Customized list of elements using etoolbox

Question

I need to have several lists with access by index (array-like). My minimum non-working example follows:

\documentclass{minimal}
\usepackage{etoolbox}

\newcounter{listAcounter}

\newcommand\addToList[3]{%
\stepcounter{#2}%
\csdef{#1\the#2}{#3}
} 

\newcommand\getFromList[2]{%
\csuse{#1#2}
}

\begin{document}

\addToList{listA}{listAcounter}{one}
\addToList{listA}{listAcounter}{two}
\addToList{listA}{listAcounter}{three}

This is element 2 from list A: \getFromList{listA}{2}.

\end{document}

I get from compiling with pdflatex:

! You can't use `the letter l' after \the.
<argument> listA\the l
                  istAcounter
l.17 \addToList{listA}{listAcounter}{one}

How can I fix this code? Why is this wrong?

Answer 1

When you type \the#2 you have an already formed token, so this will not be merged into a single command name such as \thelistAcounter. You have to build the name yourself:

\documentclass{article}
\usepackage{etoolbox}

\newcounter{listAcounter}

\newcommand\addToList[3]{%
  \stepcounter{#2}%
  \csdef{#1\csuse{the#2}}{#3}
}

\newcommand\getFromList[2]{%
  \csuse{#1#2}%
}

\begin{document}

\addToList{listA}{listAcounter}{one}
\addToList{listA}{listAcounter}{two}
\addToList{listA}{listAcounter}{three}

This is element 2 from list A: \getFromList{listA}{2}.

\end{document}

Note the % after \csuse{#1#2}.

---

An easier implementation with expl3, that avoids defining a counter.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\newList}{m}
 {
  \seq_new:c { l_kees_list_#1_seq }
 }
\NewDocumentCommand{\addToList}{mm}
 {
  \seq_put_right:cn { l_kees_list_#1_seq } { #2 }
 }
\NewDocumentCommand{\getFromList}{mm}
 {
  \seq_item:cn { l_kees_list_#1_seq } { #2 }
 }
\ExplSyntaxOff

\begin{document}

\newList{listA}
\addToList{listA}{one}
\addToList{listA}{two}
\addToList{listA}{three}

This is element 2 from list A: \getFromList{listA}{2}.

\end{document}