Indexing issues with 2D arrays

Question

I have a tikz code using values stored in a 1D array, here is a MWE :

\documentclass{standalone}
\usepackage{tikz}

\def \myArray {{0,1,2,3}}

\begin{document}
    \begin{tikzpicture}

        \pgfmathsetmacro\x{\myArray[0]}
        \draw (0,0) node {\x};

    \end{tikzpicture}
\end{document}

So this one works OK for my complex code, but I could really improve it using 2D arrays and loops. Here is a Minimum Not Working Example of what it could look like :

\documentclass{standalone}
\usepackage{tikz}

\def \myArray {{0,1}{2,3}}

\begin{document}
    \begin{tikzpicture}

        \pgfmathsetmacro\x{\myArray[1][1]}
        \draw (0,0) node {\x};

    \end{tikzpicture}
\end{document}

with this code, I'm expecting to have the value "3" displayed (2nd line, 2nd column).

Is there a way to do 2D array storage and indexing with latex/tikz ?

Yes, you can look at arrayjobx. I wrote a few answers using it here and here. — Tom Bombadil, Nov 05 '15 at 19:21
Or you can go old school and use \csname myArray\x,\y\endcsname. (I suspect trying to use [\x][\y] will be much harder.) — John Kormylo, Nov 05 '15 at 20:38
An alternative solution could be: How to use values from a 2d array in a tikz picture ? — Bobyandbob, Aug 17 '17 at 19:13

egreg · Accepted Answer · 2023-06-02T21:25:55.227

4

If you are keen on a change of syntax, with expl3 it can be easily done:

\documentclass{standalone}
\usepackage{tikz}
\ExplSyntaxOn
\NewDocumentCommand{\definearray}{mm}
 {
  \seq_gclear_new:c { g_thomas_array_#1_seq }
  \seq_set_split:cnn { g_thomas_array_#1_seq } { ; } { #2 }
 }
\NewExpandableDocumentCommand{\retrieve}{mmm}
 {
  \clist_item:en { \seq_item:cn { g_thomas_array_#1_seq } { #2 + 1 } } { #3 + 1 }
 }
\cs_generate_variant:Nn \seq_set_split:Nnn { c }
\cs_generate_variant:Nn \clist_item:nn { e }
\ExplSyntaxOff
\definearray{myarray}{0,1;2,3}
\begin{document}
\begin{tikzpicture}
\draw (0,0) node {\retrieve{myarray}{1}{1}};
\end{tikzpicture}
\end{document}

The two +1 are for starting indexing at 0, as usual for PGF arrays (expl3 sequences and comma separated lists start at 1).

A different approach, with a different syntax:

\documentclass{standalone}
\usepackage{tikz}
\ExplSyntaxOn
\NewDocumentCommand{\definearray}{mm}
 {
  \tl_gclear_new:c { g_thomas_array_#1_tl }
  \tl_gset:cn { g_thomas_array_#1_tl } { #2 }
 }
\NewExpandableDocumentCommand{\retrieve}{mmm}
 {
  \clist_item:en { \tl_item:cn { g_thomas_array_#1_tl } { #2 + 1 } } { #3 + 1 }
 }
\cs_generate_variant:Nn \clist_item:nn { e }
\ExplSyntaxOff
\definearray{myarray}{{0,1}{2,3}}
\begin{document}
\begin{tikzpicture}
\draw (0,0) node {\retrieve{myarray}{1}{1}};
\end{tikzpicture}
\end{document}

In either case, here's the output:

edited Jun 02 '23 at 21:25

answered Nov 05 '15 at 23:49

egreg

1,121,712

Is there a way to do the same using nested comma lists? Perhaps start with \def\matrix{{1,2},{3,4}} and then use clist_item:cn {#1} {#2} to get, for example, {1,2} and then after setting some new clist variable equal to this use #3 to get one of the two elements. Something like this could be used to access elements from multidimensional arrays stored as nested comma lists... – Ted Black Jun 02 '23 at 20:54
@TedBlack I'll add it. – egreg Jun 02 '23 at 21:18
Many thanks. Sorry for the repeated novice questions: no matter how hard I try something like \clist_set:Nn \l_tmpa_clist {\clist_item:cn {#1} {#2}} and then use \clist_item:Nn \l_tmpa_clist {#3} does not work. The program treats \l_tmpa_clist as a single token. My understanding of clists is obviously very poor but can you help me understand the main problem with my thinking? – Ted Black Jun 02 '23 at 21:34
@TedBlack Sorry, but \clist_set:Nn \l_tmpa_clist {\clist_item:cn {#1} {#2}} is not going to work. For several reasons. Actually this is understatement… – egreg Jun 02 '23 at 22:24
Yes it definitely does not work! Sounds like I need to study this a lot more... – Ted Black Jun 02 '23 at 22:39

Indexing issues with 2D arrays

1 Answers1