Problem with dynamic table creation

Question

I've wandered into uncharted territory with a package I'm writing. At one point I need to create a table of 'n' columns (n supplied by user in doc). Creating a variable in the preamble of the tabular declaration will handle the number of rows and the justification, but I can't seem to figure out how to create a row of labels with 'n' cells. Hopefully this MWE example will clarify (I've massively simplified my code to focus on, what I think, is the problem). This will generate the error.

    \documentclass{memoir}
    \begin{document}

     \ newcounter{n}
      \newcommand{\repeatntimes}[2]{ % usage:\repeatntimes{c}{3} -> ccc
      \setcounter{n}{0}%
      \loop\addtocounter{n}{1}{#1}%
      \ifnum\value{n}<#2\repeat}

      \newcommand{\sublabels}{\repeatntimes{blah &}{2}}

      \begin{tabular}[t]{ccc}
      \sublabels blah \\
      \end{tabular}
    \end{document}

I need \sublabels to expand such that what gets typeset is

\begin{tabular}[t]{ccc}
blah & blah & blah \\
\end{tabular}

I know that this is an expansion problem, but there's another problems also, which is that the '&' is one of the 10 special characters that do funny things. The error message is

! Missing } inserted.
<inserted text>
                }

I recall Knuth talking about this in the TeXbook, but I spend a long time today trying to find it and couldn't. I've tried all sorts of ways to sequence the expansion, but none work, I think because of the way '&' functions, but I'm not sure.

I would appreciate any ideas. TIA

Answer 1

You can fix \repeatntimes like this:

\newcommand{\repeatntimes}[2]{ % usage:\repeatntimes{c}{3} -> ccc
   \newcount\i
   \newtoks\ans
   \loop
      \ans=\expandafter{\the\ans#1}
      %\showthe\ans% uncomment to watch this work
      \advance\i by 1
      \ifnum\i<#2\repeat
   \the\ans
}

Then

\newcommand{\sublabels}{\repeatntimes{blah &}{2}}
\begin{tabular}[t]{ccc}
\repeatntimes{blah &}{2} blah \\
\end{tabular}

gives you what you said you wanted.

But maybe there's something more elegant, depending on what you really want. Do you want a row of cells in a table to all be the same thing? Maybe you could read the column specification to find the correct n.

Answer 2

I'd use the expandable \prg_replicate:nn function from expl3:

\documentclass{memoir}
\usepackage{expl3}
\begin{document}
  \ExplSyntaxOn
  \newcommand*{\sublabels}{\prg_replicate:nn{2}{blah &}}
  \ExplSyntaxOff
  \begin{tabular}[t]{ccc}
  \sublabels blah \\
  \end{tabular}
\end{document}

You could of course take the code behind \prg_replicate:nn and implement it yourself without expl3 if you want (after all, it is ultimately just TeX programming). However, I'd go with the 'pre-packaged' version (not surprising, although I'd point out that \prg_replicate:nn pre-dates my involvement with LaTeX3 by a long way).