Iterating through comma-separated arguments

Question

I've looked all over and can't seem to find a succinct answer to this question. Is it possible (and if so, how) to create a command that will iterate through its `arguments' of comma-separated values and act upon them?

Example mostly stolen from "For loop" in newcommand:

\newcommand{\pdfappendix}[1]{
    for \image in #1
    {
       \includegraphics[scale=0.6]{\image.pdf}
    }
}

The goal here is to create a command that can print a file menu structure (or some arbitrary path) like so:

\ppath{Command,TeXing Options,Generate PDF (C-c C-t C-p)}

potentially with an optional argument for a delimiter (defaulting to \to or something)

which would produce

Command -> TeXing Options -> Generate PDF (C-c C-t C-p)

In the words of holy ed, ?

Answer 1

etoolbox's list processing capabilities are straight forward:

\documentclass{article}
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\newcommand{\ppath}[2][$\;\triangleright\;$]{%
  \def\nextitem{\def\nextitem{#1}}% Separator
  \renewcommand*{\do}[1]{\nextitem\textsf{##1}}% How to process each item
  \docsvlist{#2}% Process list
}
\begin{document}
A decent file path is \ppath{File,New,Document}.
\end{document}

The separator \nextitem is defined to do nothing during its first use. \do defines how each item is processed, while \docsvlist processes a comma-separated list. See Cunning (La)TeX tricks for a short discussion on the use of \nextitem.

Answer 2

Found an answer in another question's answer. With @Werner's help (specifically the deferred \def trick), the pure TeX solution works without the need for extra packages. What follows is a minimal working example of what I was looking for.

\documentclass{article}

\makeatletter
\newcommand{\ppath}[2][ $\triangleright$ ]{%
  \def\nextitem{\def\nextitem{#1}}%
  \@for \el:=#2\do{\nextitem\el}%
}
\makeatother

\begin{document}
  A decent file path is \ppath{File,New,Why}.  I said, Why.
\end{document}

Output:

(Thanks @Peter, @jon.)

---

Explanation for the comments:

\def\nextitem{\def\nextitem{#1}} here is defining \nextitem to, when it is used, redefine \nextitem to expand to the argument it was given the first time. This is a pretty tricky operation, so read that again.

So, if I call \nextitem{foo}, that expands to \def\nextitem{foo}. The \nextitem token, having been consumed and expanded, is gone. Note that \nextitem goes from having one argument to having no arguments; after using \nextitem{foo}, that itself redefines \nextitem to foo -- and doesn't emit foo the first time (since the first time is just the redefinition.

This is heart of the trick: the for loop provides a way to output >File>New>Why -- the \nextitem trickery is a way to make the first > 'disappear' -- because the first time \nextitem is used, all it's doing is a redefinition.

Answer 3

Here is a solution with LaTeX3, which might be simpler

Since may 2021

\clist_use:nn and \clist_use:nnnn are available as pointed out by egreg in the comments.

\documentclass{standalone}
\ExplSyntaxOn
\NewDocumentCommand \ppath { O{$\;\triangleright\;$} m } {
  \clist_use:nn { #2 } { #1 }
}
\ExplSyntaxOff
\begin{document}
A decent file path is \ppath{File,New,Document}.
\end{document}

The output is

Before may 2021

The instruction \clist_use:nn { #2 } { #1 } should be replaced by

  \group_begin:
  \clist_set:Nn \l_tmpa_clist { #2 }
  \clist_use:Nn \l_tmpa_clist { #1 }
  \group_end:

Some explanations:

In the second example,

1. We wrap the commands in a group to use local variables
2. We store the mandatory argument in a scratch variable named \l_tmpa_clist of type clist
3. We "use" that variable to display the list with the given separator

Notice the use of \clist_use:Nn instead of \clist_use:nn.

The LaTeX3 commands documentation is available on CTAN as interface3.pdf and may be available locally with texdoc interface3.

Answer 4

In OpTeX we have \foreach command with the syntax

\foreach <list of object>\do <parameter-mask for sigle object>{<what to do>}

For example

\foreach a,b,c,d,\do #1,{do something with #1}

does (expadable) loop over the given list of objects separated by comma. Your task should be solved by the following macro:

\def\ppath#1{\ppathA#1,\end}
\def\ppathA#1,#2\end{#1\foreach #2\do ##1,{$\,\triangleright\,$##1}}

\ppath{Command,TeXing Options,Generate PDF (C-c C-t C-p)}

\ppath{Cmd}

\bye