dynamically generating tabular columns

Question

Consider a table (regular tabular environment) with a fixed number of columns. I want to create a command that accepts a comma-separated list of strings, where the number of strings may differ between zero and the number of columns.

\begin{tabular}{llll}
  \outputrow{ab cd,foo,3,5}\\ %all columns used
  \outputrow{}\\ %no column used
  \outputrow{1,2}\\ %two columns used
\end{tabular} 

\outputrow{cvs-input} should, in all cases, generate all columns (4 in this example). The actual column value should be generated by two additional commands: \emptycol just outputs the content of an empty column, whereas \usercol{content} generates the corresponding output of the provided user input. So the result of the above example should be

\begin{tabular}{llll}
  \usercol{ab cd} & \usercol{foo} & \usercol{3} & \usercol{5} \\
  \emptycol & \emptycol & \emptycol & \emptycol \\
  \usercol{1} & \usercol{2} & \emptycol & \emptycol \\
\end{tabular} 

The number of columns is static for a single table, but I need this solution for different cases with a different number of columns.

I first thought of using a package like etoolbox, but I'm not sure how I can emit a single column using a loop or something like this.

Do you have any advice how this can be achieved?

Answer 1

This can be done by using some advanced TeX features, i.e. \def with a scantext instead of \newcommand. This way you can extract all text between the commas.

One way to do this would be:

\documentclass{article}

\makeatletter
\newcommand\outputrow[1]{%
    \@outputrow#1,,,,,\@nil
}
\def\@outputrow#1,#2,#3,#4,#5\@nil{%
    \@usercol{#1} & \@usercol{#2} & \@usercol{#3} & \@usercol{#4} \\%
}
\def\@usercol#1{%
    \expandafter\ifx\expandafter\@nnil\unexpanded{#1}\@nnil
        \emptycol
    \else
        \usercol{#1}%
    \fi
}
\makeatother

\newcommand\usercol[1]{\fbox{#1}}
\newcommand\emptycol{(empty)}


\begin{document}

\begin{tabular}{llll}
  \outputrow{ab cd,foo,3,5}\\ %all columns used
  \outputrow{}\\ %no column used
  \outputrow{1,2}\\ %two columns used
\end{tabular} 
\end{document}

The \outputrow calls an internal macro \@outputrow and places the argument behind it without { } but with several commas and an endmarker (\@nil). Then \@outputrow scans for the four arguments and consumes the remaining commas as fives argument which is then discarded. The four arguments as used with \@usercol which checks if the argument is empty and then uses \emptycol or \usercol{#1}.

Note that this can be improved as well, e.g. with a loop so it works with an large number of columns.