Clash somewhere among \fpeval, \or, and \CountToken

Question

(This is my first question here, sorry for any "side effects" of it...)

I'd like to implement that, depending on the value of \MyR (a number between 0 and 1, 1 excluded), the first/second/... item of a set would be chosen. In the MWE below, a set is realized in the format {item1\or item2\or item3 ...}. If, for example, a set has two elements: {item1\or item2} then, for values of \MyR below 0.5, item1 would be returned, and for values above 0.5, item2 is to be returned. From a set {item1\or item2\or item3}, 0.33 would yield item1 while 0.34 would lead to item2. Below I use \ifcase, and compute which of its branches should be returned.

Version (*) (see below, implementation via \newcommandFromSet) seems to work. Hovewer, the variants (**) and (***) (two attempts for \FromSet) fail when we uncomment the last (but one) line %\fpeval.... The success of (*) seems to justify the basic concept, but how could I cure the definition of \FromSet? Thank you!

(Link to @egreg's \CountToken.)

[A format {item1\SomeShortToken item2\SomeShortToken item3 ...} would be easy to type, and calculating the number of items would also be for user convenience.]

\documentclass{article}  %% MWE
\usepackage{xfp}
\def\finish{\finish}                    %% Frequency of token #1 (stored
\def\CountToken[#1,#2]{%                %% in counter #2) in what's
  \def\TestedToken{#1}%                 %% between \CountToken[#1,#2]
  \ifdefined#2#2=0 \else\newcount#2\fi  %% and \finish. Code copied from
  \let\TotalOccurrence#2%               %% https://tex.stackexchange.com
  \let\next\TestNext\next               %% questions/525556/token-
}                                       %% counter-strange-behaviour-
\long\def\TestNext#1{%                  %% of-math-mode (egreg's version)
  \def\CurrentToken{#1}%                %%
  \ifx\CurrentToken\finish              %%
    \let\next\relax                     %%
  \else                                 %%
    \ifx\CurrentToken\TestedToken       %%
      \advance\TotalOccurrence by 1     %%
    \fi                                 %%
  \fi                                   %%
  \next                                 %%
}                                       %%
\newcommand{\MyR}{0.67}  %% my "global variable".
\newcommand{\newcommandFromSet}[2]{\CountToken[\or,\cnti]#2\finish  %% ()
  \xdef#1{\ifcase\fpeval{floor(\MyR(1+\cnti))} #2\fi}}
\newcommand{\FromSet}[1]{\CountToken[\or,\cnti]#1\finish  %% ()
          \ifcase\fpeval{floor(\MyR*(1+\cnti))} #1\fi}
%\renewcommand{\FromSet}[1]{\newcommandFromSet{\MyTemp}{#1}\MyTemp}  %% (*)
\begin{document}
\newcommandFromSet{\MyText}{first\or second\or third}
\newcommandFromSet{\MyNumber}{11\or 12\or 13}
The \MyText\ number is \MyNumber, I repeat,
\fpeval{1*\MyNumber}.  %% everything seems to be good.
The \FromSet{first\or second\or third} number is  %% this line seems to work.
\FromSet{11\or 12\or 13}, I repeat,  %% this line also seems to work.
%\fpeval{1*\FromSet{\or 11\or 12\or 13}}.  %% ! Extra \or. \TestedToken ->\or
\end{document}

Answer 1

You can't use \or in that context, I'm afraid. But the code can be made much simpler.

I'd prefer comma separated items, rather than separated by \or, so the \FromSet macro is fully expandable.

With \clist_count:n we can access expandably the number of items, so we can scale the variable by it; then truncate and add 1.

\documentclass{article}
\usepackage{xfp}
\ExplSyntaxOn
\NewDocumentCommand{\newcommandFromSet}{O{\MyR}mm}
 {
  \cs_new:Npx #2
   {
    \clist_item:nn { #3 }
     {
      \fp_eval:n { trunc(#1\clist_count:n { #3 },0) + 1 }
     }
   }
 }
\NewExpandableDocumentCommand{\FromSet}{O{\MyR}m}
 {
  \clist_item:nn { #2 }
   {
    \fp_eval:n { trunc(#1\clist_count:n { #2 },0) + 1 } }
 }
\ExplSyntaxOff
\newcommand{\MyR}{0.67}  %% my "global variable".
\newcommand{\MyS}{0.1}  %% another "global variable".
\begin{document}
\newcommandFromSet{\MyText}{first,second,third}
\newcommandFromSet{\MyNumber}{,11,12,13}
The \MyText\ number is \MyNumber, I repeat,
\fpeval{1*\MyNumber}.
The \FromSet{first,second,third} number is
\FromSet{11,12,13}, I repeat,
\fpeval{1*\FromSet{11,12,13}}.
\newcommandFromSet[\MyS]{\MyTextA}{first,second,third}
\newcommandFromSet[\MyS]{\MyNumberA}{11,12,13}
The \MyTextA\ number is \MyNumberA, I repeat,
\fpeval{1*\MyNumberA}.
The \FromSet[\MyS]{first,second,third} number is
\FromSet[\MyS]{11,12,13}, I repeat,
\fpeval{1*\FromSet[\MyS]{11,12,13}}.
\end{document}

Note that you can use different variables; the default is \MyR.