Trouble with expansion

Question

I am trying to create an easier way to write math-formulas in tex for multiple assignments. At the current time, I am almost there. This example might not seem very intuitive, but I had to leave out a great deal to make it small enough.

What I am trying to accomplish in this example, is simply some string replacement. For every occurrence of string defined in \allvars, replace them with whatever \varprint{string for \allvars} outputs. However, I have messed up the expansion here, and have tried a few other ways, but I just don't quite understand really. The string-replacement works as expected, but I need StrSubstitute to rerun with its own output on the next index in allvars. Any help on this is greatly appreciated.

\documentclass[11pt]{article}
\usepackage{xinttools, xstring}
\newcommand{\allvars}{sideA,sideB,sideC}
\newcommand{\varPrint}[1]{%
great(#1)
} % just a command used in this example. in real document, a more advanced output is to be expected from this.
\newcommand{\matte}[1]{%
    \edef\mattetempB{#1}
        \xintFor ##1 in {\allvars}\do%
     {\def\mattetempB{%
        \StrSubstitute{\mattetempB}{##1}%
        {\varPrint{##1}}%
        }%
     }%
     \mattetemp
    }%


\begin{document}
\matte{sideA+sideB=sideC},\\

Would like to see something like the following:\\

great(sideA)+great(sideB)=great(sideC)\\

But only sees:\\

sideA+sideB=great(sideC) ,

\end{document}

Answer 1

The macros in the xstring package output text directly, which can't be used to feed into other macros. To return text that can be used for further manipulation you can use the trailing optional parameter which will save the output in the macro specified there. So instead of something like

\newcommand{\MySubst}{\StrSubstitute{abracadabra}{a}{o}}

you'll first need to save the output and do your definition afterwards:

\StrSubstitute{abracadabra}{a}{o}[\MyOutput]
\newcommand{\MySubst}{\MyOutput}

Answer 2

Here's a more generic version, based on l3regex.

\documentclass{article}
\usepackage{xparse,l3regex,siunitx}

\ExplSyntaxOn

\prop_new:N \g_runart_variables_prop
\tl_new:N \l__runart_variables_matte_tl
\tl_new:N \l__runart_variables_item_tl

\NewDocumentCommand{\definevariable}{mm}
 { % #1 is the name, #2 is the formatting
  \prop_gput:Nnn \g_runart_variables_prop { #1 } { #2 }
 }

\NewDocumentCommand{\removevariable}{m}
 { % #1 is the name
  \prop_gremove:Nn \g_runart_variables_prop { #1 }
 }

\NewDocumentCommand{\matte}{m}
 { % #1 is the expression to output
  \tl_set:Nn \l__runart_variables_matte_tl { #1 }
  \prop_map_inline:Nn \g_runart_variables_prop
   {
    \tl_set:Nn \l__runart_variables_item_tl { ##2 }
    \regex_replace_all:nnN
     { ##1 }
     { \u{l__runart_variables_item_tl} }
     \l__runart_variables_matte_tl
   }
  \tl_use:N \l__runart_variables_matte_tl
 }

\cs_new_protected:Nn \runart_variable_use:n
 {
  \prop_item:Nn \g_runart_variables_prop { #1 }
 }
\ExplSyntaxOff

\definevariable{a_car}{\SI{20}{\metre\per\second}}
\definevariable{v_car}{\SI{40}{\metre}}
\definevariable{t_car}{\SI{2}{\second}}

\begin{document}

\[
\matte{
  a_car=\frac{v_car}{t_car}
}
\]

\end{document}