How to use xstring IfEq inside command?

Question

I wrote a command for accessing a hardcoded string ("For Reasons™", because xstring StrMid is incredibly slow on large strings) and wonder now

• Why can the results be printed but not reused?
• How would I write this command so that it can be further used?

This snippet runs as expected and generates the following pdf.

  debug for part two
  \newcounter{debugctr}
  \setcounter{debugctr}{-1}
    {\loop
    \stepcounter{debugctr}
    \edef\tmpinput{\the\numexpr \value{debugctr} - 0}
    \def\tmpchar{\expandafter\lucidcharat\tmpinput}
    (\tmpinput)[\tmpchar],
    %\edef\tmpcharr{\tmpchar}
    \show\tmpchar
    %\typeout{tmpchar is \tmpchar}
    \ifnum \value{debugctr}<3
    \repeat }

The command lucidcharat is defined as follows:

\newcommand{\lucidcharat}[1]{
\edef\tmpp{#1}
\IfEq{\tmpp}{0}{1}{}%
\IfEq{\tmpp}{1}{2}{}%
\IfEq{\tmpp}{2}{2}{}%
}

and is input to the other file with \input{sneakycode}.

I assume this all has to do with how fickle xstring is with regard to evaluation. To demonstrate what I mean with "results can not be further used" uncomment one of the commented lines above and you'll see that neither typeout nor edef work. They'll error out with

!  Undefined control sequence.
\reserved@a ->\def \xs@dessep
{,}\@xs@readdecimalpart
l.617 \repeat

Please help me understand how a command using IfEq internally would be written so that it can be reused e.g. for comparison of the output and why it is not working currently.

Here's the complete MCVE for copy-pasting again (It's the same as above, just put together into one file):

\documentclass{article} \usepackage[utf8]{inputenc}
\usepackage{xstring}

\newcommand{\lucidcharat}[1]{
\edef\tmpp{#1}
\IfEq{\tmpp}{0}{1}{}%
\IfEq{\tmpp}{1}{2}{}%
\IfEq{\tmpp}{2}{2}{}%
}

\begin{document}
  debug for part two
  \newcounter{debugctr}
  \setcounter{debugctr}{-1}
    {\loop
    \stepcounter{debugctr}
    \edef\tmpinput{\the\numexpr \value{debugctr} - 0}
    \def\tmpchar{\expandafter\lucidcharat\tmpinput}
    (\tmpinput)[\tmpchar],
    %\edef\tmpcharr{\tmpchar}
    \show\tmpchar
    %\typeout{tmpchar is \tmpchar}
    \ifnum \value{debugctr}<3
    \repeat }
\end{document}

Answer 1

It is not clear what is your intended output, but as you need it to work in an edef, you need expandable tests, this uses \ifcase to test for the argument being 0 1 or 2 and outputs 1 2 2 respectively.

\documentclass{article} \usepackage[utf8]{inputenc}


\newcommand{\lucidcharat}[1]{%
\ifcase\numexpr#1\relax
1\or2\or2\else\fi}

\begin{document}
  debug for part two
  \newcounter{debugctr}
  \setcounter{debugctr}{-1}
    {\loop
    \stepcounter{debugctr}%
    \edef\tmpinput{\the\numexpr \value{debugctr} - 0}%
    \def\tmpchar{\expandafter\lucidcharat\tmpinput}%
    (\tmpinput)[\tmpchar],
    \edef\tmpcharr{\tmpchar}%
    \show\tmpcharr
    \typeout{tmpchar is \tmpchar}%
    \ifnum \value{debugctr}<3
    \repeat }
\end{document}