What is the correct way to calculate \multicolumn size when using tabularx?

Question

What is the correct way to determine the correct size for use with p{} needed by \multicolumn to get the size correct?

The problem is this: I want to use tabularx and also use \multicolumn. To do that, I have to give \multicolumn an exact size to use. And this is the problem. In complicated table, where I can have one table inside another, and a row that extends to few columns, the calculations are getting hard.

It become harder to figure the correct size, and if I change the table later, I have to change the code again.

The first example below shows that X can't be used with \multicolumn to make it extend to more than one column:

\documentclass[10pt,notitlepage]{article}%
\usepackage{tabularx}
\usepackage{array}
\newcolumntype{Y}{>{\centering\arraybackslash}X}
\usepackage{lipsum}
\setlength{\parindent}{0pt}
\begin{document}

\begin{tabularx}{\textwidth}{@{}|l|Y|@{}}\hline
 column 1 &
 {\begin{tabularx}{\linewidth}{@{}Y|Y|Y@{}}\hline
      \lipsum[75] & \lipsum[75] &  \lipsum[75] \\\hline
      \multicolumn{3}{X}{\lipsum[75]}
  \end{tabularx}
 }
 \\\hline
\end{tabularx}
\end{document}

So have to use p{}. Nothing else would let it extend to the other columns.

But p wants a size. after some trial and error, here is one that gets close to the size needed

\documentclass[10pt,notitlepage]{article}%
\usepackage{tabularx}
\usepackage{array}
\newcolumntype{Y}{>{\centering\arraybackslash}X}
\usepackage{lipsum}
\setlength{\parindent}{0pt}
\begin{document}

\begin{tabularx}{\textwidth}{@{}|l|Y|@{}}\hline
 column 1 &
 {\begin{tabularx}{\linewidth}{@{}Y|Y|Y@{}}\hline
       \lipsum[75] &  \lipsum[75] &   \lipsum[75] \\\hline
      \multicolumn{3}
        %this calculation below is needed, and not sure if it is correct now
        {p{.9\dimexpr \textwidth-4\arrayrulewidth-8\tabcolsep\relax}}{\lipsum[75]}
  \end{tabularx}
 }
 \\\hline
\end{tabularx}
\end{document}

Ok, it is better now. But if I change something later, I have to go change .9 to something else I am sure.

The question is: Is this really how one is supposed to do this in Latex? It seems that Latex should have a way to do this automatically. Is there a better approach I should be looking at instead of what I am doing above?

The above is a small example. I want to use tabularx and have other tabularx tables inside as well. So this can get complicated very quickly.

Answer 1

You don't need multicolumn here, your text is just a paragraph the full width of the outer table column. (Incidentally as I've commented before I wouldn't use tabularx for the inner column: in a table where every column is an X column, you know in advance the column widths, so there is no need to make tabularx work them out (although it works).

\documentclass[10pt,notitlepage]{article}%
\usepackage{tabularx}
\usepackage{array}
\newcolumntype{Y}{>{\centering\arraybackslash}X}
\usepackage{lipsum}
\setlength{\parindent}{0pt}
\begin{document}

\begin{tabularx}{\textwidth}{@{}|l|Y|@{}}\hline
 column 1 &
 {\begin{tabularx}{\linewidth}{@{}Y|Y|Y@{}}\hline
       \lipsum[75] &  \lipsum[75] &   \lipsum[75] \\\hline
  \end{tabularx}
 }

\lipsum[75]
 \\\hline
\end{tabularx}
\end{document}

Answer 2

This is a way to achieve the objective. The default size of each single column is \hsize. To change it, use >{\setlength\hsize{1\hsize}}. As to the 3-column width, one needs to set it to 3 times the basic \hsize because it takes up 3 columns.

Code:

\documentclass[10pt,notitlepage]{article}%
\usepackage{tabularx}
\usepackage{array}
\newcolumntype{Y}{>{\centering\arraybackslash}X}
\usepackage{lipsum}
\setlength{\parindent}{0pt}
\begin{document}

\begin{tabularx}{\textwidth}{@{}|l|Y|@{}}\hline
 column 1 &{
\begin{tabularx}{\linewidth}{Y|Y|Y}%
\lipsum[75] &\lipsum[75] & \lipsum[75] \\ \hline
\multicolumn{3}{>{\centering\setlength\hsize{3\hsize}}Y}{\lipsum[75]}
\end{tabularx}}\\ \hline
\end{tabularx}
\end{document}