Can one pass a pgf array to a pgf math function?

Question

I'd like to define:

 anglearray(\A,\I,\L) = array(\A,Mod(\I,\L)) + 360*floor((\I+0.1)/\L);

but I get strange errors about "! Missing number, treated as zero." and the "array" part appears to evaluate to 0. Here is a fairly minimal example that shows a really dumb-looking work-around (that more or less loses any of the advantages of writing it as a function).

\documentclass{article}
\usepackage{tikz}
\begin{document}
\newcommand{\showB}[1]{\typeout{\string#1=\meaning#1}}.
\begin{tikzpicture}[
    declare function={
      anglearray1(\AI,\I,\L) = \AI + 360*floor((\I+0.1)/\L);
      anglearray2(\A,\I,\L) = array(\A,Mod(\I,\L)) + 360*floor((\I+0.1)/\L);
    }
]
% Spin around a circle stopping at 10 and 190 degrees
\foreach \i in {-10,...,10} {
  \pgfmathsetmacro{\X}{anglearray1(array({10,190},Mod(\i,2)),\i,2)}
  \showB\X
}
% Spin around a circle stopping at 10 and 190 degrees; doesn't work
\foreach \i in {-10,...,10} {
  \pgfmathsetmacro{\X}{anglearray2({10,190},\i,2)}
  \showB\X
}
\end{tikzpicture}
\end{document}

I'm using PGF 2.10 from TexLive 2011 of today.

Answer 1

As @JackSchmidt pointed out, with public pgf functions, a pgf array {1,2,3} is parsed and transformed to {1}{2}{3} before being handled by private pgf functions. So you need to access those private functions (see the pgfmanual, Math engine, Customizing the math engine).

\documentclass{article}
\usepackage{tikz}
\begin{document}

\tikzset{%
  declare function={%
    anglearray1(\AI,\I,\L) = \AI + 360*floor((\I+0.1)/\L);}}

% Spin around a circle stopping at 10 and 190 degrees
\foreach \i in {-10,...,10} {%
  \pgfmathsetmacro{\X}{anglearray1(array({10,190},Mod(\i,2)),\i,2)}
  \X\par}

\noindent\hrulefill

\makeatletter
\pgfmathdeclarefunction{anglearray2}{3}{%
  % #1 an array (represented in pgfmath internal format, ie 
  % {<index 0>}{<index 1>}...{<index N-1>})
  % #2 \I
  % #3 \L
  \pgfmathparse{Mod(#2,#3)}
  % The @ is needed (see pgfmanual, math engine, custumizing the math
  % engine) 
  \pgfmatharray@{#1}{\pgfmathresult}
  \pgfmathparse{anglearray1(\pgfmathresult,#2,#3)}}
\makeatother

% Spin around a circle stopping at 10 and 190 degrees; doesn't work
\foreach \i in {-10,...,10} {
  \pgfmathsetmacro{\X}{anglearray2({10,190},\i,2)}
  \X\par}

\end{document}

Answer 2

Here is a goofy answer. It appears that by the time your code in declare function is called, the array has already been turned into a sequence of braced entries, so the PGF array {1,2,3,4} becomes the sequence of macro aguments {1}{2}{3}{4}. Obviously none of the PGF math functions work on such things, so one has to use more basic tex programming. Such programming appears to be well beyond me, so instead of worrying about the "tail end" of things, I instead work on the "head":

\def\anglearray(#1,#2,#3){array({#1},Mod(#2,#3)) + 360*floor((#2+0.1)/#3)}

This is used as:

\pgfmathsetmacro{\X}{\anglearray({10,190},\i,2)}

Notice the leading backslash since this is a tex macro, not a pgf math function.