This answer may be more generic than specifically relating to TikZ/PGF.
(La)TeX is a macro-based language, so it does not work as expected compared to other languages when dealing with "arrays". For example, while \names[2] should yield Laura where
\def\names{Katie, Frank, Laura, Joe}
(indexing from 0), (La)TeX considers [2] to have no connection to \names. As such, you're more likely to obtain the output Katie, Frank, Laura, Joe[2] - a concatenation of \names (as it is defined) and [2].
In order to allow for indexing like one typically would using arrays, you would need some other functionality. Here's an example of a list parser that works like you would expect arrays do:
\documentclass{article}
\usepackage{xparse}% http://ctan.org/pkg/xparse
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\newcounter{listtotal}\newcounter{listcntr}%
\NewDocumentCommand{\names}{o}{%
\setcounter{listtotal}{0}\setcounter{listcntr}{-1}%
\renewcommand*{\do}[1]{\stepcounter{listtotal}}%
\expandafter\docsvlist\expandafter{\namesarray}%
\IfNoValueTF{#1}
{\namesarray}% \names
{% \names[<index>]
\renewcommand*{\do}[1]{\stepcounter{listcntr}\ifnum\value{listcntr}=#1\relax##1\fi}%
\expandafter\docsvlist\expandafter{\namesarray}}%
}
\begin{document}
\newcommand{\namesarray}{Katie, Frank, Laura, Joe}%
\verb|\names:|\ \names \par
\verb|\names[2]:|\ \names[2] \par
\verb|\names[0]:|\ \names[0] \par
\verb|\names[5]:|\ \names[5]
\end{document}
The idea here is to store the names in an array \namesarray and then define a macro (or "function") that takes an optional argument. If no argument is supplied (i.e., you just use \names), then you print the entire \namesarray. If an argument is supplied (of the form \names[<index>]), parse the list sequentially to find that item that matches <index> and print it.
The list parser relies on etoolbox's \docsvlist and enumerator \do.
The question seems a bit ill-defined, but one can indeed parse a general list in a nested way, and do different things based on the list length of each successive rank-1 list. Here
\loopy{1,2: 2,3,7: 2,4,4,5}
will place a 2 at location (1,2), since it was a 2-element list.
It will place a 3_7 at location (2,3), since 7 was the third element of the list starting with the coordinates (2,3).
Finally, it will place a 4_1 and 4_2 at (2,4) and (4,5) since the 4 element list began with 2,4 and concluded with 4,5.
\documentclass{article}
\usepackage{listofitems,tikz}
\newcommand{\loopy}[1]{%
\setsepchar{:/,}%
\readlist\mylist{#1}%
\begin{tikzpicture}
\foreachitem\x\in\mylist[]{%
\ifnum\listlen\mylist[\xcnt]=2\relax
\node at (\mylist[\xcnt,1],\mylist[\xcnt,2]){$2$};
\else
\ifnum\listlen\mylist[\xcnt]=3\relax
\node at (\mylist[\xcnt,1],\mylist[\xcnt,2]){$3_{\mylist[\xcnt,3]}$};
\else
\ifnum\listlen\mylist[\xcnt]=4\relax
\node at (\mylist[\xcnt,1],\mylist[\xcnt,2]){$4_1$};
\node at (\mylist[\xcnt,3],\mylist[\xcnt,2]){$4_2$};
\fi
\fi
\fi
}
\end{tikzpicture}
}
\begin{document}
\loopy{1,2: 2,3,7: 2,4,4,5}
\end{document}
Best Answer
The TikZ
\foreachmacro is handy for situations like this:See section 56 ("Repeating Things: The Foreach Statement") in the TikZ/PGF v2.10 manual. The
countoption was introduced in TikZ v2.10, so you might need to update TikZ/PGF. The reason for usingcountinstead of LaTeX counters in that counters are defined globally, and I do not know how to easily get local counters (otherwise a second call to the macro will produce analready definederror).