Filling a table with list data: ! Extra }, or forgotten \endgroup

Question

I'm trying to fill a table by iterating through predefined data and experience errors with both & (next column) and \\ (next row) using code like follows:

\begin{document}
%   \foreach \s in \schedule {
%       \textbf{\GetListMember{\s}{1}:} \GetListMember{\s}{2} \\
%   }
    \begin{tabular}{ll}
        \foreach \s in \schedule {
            \textbf{\GetListMember{\s}{1}} & \GetListMember{\s}{2} \\
        }
    \end{tabular}
\end{document}

The complete example:

\documentclass[12pt, a4paper]{article}%
\usepackage[T1]{fontenc}%
\usepackage[utf8]{inputenc}%
\usepackage{pgffor}%
\usepackage{xstring}%
%
%\tracingall%
%
\setlength{\parindent}{0in}%
%
\newcommand*{\GetListMember}[2]{%
    \edef\dotheloop{%
        \noexpand\foreach \noexpand\a [count=\noexpand\i] in {#1} {%
            \noexpand\IfEq{\noexpand\i}{#2}{\noexpand\a\noexpand\breakforeach}{}%
    }}%
    \dotheloop%
}%
%
\edef\schedule {%
    %{{Title},{Description}},%
    {{Ipsum},{Pellentesque mauris mauris, feugiat ut.}},%
    {{Lorem},{Integer viverra blandit magna, in imperdiet sapien sollicitudin at.}},%
    {{Presen},{Nunc eu euismod nunc. Sed tincidunt, sem eu facilisis rhoncus.}},%
    {{Magna},{Phasellus velit dui, tristique at sem et, finibus volutpat sapien.}},%
    {{Vide},{Fusce in sapien mi. Vestibulum sed venenatis risus. Ut turpis dui.}},%
    {{Albi},{Proin dolor mauris, pellentes sem convallis. Suspendisse elementum.}},%
    {{Cosco},{Integer in molestie diam. Donec sapien, porta lacinia gravida ac.}}%
}%
\begin{document}
%   \foreach \s in \schedule {
%       \textbf{\GetListMember{\s}{1}:} \GetListMember{\s}{2} \\
%   }
    \begin{tabular}{ll}
        \foreach \s in \schedule {
            \textbf{\GetListMember{\s}{1}} & \GetListMember{\s}{2} \\
        }
    \end{tabular}
\end{document}

pdflatex states:

! Extra }, or forgotten \endgroup.
<template> \unskip \hfil }
                          \hskip \tabcolsep \endtemplate 
l.36        }

Unfortunately, tracing the build with \tracingall runs endlessly and I'm not familiar with macros. The build succeeds with documents like:

\begin{document}
%   \foreach \s in \schedule {
%       \textbf{\GetListMember{\s}{1}:} \GetListMember{\s}{2} \\
%   }
    \begin{tabular}{l}
        \foreach \s in \schedule {
            \textbf{\GetListMember{\s}{1}} \GetListMember{\s}{2}
        }
    \end{tabular}
\end{document}

and:

\begin{document}
    \foreach \s in \schedule {
        \textbf{\GetListMember{\s}{1}:} \GetListMember{\s}{2} \\
    }
%   \begin{tabular}{ll}
%       \foreach \s in \schedule {
%           \textbf{\GetListMember{\s}{1}} & \GetListMember{\s}{2} \\
%       }
%   \end{tabular}
\end{document}

You can find the discussion about GetListMember here: Macro to access a specific member of a list

I'm happy with every solution that enables building a table by iterating and accessing two dimensional data.

Answer 1

\foreach cannot straddle table cells.

There is a simpler way to do the task, with expl3.

I define a \listloop command that takes as argument a list name and a template for what to do to each list item.

In this case, each item in the list consists of a pair of braced items, so we can define \makerow to do \textbf{#1} & #2 \\ and the template will be

\makerow#1

so that we pass exactly the two arguments to \makerow. In the template #1 stands for the current list item being processed.

Another similar example is to build a description list.

The trick is that \clist_map_function:cN delivers the result altogether; the job is done in the first cell (in the tabular case, but then the & and \\ will build the cells. A scratch function \__walfi_list_loop:n function is redefined at each call.

\documentclass[12pt, a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand{\newlist}{mm}
 {
  \clist_clear_new:c { l_walfi_list_#1_clist }
  \clist_set:cn  { l_walfi_list_#1_clist } { #2 }
 }

\NewDocumentCommand{\looplist}{mm}
 {
  \cs_gset:Nn \__walfi_list_loop:n { #2 }
  \clist_map_function:cN  { l_walfi_list_#1_clist } \__walfi_list_loop:n
 }

\ExplSyntaxOff

\newlist{schedule}{
  %{Title}{Description},
  {Ipsum}{Pellentesque mauris mauris, feugiat ut.},
  {Lorem}{Integer viverra blandit magna, in imperdiet sapien sollicitudin at.},
  {Presen}{Nunc eu euismod nunc. Sed tincidunt, sem eu facilisis rhoncus.},
  {Magna}{Phasellus velit dui, tristique at sem et, finibus volutpat sapien.},
  {Vide}{Fusce in sapien mi. Vestibulum sed venenatis risus. Ut turpis dui.},
  {Albi}{Proin dolor mauris, pellentes sem convallis. Suspendisse elementum.},
  {Cosco}{Integer in molestie diam. Donec sapien, porta lacinia gravida ac.}
}

\newcommand{\makerow}[2]{\textbf{#1} & #2 \\}

\newcommand{\makeitem}[2]{\item[#1] #2}

\begin{document}

\begin{tabular}{lp{8cm}}
  \looplist{schedule}{\makerow#1}
\end{tabular}

\begin{description}
\looplist{schedule}{\makeitem#1}
\end{description}

\end{document}