Is there a way to step through two lists of arguments pairwise?

Question

Is there a way to write a macro that steps through two lists of arguments pairwise, like what Python's zip function does? For example, I'd like to write a macro like this (using xparse because that's what I've been experimenting with; I'm open to other options):

\NewDocumentCommand{\Zip}{ >{\SplitList{,}}m >{\SplitList{,}}m }{%
    % something that calls \myfunc in an itemize environment
}
\newcommand{\myfunc}[2]{\item #1, #2}

So that calling Zip{zebra,frog,jay}{mammal,amphibian,bird} is equivalent to

\begin{itemize}
    \item zebra, mammal
    \item frog, amphibian
    \item jay, bird
\end{itemize}

Is this possible? I've looked into xparse but it seems to only allow stepping through two lists one at a time, not zipping them together. There was a similar question here back in 2011, but I hope there's a nicer solution now.

See also https://tex.stackexchange.com/a/554381/4427 – egreg Oct 28 '20 at 18:23 — egreg, Oct 28 '20 at 18:23

score 3 · Accepted Answer · answered Oct 27 '20 at 16:23

3

You can use \seq_mapthread_function:NNN to iterate over two seq variables like that.

\documentclass{article}
\ExplSyntaxOn
\seq_new:N \l__gil_tmpa_seq
\seq_new:N \l__gil_tmpb_seq
\NewDocumentCommand \Zip { m m }
  {
    \seq_set_from_clist:Nn \l__gil_tmpa_seq {#1}
    \seq_set_from_clist:Nn \l__gil_tmpb_seq {#2}
    \seq_mapthread_function:NNN
      \l__gil_tmpa_seq \l__gil_tmpb_seq \myfunc
  }
\ExplSyntaxOff
\newcommand{\myfunc}[2]{\item #1, #2}
\begin{document}
\begin{itemize}
  \Zip{zebra,frog,jay}{mammal,amphibian,bird}
\end{itemize}
\end{document}

answered Oct 27 '20 at 16:23

Phelype Oleinik

70,814

Nice, exactly what I need! I'll see if someone has a higher-level solution (I was kind of expecting there to be one) but if not, I will accept your answer. – gil Oct 27 '20 at 18:05
@gil How higher-level do you mean? \seq_mapthread_function:NNN is a documented interface in expl3, so it is really not “low level”. If you mean something built into xparse, then the answer is no, it doesn't have that. – Phelype Oleinik Oct 27 '20 at 18:14
I mean if some package offers an easy, xparse-like interface for zipping (which xparse doesn't do, I know), I'd prefer that. Maybe I was hoping for too much. – gil Oct 27 '20 at 21:23
@gil It's rather specific, so I don't think so (but there are thousands of packages out there, so there might as well be, but none that I know of). There's \foreach from the pgffor package, which is quite powerful and maybe has something to do this (I know it can do a for loop with two counters, like C's for; I don't know about arbitrary lists). – Phelype Oleinik Oct 27 '20 at 21:32
This is essentially identical to my answer to the question marked as duplicate, although not as powerful. – egreg Oct 28 '20 at 18:20
How could I make this work in case I put the two lists in a command first? So e.g. having \newcommand{\listone}{zebra,frog,jay} and \newcommand{\listtwo}{mammal,amphibian,bird}, then calling \Zip{\listone}{\listtwo}? – Steven Mar 15 '23 at 16:15
@Steven Replace the two \seq_set_from_clist:Nn with \seq_set_from_clist:NN (:NN instead of :Nn) – Phelype Oleinik Mar 15 '23 at 16:24
@PhelypeOleinik thanks for the quick reply, works like a charm! – Steven Mar 15 '23 at 17:13

score 0 · Answer 2 · answered Oct 29 '20 at 06:30

Using \foreach from pgffor, to illustrate as per the comment.

Traverse the diagonal of the matrix of pairwise combinations:

\foreach \x [count=\xi] in {#1}{%
\foreach \y [count=\yi] in {#2}{%
\ifnum \xi=\yi%
\item \x , \y 
\fi
}
}

MWE


\documentclass{article}
\usepackage{xparse}
\usepackage{pgffor}
\ExplSyntaxOn
\seq_new:N \l__gil_tmpa_seq
\seq_new:N \l__gil_tmpb_seq
\NewDocumentCommand \Zip { m m }
  {
    \seq_set_from_clist:Nn \l__gil_tmpa_seq {#1}
    \seq_set_from_clist:Nn \l__gil_tmpb_seq {#2}
    \seq_mapthread_function:NNN
      \l__gil_tmpa_seq \l__gil_tmpb_seq \myfunc
  }
\ExplSyntaxOff
\newcommand{\myfunc}[2]{\item #1, #2}
\newcommand{\myfuncb}[2]{%
\begin{itemize}
\foreach \x [count=\xi] in {#1}{%
\foreach \y [count=\yi] in {#2}{%
\ifnum \xi=\yi%
\item \x , \y 
\fi
}
}
\end{itemize}
}
\begin{document}
Expl3:
\begin{itemize}
  \Zip{zebra,frog,jay}{mammal,amphibian,bird}
\end{itemize}
\textbackslash foreach:
\myfuncb{zebra,frog,jay}{mammal,amphibian,bird}
\end{document}

Is there a way to step through two lists of arguments pairwise?

2 Answers2