Notation and output in siunitx

Question

Consider the following example:

\documentclass{article}

\usepackage{amsmath}
\usepackage[locale = DE]{siunitx}

\newcommand*\something[3]{
  \SIlist[
    list-separator = #1,
    list-final-separator = #1,
    list-units = brackets
  ]{#2}{#3}
}

\begin{document}

\begin{align*}
  l
  &= \SI{38.0}{\cm} + 2 \cdot \SI{26.2}{\cm} + \SI{32.6}{\cm}\\
  &= \something{+}{38; 52.4; 32.6}{\cm}\\
  &= \SI{123.0}{\cm}.
\end{align*}

\end{document}

The \something command works just fine when there is only either + or - but can I create a similar command when both plus and minus are used?

Answer 1

This might be what you want:

\documentclass{article}

\usepackage{amsmath}
\usepackage[locale = DE]{siunitx}

\ExplSyntaxOn
\NewDocumentCommand\SIexpr{ m m }
 {
  \svend_siexpr:nn { #1 } { #2 }
 }

\cs_new_protected:Npn \svend_siexpr:nn #1 #2
 {
  \seq_set_split:Nnn \l_svend_siexpr_input_seq { ; } { #1 }
  \seq_pop_left:NN \l_svend_siexpr_input_seq \l_svend_siexpr_first_tl
  \seq_clear:N \l_svend_siexpr_output_seq
  \seq_put_right:Nx \l_svend_siexpr_output_seq
   {
    \fp_compare:nTF { \l_svend_siexpr_first_tl > 0 }
      { \num { \l_svend_siexpr_first_tl } }
      { - \num { \tl_tail:V \l_svend_siexpr_first_tl } }
   }
  \seq_map_inline:Nn \l_svend_siexpr_input_seq
   {
    \seq_put_right:Nx \l_svend_siexpr_output_seq
     {
      \fp_compare:nTF { ##1 > 0 }
        { + \num { ##1 } }
        { - \num { \tl_tail:n { ##1 } } }
     }
   }

  \SI[parse-numbers=false]
   {
    \sisetup{parse-numbers}
    ( \seq_use:Nn \l_svend_siexpr_output_seq { } )
   }
   { #2 }
}
\ExplSyntaxOff

\begin{document}

\begin{align*}
  l
  &= \SI{38.0}{\cm} + 2 \cdot \SI{26.2}{\cm} + \SI{32.6}{\cm}\\
  &= \SIexpr{38.0; 52.4; 32.6}{\cm}\\
  &= \SI{123.0}{\cm}.\\
  l
  &= \SI{38.0}{\cm} - 2 \cdot \SI{26.2}{\cm} + \SI{32.6}{\cm}\\
  &= \SIexpr{38.0; -52.4; 32.6}{\cm}\\
  &= \SI{18.2}{\cm}.\\
  l
  &= \SI{-38.0}{\cm} + 2 \cdot \SI{26.2}{\cm} + \SI{32.6}{\cm}\\
  &= \SIexpr{-38.0; 52.4; 32.6}{\cm}\\
  &= \SI{47.0}{\cm}.
\end{align*}

\end{document}

The argument to \SIexpr is split into components.

• The first component is detached to be treated separately.

• Each component is examined for seeing if it is positive or negative: if negative we add -\num{<absolute value>} else +\num{<absolute value>}.

• However, a leading + is not added by doing a special examination to the first component.

To have the separator in the first mandatory argument to \SIexpr selectable with an optional argument, change the main code into

\ExplSyntaxOn
\NewDocumentCommand\SIexpr{ O{;} m m }
 {
  \svend_siexpr:nnn { #1 } { #2 } { #3 }
 }

\cs_new_protected:Npn \svend_siexpr:nnn #1 #2 #3
 {
  \seq_set_split:Nnn \l_svend_siexpr_input_seq { #1 } { #2 }
  \seq_pop_left:NN \l_svend_siexpr_input_seq \l_svend_siexpr_first_tl
  \seq_clear:N \l_svend_siexpr_output_seq
  \seq_put_right:Nx \l_svend_siexpr_output_seq
   {
    \fp_compare:nTF { \l_svend_siexpr_first_tl > 0 }
      { \num { \l_svend_siexpr_first_tl } }
      { - \num { \tl_tail:V \l_svend_siexpr_first_tl } }
   }
  \seq_map_inline:Nn \l_svend_siexpr_input_seq
   {
    \seq_put_right:Nx \l_svend_siexpr_output_seq
     {
      \fp_compare:nTF { ##1 > 0 }
        { + \num { ##1 } }
        { - \num { \tl_tail:n { ##1 } } }
     }
   }

  \SI[parse-numbers=false]
   {
    \sisetup{parse-numbers}
    ( \seq_use:Nn \l_svend_siexpr_output_seq { } )
   }
   { #3 }
}
\ExplSyntaxOff

Now you can do

\SIexpr{38.0; 52.4; 32.6}{\cm}

or

\SIexpr[,]{38.0, 52.4, 32.6}{\cm}

or even

\SIexpr[?]{38.0 ? 52.4 ? 32.6}{\cm}

If you prefer a comma by default, instead of a semicolon, you should have

\NewDocumentCommand\SIexpr{ O{,} m m }

The default separator can't be a colon, but you can select a colon with the optional argument.