colortbl has different behavior when tabular contents are \input

Question

I'm typesetting a complex table with the contents created by an external program. When I use \input I get unexpected behavior from \rowcolor. See the MWE below

\documentclass{article}
\usepackage{xcolor}
\usepackage{colortbl}
\begin{document}
Works\par
\begin{tabular}{ll}
  \rowcolor{blue!20}%
  A & B
\end{tabular}

Doesn't work\par
\begin{tabular}{ll}
  % Uncomment line below to get the unexpected behavior
  %\input{mwe-bit.tex}
\end{tabular}
\end{document}

where mwe-bit.tex simply contains

\rowcolor{blue!20}%
A & B

being the same two lines as directly included above. When I run this, the first part works, but the second generates (when uncommented):

! Misplaced \noalign.
\rowcolor ->\noalign 
                     {\ifnum 0=`}\fi \global \let \CT@do@color \CT@@do@color...
l.1 \rowcolor
             {blue!20}

I added the % after the \rowcolor in an abundance of caution, but it doesn't seem to make a difference.

I'm still something of a newbie when it comes to posting here, so apologies for any missteps.

(Update, original post had a pair of \rowcolor instructions for each attempt. I'd been thinking it was something to do with changing an already-specified color, but actually, just having one inside the inputted file is sufficient to generate the error)

Answer 1

If it must be subject to \rowcolor, you need to treat the first tabular line out of your \input file (or do something else like putting also the tabular preamble in the \input file). This is because \rowcolor expands to tokens, one of which is \noalign, and \noalign can only happen at very specific places in a TeX alignment such as tabular. See here for a detailed explanation about this (this is also explained below).

The following works:

main.tex:

\documentclass{article}
\usepackage{xcolor}
\usepackage{colortbl}
\begin{document}
Works\par
\begin{tabular}{ll}
  \rowcolor{blue!20}%
  A & B
\end{tabular}

Also works\par
\begin{tabular}{ll}
  \rowcolor{green!20}% first line must be colored out of the \input file
  foobar
  \input{mwe-bit.tex}
\end{tabular}
\end{document}

mwe-bit.tex:

\\
\rowcolor{blue!20}%
A & B

In your case, you get the dreaded “Misplaced \noalign” error because \input executes \@ifnextchar, which executes \let (at least, this is one reason why this error must happen; there might be others quite similar, e.g., from some command other than \@ifnextchar). \let being a non-expandable, non-space token, TeX stops his expansion of tokens looking for \noalign or \omit at the beginning of the tabular or after the end of a tabular line (after the \cr or \crcr command used internally by tabular). Because of this, it doesn't expand \rowcolor at this point, and when it eventually does, it is already processing the first tabular entry contents, where \noalign is invalid. \noalign is only valid during the special expansion step looking for \noalign or \omit at the start of a tabular¹ or after a \cr or \crcr, special step which stops at the first non-expandable, non-space token.

My example works because:

1. The first \rowcolor at the very beginning of the tabular gets expanded during the “special step” I just described, and the first non-expandable, non-space token is \noalign (from \rowcolor's definition), which allows \rowcolor to work as designed.

2. Other \rowcolor commands from mwe-bit.tex occur right after \\, and if we look at the definition of \\ in tabular, it is probably not very different from \crcr (I guess there is an \unskip before, but “before” contents can't cause any problem: what counts is what there is between \cr or \crcr and \noalign material). When performing the “special expansion step” started by \\, TeX finds no intervening non-expandable, non-space token before arriving at the \noalign from \rowcolor; therefore this one can also work as designed (i.e., put the color \special material where it belongs).

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Footnote

1. More generally, a TeX alignement such as \halign or \valign.