[Tex/LaTex] Misplaced \omit. \multispan with \newcommand

columnserrorsmacrostables

I'm trying to build my own cookbook-style, which has an ingredient makro. Sometimes the ingredients should be separated and have a headline, so I tried with an optional argument. A minimum example looks like this:

\documentclass{scrartcl}
\newcommand{\ingredient}[3][]{
    #1 #2 & #3 \\
}
\begin{document}
    \begin{tabular}{r|l}        
        \ingredient[\multicolumn{2}{l}{Head} \\]{test1}{test2}
        \ingredient[test3]{test4}{test5}
        \ingredient{test6}{test7}
    \end{tabular}
\end{document}

What's wrong with this code? I get the following error message:

! Misplaced \omit.
\multispan ->\omit 
                   \@multispan 
l.7 ...[\multicolumn{2}{l}{Head} \\]{test1}{test2}

Best Answer

Tabular material is really picky about what is allowed to go "before" a line. In particular, it has to be fully expandable.

Hence, even the assignment which \ingredient needs to do to check for its optional argument is enough to start the line, and then \multicolumn is no longer allowed to appear.

The easiest way to remedy this is to make the first argument of \ingredient mandatory:

\documentclass{scrartcl}
\newcommand{\ingredient}[3]{
    #1 #2 & #3 \\
}
\begin{document}
    \begin{tabular}{r|l}        
        \ingredient{\multicolumn{2}{l}{Head} \\}{test1}{test2}
        \ingredient{test3}{test4}{test5}
        \ingredient{}{test6}{test7}
    \end{tabular}
\end{document}
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