How to create and use an array of strings?

Question

How can I index into an array of "strings" (text to be typeset) in Latex? If I were to write something in Python, it might look like this:

dow = [
  "Sunday",
  "Monday",
  "Tuesday",
  "Wednesday",
  "Thursday",
  "Friday",
  "Saturday"]

def getdow(daynum):
    return dow[daynum]

But the best I can do in Latex is the following. The syntax gets rather unwieldy if there are more than about half a dozen entries. Is there a cleaner way? Does Latex have any sort of indexable arrays? A package is OK, but if going that route I'd prefer a lighter package.

(Note that I'm using days of the week just as an example; I'm aware that there are date packages.)

\documentclass{article}
\newcommand\getdow[1]{%
  \ifnum #1=0Sunday\else
  \ifnum #1=1Monday\else
  \ifnum #1=2Tuesday\else
  \ifnum #1=3Wednesday\else
  \ifnum #1=4Thursday\else
  \ifnum #1=5Friday\else
  Saturday%
  \fi\fi\fi\fi\fi\fi}
\begin{document}
Today is \getdow{5}.
\end{document}

Addendum: I benchmarked the solutions below. I used lualatex to compile the test documents, performed 100,000 accesses with paragraph breaks every 100, and chose the best of three trials:

Expl syntax: 3.20s (1.42s with pdflatex)
listofitems: 3.25s (1.53s with pdflatex)
Lua: 2.10s

So the Lua solution is noticably faster, assuming lualatex. The other two solutions are faster still with pdflatex.

Neat edit with the compilation speeds. Just out of idle curiosity: How did you program the 100K repeats? E.g., did you set up a Lua for loop for all three test documents when you ran them under LuaLaTeX? — Mico
– Mico, Commented Mar 22 at 6:45
@Mico Nothing so fancy, I just nested macros of 10 instances each. Came to 261 pages. — dedded
– dedded, Commented Mar 22 at 15:01

David Carlisle · Accepted Answer · 2025-03-22 14:41:16Z

8

You don't need a package, you can use the L3 programming layer provided by LaTeX.

\documentclass{article}

\ExplSyntaxOn
\let\mysetlist\clist_const:Nn
\let\myuseitem\clist_item:Nn
\ExplSyntaxOff

\begin{document}

\mysetlist\days{
  Sunday,
  Monday,
  Tuesday,
  Wednesday,
  Thursday,
  Friday,
  Saturday}

day 3 is \myuseitem\days{3}.


\end{document}

edited Mar 22 at 14:41

answered Mar 21 at 20:30

David Carlisle

826k74 gold badges1.7k silver badges2.6k bronze badges

1

Great! I don't even need to understand the L3 syntax :-). I notice that the list is 1-based, not 0-based.

dedded
– dedded

2025-03-21 20:43:57 +00:00
Commented Mar 21 at 20:43
3

@dedded all sensible systems are 1-based:-) English, Fortran, XPath, don't be fooled by C into thinking saying the second item is item1 is at all natural:-)

David Carlisle
– David Carlisle

2025-03-21 20:47:03 +00:00
Commented Mar 21 at 20:47
1

@dedded for the same reason that \documentclass[ twoside, 12pt]{article} doesn't give an unknown option error on _12pt the comma parser strips spaces, you can use { aaa, bbb} to make an entry starting with a space and containing a comma

David Carlisle
– David Carlisle

2025-03-21 20:48:27 +00:00
Commented Mar 21 at 20:48
2

you haven't got an error from a space in the option comma list since 1993 (when I added code to strip spaces, as one of the first changes made to make latex2e from latex2.09) @dedded (the L3 keyval parser is actually different code but all similar systems ignore space around , and =)

David Carlisle
– David Carlisle

2025-03-21 20:52:39 +00:00
Commented Mar 21 at 20:52
1

Good to know, thanks. (I used Latex in '87-88, and then not again* till 2004 or so, so didn't notice changes in '93 :-). *not again directly. I used Vax Document, which was Tex based, in the 90s.

dedded
– dedded

2025-03-21 20:56:42 +00:00
Commented Mar 21 at 20:56

| Show 3 more comments

John Kormylo · Accepted Answer · 2025-03-22 02:16:19Z

For completeness:

