引用由 listadd 创建的数组元素

Question

鉴于 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}

实现以下目标的最简单方法是什么？

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

目标是仅显示选定的元素，在本例中是元素编号 1 和 3。

Answer 1

\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}

Answer 2

我可以建议一种基于的相当通用的方法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}

第二个参数\listextract是如何使用存储项目的模板，其中#1代表循环中的当前项目。

\addpunct只是为了举例说明如何使用模板。