Split single argument in multiple parts

Question

I would like to define the command A\!\!_B\!C in a math mode as a macros, and I am using the following:

\newcommand{\cmp}[3]{#1\!\!_#2\!#3}

Unfortunately I get an error that there are {} missing. Any hint is appreciated.

Is there any possibility to get a Mathematica-like syntax with only one argument, splitting it a the comma?

Answer 1

For example:

\def\cmp#1{\cmpA#1,}
\def\cmpA#1,#2,#3,{{#1}\!\!_{#2}\!{#3}}

$a = \cmp{A,B,C}$

But I don't understand, why do you need the ugly clump of letters.

Answer 2

With your original command you have to remember to give it three arguments.

With package xparse you can split one argument at a given symbol (here the comma) and hence stick with the syntax you know from Mathematica.

The output looks a little bit odd though, but this is a different matter.

\documentclass{article}
\usepackage{xparse}
\newcommand{\cmp}[3]{#1\!\!_{#2}\!#3}
\NewDocumentCommand{\cmpA}{  >{\SplitArgument{2}{,}} m}
{\cmp#1}%<- feeding the three arguments to the original command

\begin{document}
\[ \cmp{A}{B}{C} \]
\[ \cmpA{A,B,C} \]
\end{document}