macro value can not be updated in tabular with multirow columns

Question

I am writing a command \newrow to generate one row in a longtable. The problem is that the tabular environment here can not see the value of \subColWid (output is 0pt). This occurs only when multirow is also used. Can anyone help me to figure out why the updated \subColWid by \setlength is invisible in the tabular (including column specification). Thanks.

\documentclass{article}

\usepackage{fullpage}
\usepackage{longtable}
\usepackage{array}
\usepackage{multirow}

% define my macros
\newlength{\firstColWid}
\newlength{\secondColWid}
\newlength{\subColWid}
\setlength\firstColWid{0.1\textwidth}
\setlength\secondColWid{0.9\textwidth}
%\setlength{\subColWid}{0.5\secondColWid} % uncomment this line to see correct result

\setlength{\tabcolsep}{0pt}

\newcolumntype{M}[1]{>{\centering\arraybackslash}m{#1}}

\newcommand{\newrow}[2]{ % a command to produce a row spanning two lines
    % set width for each cell in the second column
    \setlength\subColWid{0.2\secondColWid} % this has effect here but not in the 
    %tabular
    % \the\subColWid
    \multirow{2}{*}{#1}  & subColWid = \the\subColWid \\ \cline{2-2}
                         & 
    % use a tabular to represent all the remaining values
    \begin{tabular}[c]{ @{\extracolsep{\fill}} *{2}{M{\subColWid} | }}
        % subColWid = \the\subColWid & #2 \\
        #2 & \the\subColWid \\
    \end{tabular}

}


\begin{document}
\begin{longtable}[c]{|M{\firstColWid}|@{\extracolsep{\fill}} M{\secondColWid}|} % 
    \hline
    % give row number and another cell
    \newrow{1}{Row1} \\ \hline
    \newrow{2}{Row2} \\
    \hline

\end{longtable}
\end{document}

Answer 1

You might think to prefix \setlength with \global and expect to get the effect you want. And in this particular case, this seems to work. But not generally. See the update below for more details.

\documentclass{article}

\usepackage{fullpage}
\usepackage{longtable}
\usepackage{array}
\usepackage{multirow}

% define my macros
\newlength{\firstColWid}
\newlength{\secondColWid}
\newlength{\subColWid}
\setlength\firstColWid{0.1\textwidth}
\setlength\secondColWid{0.9\textwidth}
%\setlength{\subColWid}{0.5\secondColWid} % uncomment this line to see correct result

\setlength{\tabcolsep}{0pt}

\newcolumntype{M}[1]{>{\centering\arraybackslash}m{#1}}

\newcommand{\newrow}[2]{ % a command to produce a row spanning two lines
    % set width for each cell in the second column
    \global\setlength\subColWid{0.2\secondColWid} % this has effect here but not in the 
    %tabular
    % \the\subColWid
    \multirow{2}{*}{#1}  & subColWid = \the\subColWid \\ \cline{2-2}
                         & 
    % use a tabular to represent all the remaining values
    \begin{tabular}[c]{ @{\extracolsep{\fill}} *{2}{M{\subColWid} | }}
        % subColWid = \the\subColWid & #2 \\
        #2 & \the\subColWid \\
    \end{tabular}

}


\begin{document}

\begin{longtable}[c]{|M{\firstColWid}|@{\extracolsep{\fill}} M{\secondColWid}|} % 
    \hline
    % give row number and another cell
    \newrow{1}{Row1} \\ \hline
    \newrow{2}{Row2} \\
    \hline

\end{longtable}
\end{document}

Of course, that will effect how \subColWid will behave in the remainder of the document. If you need the old value, you should do something like

\newlength\myoldlength
\setlength\myoldlength{\subColWid}

and once you're done with your table you can restore the old value using

 \setlength\subColWid{\myoldlength}

UPDATE

After reading around a bit here (on account of what @egreg wrote regarding the use of \global and \setlength here [see the end of his discussion to see the relevant comment]), my use of \global\setlength above works only idiosyncratically within LaTeX. A better solution is offered by @AndrewSwann here. So it seems a better approach to getting \subColWid to have the desired value, when changing it from within the table, is to do define your command as follows:

\newcommand{\newrow}[2]{ % a command to produce a row spanning two lines
    % set width for each cell in the second column
    \global\subColWid=\dimexpr0.2\secondColWid\relax % this has effect here but not in the 
    %tabular
    % \the\subColWid
    \multirow{2}{*}{#1}  & subColWid = \the\subColWid \\ \cline{2-2}
                         & 
    % use a tabular to represent all the remaining values
    \begin{tabular}[c]{ @{\extracolsep{\fill}} *{2}{M{\subColWid} | }}
        % subColWid = \the\subColWid & #2 \\
        #2 & \the\subColWid \\
    \end{tabular}
}