Aligning text in S columns in a way that respects the decimal marker for numerical entries

Question

As I understand it, using various options for the S column in siunitx, I may produce columns containing both numerical and nonnumerical entries. I may have the numerical entries aligned at their decimal points. I may have the text entries either left-, right-, or center- justified with respect to the column's width.

Is it possible to have the numerical entries aligned by their decimal, while controlling the text entries' alignment as it relates to the decimal marker?

To complicate things further, is it possible to apply alignments differently row by row? In particular, could I leave column-centered alignment for the head entry of a column, while making my special decimal marker-based alignment apply to later entries?

The following MWE sort of produces what I want with exaggerated numbers, but I have manually forced some space in the last two entries to make it happen. I want to not have to do this.

\documentclass{article}

\usepackage{siunitx}


\newcommand*\heading[1]{\multicolumn{1}{c}{#1}}

\begin{document}


\begin{tabular}{S[table-format=9.3,table-text-alignment=center]}
\hline
\heading{Title}\\
\hline
1\\
2.000\\
6\\
24.0\\
123456789\\
{\phantom{123\,456\,78}\vdots\phantom{\,000}}\\
{\phantom{123\,456\,7}$f(n)$\phantom{\,00}}
\end{tabular}

\end{document}

Note that I do want a centered title, and that ideally I actually want the remaining text entries centered with respect to a point that is roughly 0.5\ex to the left of the decimal marker. Also, note that since I have such asymmetry between integer an fractional parts, I do not think that table-number-alignment=center-decimal-marker is helpful.

Answer 1

Let me say two things before:

1. This is not an only-siunitx solution. Sadly, table-text-alignment doesn't take decimal-marker :(
2. I provide two macros:
   1. \widestparts{<digits before …>}{<… and after decimal marker>}^†
   2. \tcenter{<text to center at the decimal marker>}

---

^† _{This works now, thanks to Peter Grill's question, Martin Scharrer's answer and Peter's hint in the chat.}

Manual aligning

You can add another \textcenter macro that takes three arguments:

1. stuff before your actual text,
2. stuff after your actual text (in the example below I also removed the decimal marker .), and
3. stuff that gets actually typeset.

For example:

\newcommand*{\textcenter}[3]{{%
    \hphantom{\num{#1}}%
    \makebox[\widthof{.}][c]{#3}\hspace{-\decimalmarkerwidth}%
    \hphantom{\num[add-decimal-zero = false,add-integer-zero = false,scientific-notation = fixed]{#2}}%
}}

Code

\documentclass{article}
\usepackage{xcolor} % only for this example (redefinition of hphantom}
\usepackage{mathtools,calc,siunitx,ifthen}
\usepackage[T1]{fontenc}
% getting the width of the decimal marker: . or ,
\newlength{\decimalmarkerwidth}\settowidth{\decimalmarkerwidth}{.} % or ,

% remove the % in the next line to get gray phantoms
%\renewcommand\hphantom[1]{{\color[gray]{.7}#1}}

\newcommand*\heading[1]{\multicolumn{1}{c}{#1}} % not used anymore ... (but can used (without the multicolumn) for other formatting (bf, color, ...)

\makeatletter
\newcounter{alex@auxctr} % (Internal)

% \widestparts takes two arguments: #1 = number of digits before the decimal marker
                                  % #2 = number of digits after the decimal marker
\newcommand*{\widestparts}[2]{%
    \alex@widestpartauxloop{\alex@widestpartleft}{#1}{9}%
    \alex@widestpartauxloop{\alex@widestpartright}{#2}{0}%
}

% (Internal) \alex@widestpartauxloop loops over #2 and adds #2 times #3 to #1.
\newcommand*{\alex@widestpartauxloop}[3]{%
    \def\alex@WidestPartTemp{}\setcounter{alex@auxctr}{0}%
    \whiledo{\value{alex@auxctr}<#2}{\g@addto@macro\alex@WidestPartTemp{#3}\stepcounter{alex@auxctr}}%
    \xdef#1{\alex@WidestPartTemp}%
}

% \tcenter takes one argument: #1 = actual text that should be typeset
\newcommand*{\tcenter}[1]{{%
    \hphantom{\num{\alex@widestpartleft}}%
    \makebox[\widthof{.}][c]{#1}\hspace{-\decimalmarkerwidth}%
    \hphantom{\num[add-decimal-zero = false,add-integer-zero = false,scientific-notation = fixed]{.\alex@widestpartright}}%
}}
\makeatother

\usepackage{booktabs} % for \top-, \mid- and \bottomrule

\begin{document}
\widestparts{9}{3}
\begin{tabular}{S[table-format=9.3,table-text-alignment=center]}
    \toprule
             \tcenter{Title}            \\ \midrule
                    1                   \\
                  2.000                 \\
                    6                   \\
                   24.0                 \\
                123456789               \\
    \tcenter{$\smash{\vdotswithin{.}}$} \\ % \smash is used to hide the oversized vertical height of \vdots
             \tcenter{$f(n)$}           \\ \bottomrule
\end{tabular}
\end{document}

Output