Printing an element of a non-numeric array as the node label

Question

I want to retrieve a member of a non-numeric array passed to a TikZ pic as an argument, to use it as a node label. \foreach easily does it while iterating over the array, but I cannot do it for the individual elements. The following are my two attempts that fail miserably:

\documentclass[tikz]{standalone}

\begin{document}

\tikzset{
   pics/mypic/.style n args={1}{
      code={
         \foreach \a [count=\i] in {#1}{
            \node at (\i, 1) {$\a$};
         }

         % Attempt 1: Doesn't work
         % \node at (1, 2) {$#1[0]$};

         % Attempt 2: Doesn't compile
         % Error: Undefined control sequence. ... Missing $ inserted. ...
         \node at (2, 2) {$\pgfmathparse{{#1}[0]}\pgfmathresult$};
      }
   }
}

\tikz{\pic {mypic={\mathtt{a},\mathtt{b},\mathtt{c}};}}
% \tikz{\pic {mypic={a,b,c};}}   % It's the same for this one.
% \tikz{\pic {mypic={0,1,2};}}   % Attempt 2 works with this one,
% \tikz{\pic {mypic={0,a,2};}}   % but not with this one ("Unknown function `a').

\end{document}

Output:

I want to be able to somehow draw a node with the label #1[0]. Regarding Attempt 2, I noticed that it yields the same errors also with the text arrays. It works fine for the all-numeric arrays, but hybrid arrays are also troubling, even if the accessed element is numeric.

Answer 1

Here, I use listofitems to read the desired data into an array \mydata. The OP suggested a useful EDIT to my original approach by including all of the listofitems processing inside the \tikzset code.

Note that the 2 in \mydata[2], calling out a single array element, can be any expandable calculation that results in an integer. Also, negative values denote "reverse index", that is, relative to the end of the array, rather than the beginning.

\documentclass[tikz]{standalone}
\usepackage{listofitems}

\begin{document}

\tikzset{
   pics/mypic/.style n args={1}{
      code={
         \foreach \a [count=\i] in {#1}{
            \node at (\i, 1) {$\a$};
         }

         \readlist\mydata{#1}
         \node at (2, 2) {$\mydata[2]$};
      }
   }
}

\tikz{\pic {mypic={\mathtt{a},\mathtt{b},\mathtt{c}};}}
\end{document}

Answer 2

The following command uses \foreach to retrieve the element at a given index, and assign it globally to the given macro.

% Does #1 = #2[#3] with 0-based indexing
\newcommand\arrayget[3]{
    \foreach \a [count=\i from 0] in {#2}{
        \ifnum #3=\i
            \xdef#1{\unexpanded\expandafter{\a}}
        \fi
    }
}

References for the curious:

• Global definition with \xdef: source 1
• \unexpanded\expandafter{...} to expand \a once: source 2

It should be able to handle anything that \foreach can. Demonstration:

\documentclass[tikz]{standalone}

\newcommand\arrayget[3]{   % Does #1 = #2[#3] with 0-based indexing
    \foreach \a [count=\i from 0] in {#2}{
        \ifnum #3=\i
            \xdef#1{\unexpanded\expandafter{\a}}
        \fi
    }
}

\begin{document}

\tikzset{
   pics/mypic/.style n args={1}{
      code={
         \foreach \a [count=\i] in {#1}{ \node at (\i, 1) {$\a$}; }

         \arrayget{\asd}{#1}{0} \node at (1.5, 2) {$\asd$};
         \arrayget{\asd}{#1}{1} \node at (2.0, 2) {$\asd$};
         \arrayget{\qwe}{#1}{2} \node at (2.5, 2) {$\qwe$};
      }
   }
}

\tikz{\pic {mypic={\mathtt{a},\mathtt{b},\mathtt{c}};}}
\tikz{\pic {mypic={a,b,c};}}
\tikz{\pic {mypic={0,1,2};}}
\tikz{\pic {mypic={0,a,2};}}

\end{document}

Output: