Usage of \greadlist* from package 'listofitems'

Question

I'm trying to define a LIST using \greadlist from the listofitems package. I would like to give a dynamic name to that list, something like LISTN, where N depends on a counter.

I tried something like

\documentclass{amsart}
\usepackage{listofitems}

\begin{document}
\newcounter{num}\setcounter{num}{1}
\setsepchar{,}
\greadlist*{\expandafter\noexpand\csname LIST\roman{num}\endcsname}{a,b,c}
\end{document}

But this just make LaTeX not stop running...

Answer 1

Expand the \csname before invoking the \greadlist.

\documentclass{amsart}
\usepackage{listofitems}

\begin{document}
\newcounter{num}\setcounter{num}{4}
\setsepchar{,}
\expandafter\greadlist\expandafter*\csname 
  LIST\roman{num}\endcsname{a,b,c}

\LISTiv[2]
\end{document}

Answer 2

You can define your own variation of the commands.

\documentclass{amsart}
\usepackage{listofitems}

\ExplSyntaxOn
\NewDocumentCommand{\readnamedlist}{smm}
 {
  \IfBooleanTF{#1}
   {\exp_args:NNc \readlist*{#2}{#3}} % * variant
   {\exp_args:Nc \readlist{#2}{#3}}
 }
\NewDocumentCommand{\greadnamedlist}{smm}
 {
  \IfBooleanTF{#1}
   {\exp_args:NNc \greadlist*{#2}{#3}} % *-variant
   {\exp_args:Nc \greadlist{#2}{#3}}
 }
\ExplSyntaxOff

\begin{document}
\newcounter{num}\setcounter{num}{1}
\setsepchar{,}

\greadnamedlist*{LIST\roman{num}}{a,b,c}

\LISTi[1],\LISTi[2],\LISTi[3]

\stepcounter{num}

\readnamedlist{LIST\roman{num}}{d,e,f}

\LISTii[1],\LISTii[2],\LISTii[3]

\end{document}

What does \exp_args:Nc do? It jumps over the next token and applies \csname...\endcsname to the contents of the following braced group. Similarly \exp_args:NNc that jumps over two tokens.

If you prefer a “classical” implementation, here's an argument reversing procedure.

\documentclass{amsart}
\usepackage{listofitems}

\makeatletter
\newcommand{\readnamedlist}{%
  \@ifstar{\@readnamedlist{\readlist*}}{\@readnamedlist{\readlist}}%
}
\newcommand{\greadnamedlist}{%
  \@ifstar{\@readnamedlist{\greadlist*}}{\@readnamedlist{\greadlist}}%
}
\newcommand{\@readnamedlist}[2]{%
  \expandafter\@@readnamedlist\csname #2\endcsname{#1}%
}
\newcommand{\@@readnamedlist}[2]{#2#1}
\makeatother

\begin{document}
\newcounter{num}\setcounter{num}{1}
\setsepchar{,}

\greadnamedlist*{LIST\roman{num}}{a,b,c}

\LISTi[1],\LISTi[2],\LISTi[3]

\stepcounter{num}

\readnamedlist{LIST\roman{num}}{d,e,f}

\LISTii[1],\LISTii[2],\LISTii[3]

\end{document}

Answer 3

I think those days expl3-solutions are state of the art, so I'd in any case prefer egreg's solution.

The following is just for exhibiting the technique of exchanging arguments after expansion:

\documentclass{amsart}
\usepackage{listofitems}

\newcommand\PassFirstToSecond[2]{#2{#1}}%

\begin{document}
\newcounter{num}\setcounter{num}{1}
\setsepchar{,}
\expandafter\PassFirstToSecond\expandafter{\csname LIST\roman{num}\endcsname}{\greadlist*}{a,b,c}
\end{document}

Alternatively a macro
\CsNameToCsToken⟨stuff not in braces⟩{NameOfCs}
→
⟨stuff not in braces⟩\NameOfCs
can help avoiding/hiding \expandafter...\csname..\endcsname-orgies:

\documentclass{amsart}
\usepackage{listofitems}

%%===============================================================================
%% Obtain control sequence token from name of control sequence token:
%%===============================================================================
%% \CsNameToCsToken<stuff not in braces>{NameOfCs}
%% ->  <stuff not in braces>\NameOfCs
%% (<stuff not in braces> may be empty.)
\csname @ifdefinable\endcsname\CsNameToCsToken{%
  \long\def\CsNameToCsToken#1#{\InnerCsNameToCsToken{#1}}%
}%
\newcommand\InnerCsNameToCsToken[2]{%
  \expandafter\exchange\expandafter{\csname#2\endcsname}{#1}%
}%
\newcommand\exchange[2]{#2#1}%
%%===============================================================================

\begin{document}
\newcounter{num}\setcounter{num}{1}
\setsepchar{,}
\CsNameToCsToken\greadlist*{LIST\roman{num}}{a,b,c}
\end{document}

(If it were implemented there, in expl3 the name of \CsNameToCsToken would probably be something like \exp_args:wc or whatever argument-type you use instead of w for an argument which must not be nested in braces but is delimited by a left brace {.)