\documentclass{article}
\newcommand\getdow[1]{\ifcase#1\relax 
  Sunday\or Monday\or Tuesday\or Wednesday\or Thursday\or Friday\else
  Saturday\fi}
\begin{document}
Today is \getdow{5}.
\end{document}

and

\documentclass{article}
\newcommand\getdow[1]{\csname dow#1\endcsname}
\expandafter\def\csname dow0\endcsname{Sunday}
\expandafter\def\csname dow1\endcsname{Monday}
\expandafter\def\csname dow2\endcsname{Tuesday}
\expandafter\def\csname dow3\endcsname{Wednesday}
\expandafter\def\csname dow4\endcsname{Thursday}
\expandafter\def\csname dow5\endcsname{Friday}
\expandafter\def\csname dow6\endcsname{Saturday}

\begin{document}
Today is \getdow{5}.
\end{document}

or equivalently

\documentclass{article}
\newcommand\getdow[1]{\csname dow#1\endcsname}
\makeatletter
\@namedef{dow0}{Sunday}
\@namedef{dow1}{Monday}
\@namedef{dow2}{Tuesday}
\@namedef{dow3}{Wednesday}
\@namedef{dow4}{Thursday}
\@namedef{dow5}{Friday}
\@namedef{dow6}{Saturday}
\makeatother 

\begin{document}
Today is \getdow{5}.
\end{document}

Steven B. Segletes · Accepted Answer · 2025-03-21 20:48:20Z

5

The listofitems package is ideal for creating and accessing array data.

\documentclass{article}
\usepackage{listofitems}
\begin{document}
\readlist*\dow{Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday}
Today is \dow[6].

The first day of the week is \dow[1].

The third from the last day in the week is \dow[-3].

There are \listlen\dow[] days in the week.
\end{document}

answered Mar 21 at 20:48

Steven B. Segletes

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Mico · Accepted Answer · 2025-03-21 21:53:14Z

5

For the sake of variety, a LuaLaTeX-based solution.

% !TEX TS-program = lualatex
\documentclass{article}
\directlua{dow = {"Sunday", "Monday", "Tuesday", "Wednesday", 
                  "Thursday", "Friday", "Saturday"}}
\newcommand\getdow[1]{\directlua{tex.sprint(dow[#1])}}
    
\begin{document}
Today is \getdow{6}.
\end{document}

answered Mar 21 at 21:53

Mico

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wipet · Accepted Answer · 2025-03-22 09:16:02Z

4

The \foreach macro can be used in OpTeX:

\tmpnum=0 \foreach
  Sunday
  Monday
  Tuesday
  Wednesday
  Thursday
  Friday
  Saturday
  \do #1 {\incr\tmpnum \sdef{dow\the\tmpnum}{#1}}

\def\getdow#1{\csname dow#1\endcsname}

Today is \getdow{6}.

\bye

answered Mar 22 at 9:16

wipet

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CarLaTeX · Accepted Answer · 2025-03-26 07:05:22Z

2

Since you gave your example in Python, why don't you use pythontex?

% arara: pdflatex
% arara: pythontex
% arara: pdflatex
\documentclass{article}
\usepackage{pythontex}

\begin{pycode}
dow = [
  "Sunday",
  "Monday",
  "Tuesday",
  "Wednesday",
  "Thursday",
  "Friday",
  "Saturday"]

def getdow(daynum):
   return dow[daynum]
\end{pycode}

\newcommand{\getdow}[1]{\py{getdow(#1)}}

\begin{document}

Today is \getdow{5}.

Tomorrow is \getdow{6}.

Yesterday was \getdow{4}.

\end{document}

Remember to compile with pdflatex (or lualatex or whatever you're using), pythontex and pdflatex again.
You need pygments installed (to check it you type use pip show pygments on your terminal).

edited Mar 26 at 7:05

answered Mar 25 at 23:15

CarLaTeX

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Well, this is interesting. Don't think I'd use it for this particular problem, but I can see wanting to call python. It's unclear to me if I have pygments installed. There's a file pygments-2.19.1-py3-none-any.whl in my installation directory, which looks like a zipfile of pygments, but I don't know if that amounts to being installed. (Also, does it have to be pdflatex, or does lualatex also work?)

dedded
– dedded

2025-03-25 23:21:50 +00:00
Commented Mar 25 at 23:21
Answering my own questions, lualatex seems to work fine with this, and pythontex seemed to work, so perhaps the zipfile is all that was needed.

dedded
– dedded

2025-03-25 23:41:52 +00:00
Commented Mar 25 at 23:41
@dedded Yes, I confirm, you can use any kind of compilation you like. To check if pygments is installed you can type pip show pygments on your terminal.

CarLaTeX
– CarLaTeX

2025-03-26 07:06:55 +00:00
Commented Mar 26 at 7:06

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