Setting \thecounter to user-defined format

Question

I'm trying to get my counter (which has a value between 1--4) to display as a character (namely, the corresponding char from the string SWOT). The following looks right to me but does not do the trick:

\usepackage{xstring}

and then

\renewcommand{\theswot}{\StrMid{SWOT}{\value{swot}}{\value{swot}}}

That is, it's fine until I want to use

\refstepcounter{swot}

at which point this starts throwing all sorts of unintelligible errors.

I've tried many variations on the use of \StrMid, including sequences of \ifthenelse{\value{swot}=1}{S}{} and so on, but this does not seem to make any difference.

Any help is appreciated: why the errors, and how do I get what I want?

Added in response to @siracusa's comment:

Here's an MWE:

\documentclass{article}
\usepackage{ifthen}
\usepackage{xstring}
\begin{document}
\newcounter{swot}
\renewcommand{\theswot}{%
  \StrMid{SWOT}{\arabic{swot}}{\arabic{swot}}%
  % \ifthenelse{\arabic{swot}=1}{S}{}% This also doesn't work
  % \ifthenelse{\arabic{swot}=2}{W}{}%
  % \ifthenelse{\arabic{swot}=3}{O}{}%
  % \ifthenelse{\arabic{swot}=4}{T}{}%
}
\stepcounter{swot}\theswot % Prints S
\stepcounter{swot}\theswot % Prints W
\stepcounter{swot}\theswot % Prints O
\refstepcounter{swot}      % Throws errors
\end{document}

Please post a small example document that can be used to reproduce the errors. — siracusa, Aug 29 '19 at 22:58
The answers you have received are a better way to implement this, but your first approach would work if you use \renewcommand{\theswot}{\protect\StrMid{SWOT}{\value{swot}}{\value{swot}}} (It also prints nothing at all for counter values > 4). — Alan Munn, Aug 31 '19 at 12:05
Thanks, @AlanMunn, for that elucidation! Indeed the answers below didn't explain why my original attempt failed - now I know, something to watch out for in the future! — Arend, Sep 01 '19 at 06:40

score 2 · Accepted Answer · answered Aug 29 '19 at 23:49

You can use the following template for that.

\documentclass{article}

\usepackage{expl3}
\ExplSyntaxOn
\newcommand{\Mod}[2]{\int_mod:nn{#1}{#2}}% https://tex.stackexchange.com/a/43209/5764
\ExplSyntaxOff

\newcounter{swot}

\renewcommand{\theswot}{%
  \ifcase\Mod{\value{swot}}{4}\relax
        T% 4
    \or S% 1
    \or W% 2
    \or O% 3
  \fi
}

\begin{document}

\stepcounter{swot}\theswot % 1 = S

\stepcounter{swot}\theswot % 2 = W

\stepcounter{swot}\theswot % 3 = O

\stepcounter{swot}\theswot % 4 = T

\stepcounter{swot}\theswot % 1 = S

\stepcounter{swot}\theswot % 2 = W

\stepcounter{swot}\theswot % 3 = O

\stepcounter{swot}\theswot % 4 = T

\end{document}

Modulo calculations is taken from How do I calculate n modulo 3 in LaTeX?.

Thanks, @Werner! In fact I used the answer in a simplified form since there's really no need for the modulo calculation - I can just do an \ifcase\value{swot} and leave the case for 0 open. — Arend, Sep 01 '19 at 06:33

score 2 · Answer 2 · answered Aug 30 '19 at 09:14

Define a proper interface to the numbering:

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn

\cs_new:Nn \arend_swot:n
 {
  \int_case:nn { \int_mod:nn { #1 } { 4 } }
   {
    {1}{S}
    {2}{W}
    {3}{O}
    {0}{T}
   }
 }

\cs_new:Npn \fromswot #1 { \__arend_fromswot:c { c@#1 } }
\cs_new:Nn \__arend_fromswot:N { \arend_swot:n { #1 } }
\cs_generate_variant:Nn \__arend_fromswot:N { c }

\ExplSyntaxOff

\newcounter{swot}
\renewcommand{\theswot}{\fromswot{swot}}
\setcounter{swot}{1}

\begin{document}

\theswot--%
\stepcounter{swot}\theswot--%
\stepcounter{swot}\theswot--%
\stepcounter{swot}\theswot--%
\stepcounter{swot}\theswot--%
\stepcounter{swot}\theswot--%
\stepcounter{swot}\theswot--%
\stepcounter{swot}\theswot--%
\stepcounter{swot}\theswot--%
\stepcounter{swot}\theswot

\end{document}

You can check that using \refstepcounter{swot} and a subsequent \label produces the expected result.

I'm impressed - this is pure magic! I took a shallow dive into xparse and expl3 to try and understand what's going on, but I can't say it all sunk in. Since I prefer an answer I actually understand I selected the simpler one above. — Arend, Sep 01 '19 at 06:37

Setting \thecounter to user-defined format

2 Answers2