Store results of a calculation automatically under a new macro name

Question

As I am now accessing values from tables via a macro to make dynamic references in the text (see here) and make calculations with it, I need a way to make the resulting variables under which the values are stored more user friendly. Because of LaTeX' limitation of not allowing numbers or any separator in a macro name (and I have many macros), I use the "\csname\dots\endcsname" approach from the TeX FAQ to create more meaningful names.

This results in the following complication, illustrated in the MWE below: I use siunitx to print numbers via \num or \SI. As these commands accept only numbers, I can't make calculations within those. I use l3fp to make the calculations and store the the result under a (temporary) name. This results in two lines of code for every single variable:

\calc{\Numtest}{500/2}% store result
\DefineRemark{num:test}{\Numtest}% label result

And within the text the result would be called as \num{\Remark{num:test}}. I want to combine the two commands into one that would be something like \newcalc{<varname>}{<calculation>}. The following does not work because \temp does not get overwritten. I would require a new random name for every single instance of \newcalc.

\newcommand{\newcalc}[2]{%
\calc{\temp}{#2}
\DefineRemark{#1}{\temp}
}

What is a way to solve this issue? On another note in the MWE I created two new commands based on \num and \SI that incorporates the \Remark macro. Is it a good idea to create these commands by copying the original definition completely?

\documentclass{scrartcl}

\usepackage{siunitx,xparse,expl3}

% simple calculation command that stores the result
\ExplSyntaxOn
\NewDocumentCommand {\calc} { m m } {
  \tl_set:Nx #1 { \fp_to_tl:n {#2} }
}
\ExplSyntaxOff

% Workaround for non-letters in macro names
\newcommand{\DefineRemark}[2]{%
  \expandafter\newcommand\csname rmk-#1\endcsname{#2}%
}
\newcommand{\Remark}[1]{\csname rmk-#1\endcsname}

% Create \Num and \NUM based on siunitx' \num and \SI to incorporate the above directly
% (so \Num is \num{\remark{.}}). Not sure if it as a good idea to do it that way
\ExplSyntaxOn
\NewDocumentCommand \Num { o m } {
  \leavevmode
  \group_begin:
    \IfNoValueF {#1}
      { \keys_set:nn { siunitx } {#1} }
    \__siunitx_number_output:n {\Remark{#2}}
  \group_end:
}
\NewDocumentCommand \NUM { o m o m } {
  \leavevmode
  \group_begin:
    \IfNoValueTF {#1}
      { \__siunitx_combined:nnnn { } {\Remark{#2}} {#3} {#4} }
      {
        \keys_set:nn { siunitx } {#1}
        \__siunitx_combined:nnnn {#1} {\Remark{#2}} {#3} {#4}
      }
  \group_end:
}
\ExplSyntaxOff

% Test if \Num and \NUM Works
\calc{\Numtest}{500/2}
\DefineRemark{num:test}{\Numtest}

% Trying to combine \calc and \DefineRemark
% This does not work :( Would need to create a new "\temp" each time
\newcommand{\newcalc}[2]{%
\calc{\temp}{#2}
\DefineRemark{#1}{\temp}
}

\newcalc{new:calc1}{250/2}
\newcalc{new:calc2}{125/2}


\begin{document}

Test Num and NUM Macros: \Num{num:test} and \NUM{num:test}{\percent}

\vspace{2ex}

Test the combination of calc and DefineRemark. The two values should be
different: \Num{new:calc1} and \Num{new:calc2}

\end{document}

Answer 1

I suggest quite a different approach:

\documentclass{scrartcl}

\usepackage{siunitx,xparse,expl3}

% simple calculation command that stores the result
\ExplSyntaxOn
\NewDocumentCommand{\calc}{ m m }
 {
  \tl_set:cx { l_jorg_rmk_#1_tl } { \fp_to_tl:n { #2 } }
 }
\NewDocumentCommand{\usekey}{ m }
 {
  \tl_use:c { l_jorg_rmk_#1_tl }
 }
% Create \Num and \NUM based on siunitx' \num and \SI to incorporate the above directly
% (so \Num is \num{\remark{.}}). Not sure if it as a good idea to do it that way
\NewDocumentCommand \Num { o m }
 {
  \leavevmode
  \group_begin:
    \IfNoValueF {#1}
      { \keys_set:nn { siunitx } {#1} }
    \__siunitx_number_output:n {\tl_use:c { l_jorg_rmk_#2_tl } }
  \group_end:
}
\NewDocumentCommand \NUM { o m o m }
 {
  \leavevmode
  \group_begin:
    \IfNoValueTF {#1}
      { \__siunitx_combined:nnnn { } { \tl_use:c { l_jorg_rmk_#2_tl } } {#3} {#4} }
      {
        \keys_set:nn { siunitx } {#1}
        \__siunitx_combined:nnnn {#1} { \tl_use:c { l_jorg_rmk_#2_tl } } {#3} {#4}
      }
  \group_end:
}
\ExplSyntaxOff

% Test if \Num and \NUM Works
\calc{num:test}{500/2}
\calc{new:calc1}{250/2}
\calc{new:calc2}{125/2}


\begin{document}

Test Num and NUM Macros: \Num{num:test} and \NUM{num:test}{\percent}

\vspace{2ex}

Test the combination of calc and DefineRemark. The two values should be
different: \Num{new:calc1} and \Num{new:calc2}

\vspace{2ex}

Here's a key used by itself: \usekey{num:test}

\end{document}

However, using \__siunitx_number_output:n and \__siunitx_combined:nnnn is an improper way to proceed, because these are commands prefixed by __ which makes them "internals", so bound to be changed without notice (thanks Bruno Le Floch for remarking it).

In my opinion siunitx should make available public interfaces to the internal commands, but at the moment there are many risks in using the commands in this way. So a better suggestion is to change the definitions of \Num and \NUM as follows

\NewDocumentCommand{\Num} { o m }
 {
   \num [ #1 ] { \tl_use:c { l_jorg_rmk_#2_tl } }
 }

\NewDocumentCommand{\NUM} { o m o m }
 {
  \SI [ #1 ] { \tl_use:c { l_jorg_rmk_#2_tl } } [ #3 ] { #4 }
 }

which is even simpler. The calls of \SI and \num should take into account by themselves the \IfNoValue... business.