how to determine the number of characters in the argument of a command

Question

I want to define a command that behaves differently for single-character arguments than it does for longer arguments. How do I retrieve the number of characters in, e.g., #1?

The specific application that I have in mind is to set variable names in italic if they are single letters and otherwise to set them in Roman. Something like

\newcommand \mathvar [1] {
    \ifthenelse {\characters{#1} = 1}
    {\mathit{#1))
    {\mathrm{#1}}     
}

And, yes, I really mean the number of nonblank characters, not the size of the rendered text.

Note that I want to be able to submit to arXiv.org, and they do not support, e.g., LuaTeX, XeTeX. I'm currently using PDFlatex via MIKTeX, and don't know whether e-TeX is acceptable to arXiv; their current build is rather old.

I tried using xstring; since it doesn't support #, I had to resort to a hack:

\documentclass{article}
\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{ifthen}
\usepackage{xstring}

\newcommand {\funcname} [1]
   {
     \edef\Parameter1{#1}
     \StrDel[0]{\Parameter1}{ }[\stripped]
     \ifthenelse {\StrLen{\stripped} > 1}
       {\mathrm{#1}}
       {#1}
   }

\begin{document}

Test StrDel \{ foo \} \{ \} = /\StrDel { foo } { }/

\edef\Parameter1{ foo }
\StrDel {\Parameter1} { }[\stripped]
Test StrLen StrDel \{ foo \} \{ \} = \StrLen{\stripped}

Test funcname T funcname Table = $\funcname{T} \funcname{Table}$

\end{document}

and that gave me

! Argument of \@xs@StrDel has an extra }.
<inserted text>
                \par
l.25 ...funcname T funcname Table = $\funcname{T}
                                                   \funcname{Table}$

Answer 1

It's easy with expl3:

\documentclass{article}
\usepackage{xparse} % loads expl3

\ExplSyntaxOn

\NewDocumentCommand{\mathvar}{m}
 {
  \int_compare:nTF { \tl_count:n { #1 } > 1 }
   {
    \mathrm{#1}
   }
   {
    #1 % or \mathit{#1}, but I wouldn't do it
   }
 }

\ExplSyntaxOff

\begin{document}

$\mathvar{V}\ne\mathvar{Var}$

\end{document}

The code should be self-explaining: \tl_count:n counts the number of items in its argument (ignoring spaces, however).

I wouldn't use \mathit for single letter variables, as normal math italic is specially tailored for the purpose.

An implementation with xstring:

\documentclass{article}
\usepackage{xstring}

\newcommand{\mathvar}[1]{%
  \begingroup\noexpandarg
  \StrLen{#1}[\temp]%
  \ifnum\temp>1
    \mathrm{#1}%
  \else
    #1%
  \fi
  \endgroup
}

\begin{document}

$\mathvar{V}\ne\mathvar{Var}$

\end{document}

Answer 2

Here's a LuaLaTeX-based solution. It defines a macro called \numchars, which returns the number of characters in its argument, and an implementation of \mathvar. Care is taken not to count whitespace characters in the argument of \mathvar.

% !TEX TS-program = lualatex
\documentclass{article}    
\usepackage{luacode} %  for "\luastring" macro and "luacode" environment
\begin{luacode}
function numchars ( s )  -- disregard any whitespace characters
   return tex.sprint ( unicode.utf8.len ( unicode.utf8.gsub ( s , "%s", "")))
end 
\end{luacode}
\newcommand\numchars[1]{\directlua{numchars(\luastring{#1})}}
\newcommand\mathvar[1]{%
   \ifnum\numchars{#1}<2\mathit{#1}\else\mathrm{#1}\fi}

\begin{document}
$\mathvar{ V }$, $\mathvar{VaR}$
\end{document}

Answer 3

Plainer solution.

\documentclass{scrartcl}

\long\def\gobbleone#1{}
\newcommand*\var[1]{\if\relax\detokenize\expandafter{\gobbleone#1}\relax#1\else\mathrm{#1}\fi}

\begin{document}

$\var{A} = \var{B} = \var{Whatever}$

\end{document}