Package xparse \SplitList last token

Question

I need to create a macro to render lists with a variable number of arguments (1+), e.g. \mylist{1,2,3} should expand to $\tilde{1}$--$\tilde{2}$--$\tilde{3}$. I'm trying to use xparse and \SplitList, but I didn't find a way to tell when I'm processing the last token (where I don't need the separator --).

\NewDocumentCommand\mylist{>{\SplitList{,}}m}
{
  \ProcessList{#1}{\myitem}
}
\newcommand\myitem[1]{$\tilde{#1}$--}

This would expand \mylist{1} to $\tilde{1}$-- instead of the desired $\tilde{1}$ .

score 15 · Accepted Answer · answered Apr 26 '13 at 16:26

Recognizing the first item is easier than the last.

There are many strategies for this.

Use a conditional

\documentclass{article}
\usepackage{xparse}
\NewDocumentCommand\mylist{>{\SplitList{,}}m}
{%
  \ProcessList{#1}{\myitem}%
  \firstitemtrue
}

\newif\iffirstitem
\firstitemtrue   
\newcommand\myitem[1]{%
  \iffirstitem
    \firstitemfalse
  \else
    --%
  \fi
  $\tilde{#1}$}

\begin{document}
\mylist{a,b,c}
\end{document}

Redefine the macro at first usage; doing the processing in a group will revert \myitem to the initial meaning.

\documentclass{article}
\usepackage{xparse}
\NewDocumentCommand\mylist{>{\SplitList{,}}m}
{%
  {\ProcessList{#1}{\myitem}}%
}

\newcommand\myitem[1]{$\tilde{#1}$\let\myitem\myitema}
\newcommand\myitema[1]{--$\tilde{#1}$}

\begin{document}
\mylist{a,b,c}
\end{document}

Use a different approach with expl3 macros, which is more flexible at the expense of some complication.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand\mylist{m}
 {
  \egreg_process_list:n {#1}
 }
% define a sequence for storing the "massaged" items
\seq_new:N \l_egreg_items_seq
\cs_new_protected:Npn \egreg_process_list:n #1
 {
  % clear the sequence
  \seq_clear:N \l_egreg_items_seq
  % cycle through the arguments, storing "\tilde{<arg>}" in the sequence
  \clist_map_inline:nn { #1 }
   {
    \seq_put_right:Nn \l_egreg_items_seq { $\tilde{##1}$ }
   }
  % output the sequence putting "--" in between elements
  \seq_use:Nnnn \l_egreg_items_seq { -- } { -- } { -- }
 }
\ExplSyntaxOff

\begin{document}
\mylist{a,b,c}
\end{document}

The output in all three cases is

enter image description here

Note that you still need protecting end of lines with % in the body of \NewDocumentCommand unless you're in an \ExplSyntaxOn context.

An alternative to the \clist_map_inline:nn construction is to store the sequence with \seq_set_split:Nnn <seq> { , } (stable, doesn't ignore blank entries) or \seq_set_from_clist:Nn <seq> (experimental, ignores empty/blank entries), then using \seq_set_map:NNn <seq> <seq> { \exp_not:n {$\tilde{##1}$} } (experimental function), then \seq_use:Nnnn as you did. It is more efficient :-), and (perhaps) clearer. — Bruno Le Floch, May 22 '13 at 09:49

score 5 · Answer 2 · edited Apr 13 '17 at 12:35

I have used this trick often (see Cunning (La)TeX tricks). The approach is to define a list separator using a "delay". That is, define it as a definition so that the first use just defines itself:

enter image description here

\documentclass{article}
\usepackage{xparse}% http://ctan.org/pkg/xparse
\NewDocumentCommand\mylist{O{--} >{\SplitList{,}}m}
{%
  \def\itemdelim{\def\itemdelim{#1}}% Define list separator with one delay
  \ProcessList{#2}{\myitem}% Process list
}
\newcommand\myitem[1]{\itemdelim$\tilde{#1}$}

\begin{document}
\mylist{a,b,c}
\end{document}

The additional optional argument to \mylist allows you to modify the list separator (or item delimiter) of the output. It defaults to --.

Package xparse \SplitList last token

2 Answers2

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