Using lengthconvert with siunitx S column

Question

I need to align numbers in a table. The numbers are generated from the \Convert command from the lengthconvert package. I want to use an S column from the siunitx package to align the numbers, but this doesn't work. Following is a MWE.

The first error reported is Argument of \tl_clear:N has an extra }. I suspect that \Convert is not fully expanded when read by the S macro. How do I fix this?

\documentclass{minimal}
\usepackage{siunitx}
\usepackage{lengthconvert}

\begin{document}
\begin{tabular}{S}
    \Convert[number-only, unit=cm]{100pt} \\
    3.14 \\
    2.71828
\end{tabular}

\end{document}

Answer 1

The \Convert command is not fully expandable, despite being defined with \DeclareExpandableDocumentCommand (which is an error in the package).

There's a simpler way using a built-in function in expl3:

\documentclass{article}
\usepackage{siunitx}
% \usepackage{xparse} % already loaded by siunitx

\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\convertdim}{mm}
 {
  \dim_to_decimal_in_unit:nn { #2 } { 1#1 }
 }
\ExplSyntaxOff

\begin{document}
\begin{tabular}{S[table-format=1.5,group-four-digits]}
    \convertdim{cm}{100pt} \\
    3.14 \\
    2.71828
\end{tabular}

\end{document}

A fancier version where you can state the number of decimal digits (with rounding):

\documentclass{article}
\usepackage{siunitx}

\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\convertdim}{O{2}mm}
 {
  \fp_eval:n { round ( \dim_to_decimal_in_unit:nn { #3 } { 1#2 } , #1 ) }
 }
\ExplSyntaxOff

\begin{document}
\begin{tabular}{S[table-format=1.5,group-four-digits]}
    \convertdim{cm}{100pt} \\
    \convertdim[0]{cm}{100pt} \\
    \convertdim[1]{cm}{100pt} \\
    \convertdim[2]{cm}{100pt} \\
    \convertdim[3]{cm}{100pt} \\
    \convertdim[4]{cm}{100pt} \\
    \convertdim[5]{cm}{100pt} \\
    3.14 \\
    2.71828
\end{tabular}

\end{document}

Note that \dim_to_decimal_in_unit:nn returns at most five decimal digits (but the last digit is not necessarily accurate).