Centering a table when I don't know the width

Question

I have a general question. I would like to center tables on the page, when I do not know the width. What is more, I am using tabularx and sometimes the width specified may be too small, meaning that the table is wider than the first argument to tabularx.

Here is some context for my question. I develop an R package which prints out tables in HTML, LaTeX etc. So, users may set the width - perhaps to a value which is too small for the content.

At the moment, a centered table uses \centering, like this (simplified version):

\begin{table}[h]
\centering
\begin{threeparttable}
\begin{tabularx}{0.5\textwidth}{p{0.5\textwidth}}

\multicolumn{1}{l}{Some content here} \tabularnewline[-0.5pt]

\end{tabularx}\end{threeparttable}

\end{table}

But that can lead to tables not being truly centered. Here's an example where the user has specified a too small table width for a wide table. As a result the table is not centered:

\documentclass{article}

\usepackage{threeparttable}
\usepackage{tabularx}
\usepackage{multirow}
\begin{document}
\begin{table}[h]
\centering
\begin{threeparttable}
\begin{tabularx}{0.2\textwidth}{p{0.2\textwidth}}

\multicolumn{1}{l}{Some very long content that goes on and on and on} \tabularnewline[-0.5pt]


\end{tabularx}\end{threeparttable}

\end{table}
\end{document}

Is there a way to make sure that tables stay centered?

Note that the multicolumn command overrides the table width specification. I know about this, and it is probably not optimal. But by default, my cells aren't wrapped. If a user wants they can turn wrapping on, in which case the multicolumn command would have a width spec of p{0.2\textwidth} or whatever. The point is, sometimes users may do things that make tables wider than tabularx thinks they are; is there a way to ensure that nevertheless, they are truly centered?

(Note 2: yes, I also know that in this case, multicolumn is not doing anything. But sometimes there are genuine multicolumn cells; the command allows per-cell left/right/top/bottom-alignment; and writing TeX programmatically from a different language is hard.)

Answer 1

A tabularx has to have at least one X column, a specification such as

\begin{tabularx}{0.2\textwidth}{p{0.2\textwidth}}

specifies the table is to be placed in a box of width .2\textwidth but has no way to make any adjustment to the width as changing the width of X columns is the only mechanism tabularx has. This table is setting particularly impossible constraints as not only is there no X column, the table specification just has a p{0.2\textwidth} column so its natural width is .2\textwidth+2\tabcolsep as there is \tabcolsep either side of the column. In the example the p column is replaced by l as all entries are spanned.

You should just use tabular here:

\documentclass{article}

\usepackage{threeparttable}
\usepackage{tabularx}
\usepackage{multirow}
\begin{document}


\noindent X\dotfill X % just to show the width

\begin{table}[htp]% never use [h] on its own
\centering
\begin{threeparttable}
\begin{tabular}{p{0.2\textwidth}}

\multicolumn{1}{l}{Some very long content that goes on and on and on} \tabularnewline[-0.5pt]


\end{tabular}
\end{threeparttable}

\end{table}
\end{document}