Reference an element of an array created by listadd

Question

Given the mwe:

\documentclass{article}
\usepackage{etoolbox}
\begin{document}
\listadd\terms{First item of the list}
\listadd\terms{Second item which has punctuation such as, commas, potentially a period and maybe even other things!}
\listadd\terms{And the last one}
\begin{enumerate}%
\forlistloop{\item}{\terms}%
\end{enumerate}%
\end{document}

What is the simplest way to achieve something like the following?

\begin{enumerate}%
\selectiveforlistloop{\item}{\terms}{1,3}%
\end{enumerate}%

Where the goal is to show only select elements, in this case, elements number 1 and 3.

score 4 · Answer 1 · answered Apr 04 '21 at 16:22

\documentclass{article}
\usepackage{etoolbox}
\makeatletter
\newcommand\selectiveforlistloop[4][]{%
\edef\iaux{0}%
\renewcommand*{\do}[1]{\edef\iaux{\the\numexpr\iaux+1}%
\expanded{\noexpand\in@{,\iaux,}{,#4,}}%
\ifin@
#2 ##1
\fi}%
\dolistloop{#3}%
}
\makeatother
\begin{document}

\listadd\terms{First item of the list}
\listadd\terms{Second item which has punctuation such as, commas, potentially a period and maybe even other things!}
\listadd\terms{And the last one}

\begin{enumerate}%
\selectiveforlistloop{\item}{\terms}{1,3}%
\end{enumerate}%
\end{document}

...care to add some mortar to the bricks? – Werner Apr 04 '21 at 17:14 — Werner, Apr 04 '21 at 17:14

score 4 · Answer 2 · answered Apr 04 '21 at 16:24

I can suggest a fairly general method based on expl3.

\documentclass{article}
\usepackage{amsthm} % for `\@addpunct
\ExplSyntaxOn
% borrow @addpunct
\cs_new_eq:Nc \addpunct { @addpunct }
\NewDocumentCommand{\listadd}{mm}
 {
  % create the sequence where storing the items if not existent
  \seq_if_exist:cF { l_johnchris_list_#1_seq } { \seq_new:c { l_johnchris_list_#1_seq } }
  % store the items
  \seq_put_right:cn { l_johnchris_list_#1_seq } { #2 }
 }
\NewDocumentCommand{\listextract}{mmo}
 {
  % define the function to use according to the template
  \cs_set_protected:Nn __johnchris_list_extract:n { #2 }
  \IfNoValueTF { #3 }
   {% no optional argument, use the whole sequence
    \seq_map_function:cN { l_johnchris_list_#1_seq } __johnchris_list_extract:n
   }
   {% the optional argument is a list of item numbers; extract them in a temporary sequence
    \seq_clear:N \l__johnchris_list_temp_seq
    \clist_map_inline:nn { #3 }
     {
      \seq_put_right:Nx \l__johnchris_list_temp_seq { \seq_item:cn { l_johnchris_list_#1_seq } {##1} }
     }
    % do the mapping on the temporary sequence
    \seq_map_function:NN \l__johnchris_list_temp_seq __johnchris_list_extract:n
   }
 }
\ExplSyntaxOff
\begin{document}
\listadd{terms}{First item of the list}
\listadd{terms}{Second item which has punctuation such as, commas, 
  potentially a period and maybe even other things!}
\listadd{terms}{And the last one}
\begin{enumerate}
\listextract{terms}{\item #1\addpunct{.}}
\end{enumerate}
\begin{enumerate}
\listextract{terms}{\item \textit{#1}}[1,3]
\end{enumerate}
\end{document}

The second argument to \listextract is a template for how to use the stored items, where #1 stands for the current item in the loop.

\addpunct is just to make an example of how the template can be used.

wouldn't it be better to give the new \listadd another name? etoolbox is used in many documents. — Ulrike Fischer, Apr 04 '21 at 16:41

Reference an element of an array created by listadd

2 Answers2