Dynamic table column generation

Question

I want to generate table columns dynamically but I have no clue how to tell LaTeX to accept or evaluate & as a column separator.

As you can see in the attached code, the problem is with \@for \el:=#1\do{\textbf{\el} &}, where & should separate the columns and the column titles. If I remove the & char, I get the whole string in the first column. That makes sense to me, because table cells are separated with &. How can I make it work with separate columns?

\documentclass{article}

\usepackage{longtable}
\usepackage{booktabs}
\usepackage{forloop}
\usepackage{xstring}

\makeatletter

% http://tex.stackexchange.com/a/107298
\newcommand*{\commalength}[1]{%
  \StrCount{#1,}{,}%
}

% http://tex.stackexchange.com/a/87420
\newcommand{\columnheader}[1]{%
    % if i remove the "&" it works as a whole string but not splitted in columns
    % HERE IS THE BUG WITH THE "&"
      \@for \el:=#1\do{\textbf{\el} &}%
}
\makeatother


\newcommand{\generatecolumns}[1]{

    % getting the column count automatically would be really nice like \commalength{#1}
    \multicolumn{5}{l}{\emph{Continuation of \tablename\ \thetable{}}} \\
    \toprule
    % the column titles should be generated here ...
    \columnheader{#1} \\ \midrule
    \endhead

    \hline
    \multicolumn{5}{l}{\emph{Continued on next page}} \\
    \bottomrule
    \endfoot
}


\begin{document}

\begin{longtable}{lllll}

    % this are the column titles
    \generatecolumns{Column1,Column2,Column3,Column4,Column5}

    \newcounter{i}
     \forloop{i}{1}{\value{i} < 100}{
         \arabic{i} & a & b & c & d \tabularnewline
     }

\end{longtable}

\end{document}

Answer 1

The timing in alignments can be tricky, the main problem here is that & isn't just an alignment mark it is a TeX group like {} so if you evaluate the loop within the table the group started in one iteration of the loop ends in the next, and most loop macros can't cope.

It is usually easier to construct the entire row in a token register or macro.

As you are using LT, it counts the columns already, so you don't need to.

\documentclass{article}

\usepackage{longtable}
\usepackage{booktabs}
\usepackage{forloop}
\usepackage{xstring}

\makeatletter


% http://tex.stackexchange.com/a/87420
\newcommand{\columnheader}[1]{%
 \gdef\thisheader##1{}%
    % if i remove the "&" it works as a whole string but not splitted in columns
    % HERE IS THE BUG WITH THE "&"
    \@for \el:=#1\do{\protected@xdef\thisheader{\thisheader&\el}%
}}



\newcommand{\generatecolumns}[1]{

    % getting the column count automatically would be really nice like \commalength{#1}
    \multicolumn{\LT@cols}{l}{\emph{Continuation of \tablename\ \thetable{}}} \\
    \toprule
\noalign{\columnheader{#1}}\thisheader \\ \midrule
    \endhead

    \hline 
    \multicolumn{5}{l}{\emph{Continued on next page}} \\
    \bottomrule
    \endfoot
}

\makeatother
\begin{document}

\begin{longtable}{lllll}

    % this are the column titles
    \generatecolumns{Column1,Column2,Column3,Column4,Column5}

    \newcounter{i}
    \forloop{i}{1}{\value{i} < 100}{
        \arabic{i} & a & b & c & d \tabularnewline
    }

\end{longtable}

\end{document}