter{it}
\begin{tabular}{ll}%
  \setcounter{it}{1}%
  \whiledo{\theit<3}{%
    Q \tand A \\%
  \stepcounter{it}%
  }%
  {\theit} \tand \\
\end{tabular}

PS: if wondering why I need to print the last one separately, the reason is that the execution leave a partial line (I am using tabular{|l|l|}).