LaTeX gives error when looping in tabular environment

Question

Package used: pgffor
Function used from package: foreach
Compiled with: latex

the minimised document:

\documentclass[a4paper]{article}
\usepackage{pgffor}
\begin{document}
\def\colourNames{red, green, blue}
\begin{tabular}{cc}
  \foreach \n in \colourNames {
    \foreach [expand list=true] \m in {\colourNames} { \n & \m \ }
  }
\end{tabular}
\end{document}

the error I receive:

! Extra }, or forgotten \endgroup.
<template> \unskip \hfil }
                          \hskip \tabcolsep \endtemplate 
l.11   }
? 
! Emergency stop.
<template> \unskip \hfil }
                          \hskip \tabcolsep \endtemplate 
l.11   }

adding \endgroup does not help. deleting the "offending" } would not help as it does close the brace on two lines before.

If I replace the \tabular environment with an \itemize environment (and do the necessary changes to fit the \item syntax, the code is OK.

[...]
\begin{itemize}
  \foreach \n in \colourNames {
    \foreach [expand list=true] \m in {\colourNames} { \item \n, \m }
  }
\end{itemize}
[...]

Most probably, the special tabular characters & and/or // cause the problem. But if so, how can they be escaped so that the compilation works? Is there any good documentation that shows the most elegant way to solve the problem, with in depth explanation of options and their pros and cons?

Note: The "update" section below has a summary of and additional info to the comments and answers I have received. I am still actively processing the information and leads I have received. When finished, I will clear up, complete, and move the "update" section into an answer as recommended by David Carlisle.

update 0: what I found useful so far:

David Carlisle's comment helped understand what caused the error.
both Qrrbrbirlbel and pascal974 point at a solution in LaTeX3 (reference to tutorial will come especially useful)
Steven B. Segletes' answer seems to offer a simpler solution (using TeX tokens), but even for that, I need some time and research to understand.
all in all: both solutions suggested so far seem good and need further study on my part, so I do not yet know which I will select in solving the original problem.

update 1: A good series to help get familiar with TeX tokens:

Some further reference(s?) I found useful while reading the series above:

for token expansion: The puzzling thing about \newtoks and the token list

update 2: The third answer (using OpTeX) is the simplest but it leads into a parallel universe (from LaTeX viewpoint). It is extremely useful for those who are just starting off and are not planning using LaTeX features. OpTeX Package documentation is excellent.

update 3: as it turns out, the LaTeX3 solution was the easier and quickest to expand to address the original problem, from which the minimum example was extracted. It accepts an additional user-defined function easier into the table. So, this is why the green tick goes there.

update 4: The opportunity of adding a new answer has been blocked unfortunately. The linked etoolbox solution is plagued by the same problem as the TeX token solution. Adding a user defined command inside can kill it. Therefore, it is still the LaTeX3 option that seems to be the most bulletproof solution. (Regardless, for reference: etoolbox Package Documentation)

a tabular cell is a group so & acts like }{ ending one group and starting another, \foreach has to be in a single group — David Carlisle, Jan 09 '24 at 22:34
PGFFor isn't made for this. There are ways to solve this with PGFFor (prepare your tabular content beforehand and store it in a macro and then only use that macro inside the environment but that can raise expansion issues. You're better off using another loop mechanism, say \int_step_inline:nnnn from Latex3. — Qrrbrbirlbel, Jan 10 '24 at 13:54
please don't edit answer comments in to the question (you could if really needed, post a summary answer) also don't just refer to "the third answer" as the order answers are displayed changes over time depending on vote scores — David Carlisle, Jan 13 '24 at 20:53

score 2 · Accepted Answer · answered Jan 12 '24 at 12:24

With expl3, the documentation: interface3 usrguide and a tutorial https://www.alanshawn.com/latex3-tutorial/

\documentclass[a4paper]{article}
%https://tex.stackexchange.com/questions/706767/latex-gives-error-when-looping-in-tabular-environment
\ExplSyntaxOn
%&lt;scope><module><description>_<type>
\seq_new:N \l_Tibor_listcolor_seq
\tl_new:N \l_Tibor_rowtab_tl
\NewDocumentCommand{\mytab}{ m }
  {
    \tl_clear:N \l_Tibor_rowtab_tl
    \seq_set_from_clist:NN \l_Tibor_listcolor_seq #1
    \seq_map_inline:Nn \l_Tibor_listcolor_seq

      {
        \seq_map_inline:Nn \l_Tibor_listcolor_seq 
          {
            \tl_put_right:Nn \l_Tibor_rowtab_tl {##1 & ####1\}
          }
      }
      \l_Tibor_rowtab_tl 
  }
\ExplSyntaxOff
\begin{document}
\def\colourNames{red, green, blue}
\begin{tabular}{| c | c |}
  \mytab{\colourNames}
\end{tabular}
\end{document}

score 2 · Answer 2 · answered Jan 12 '24 at 15:10

Use the \foreach loop to collect the tokens; then, regurgitate them all at once inside the tabular.

\documentclass[a4paper]{article}
\usepackage{pgffor}
\newtoks\mytoks
\newcommand\addtomytoks[1]{\global\mytoks\expandafter{\the\mytoks#1}}
\newcommand\xaddtomytoks[1]{\expandafter\addtomytoks\expandafter{#1}}
\begin{document}
\def\colourNames{red, green, blue}
\mytoks{}%
\foreach \n in \colourNames {%
  \foreach [expand list=true] \m in {\colourNames} { %
  \xaddtomytoks{\n &}\xaddtomytoks{\m \} }%
}%
\begin{tabular}{cc}
  \the\mytoks
\end{tabular}
\end{document}

score 2 · Answer 3 · answered Jan 13 '24 at 11:55

2

When we are using OpTeX then we have no problems with \foreach inside \table:

\def\colourNames{red, green, blue}
\table{cc}{
   \ea\foreach \colourNames, \do#1, {%
      \ea\foreach \colourNames, \do##1, {#1 & ##1 \cr}
   }
}
\bye

answered Jan 13 '24 at 11:55

wipet

74,238

LaTeX gives error when looping in tabular environment

3 Answers3