Building evaluated arrays

Question

I'm trying to build an array dynamically in latex then use it in tikz.

I'm a little confused about how to create an array and add evaluated mathematical expressions to it (I'm trying arrayjob).

Here's what I have so far; for some reason, I cannot access the values in the second foreach, getting the error (! Undefined control sequence. \pntAngles1~ ->\val), the first commented print in the first foreach works btw:

\documentclass{article}

\usepackage{tikz}

\usepackage{calc}
\usepackage{fp}
\usepackage{arrayjob}
\usepackage{ifthen}
\usepackage{amsmath}
% \usetikzlibrary{arrows,intersections}

\begin{document}

\def\n{8} % number of points
\FPeval{\lastn}{\n-1}
\FPeval{\angleOffset}{clip(360/\n)} % angle offset between dots

\newarray\pntAngles

\foreach \i in {0,...,\lastn} {
  \FPeval{\val}{clip(\i*\angleOffset)}
  \pntAngles(\i)= {\val}
  % \pntAngles(\i)
}

\foreach \i in {0,...,\lastn} {
  \pntAngles(\i)
}

\end{document}    

Would appreciate any advice. Thanks

Answer 1

The arrays defined by arrayjobx (which should be used in LaTeX instead of arrayjob) are local, so if they are assigned values in a group (or a scope, if you prefer), the value will be lost when the group ends.

Also, it seems that arrayjobx doesn't like 0 as an index.

A feature of \foreach is that it executes its cycles in a group, so you're in a blind alley.

You could define a new array type, let's call it a “global array”, but some other tricks are needed; a syntax like

\pntAngles(\i)={\val}

will assign \val to the correspondent array element, not the value of \val.

\documentclass{article}

\usepackage{tikz}

\usepackage{fp}
\usepackage{arrayjobx}
\usepackage{etoolbox}

% define a new data structure
\let\newglobalarray\newarray
\patchcmd{\newglobalarray}{\edef}{\xdef}{}{}

\begin{document}

\def\lastn{8} % number of points
\FPeval{\angleOffset}{clip(360/\lastn)} % angle offset between dots

\newglobalarray\pntAngles

\foreach \i in {1,...,\lastn} {
  \FPeval{\val}{clip((\i-1)*\angleOffset)}
  % expand \val before doing the assignment
  \begingroup\edef\x{\endgroup\noexpand\pntAngles(\i)= {\val}}\x
}

\foreach \i in {1,...,\lastn} {
  \pntAngles(\i)
}

\end{document}

A different approach with expl3 and property lists; sequences are more efficient, but this better emulates arrayjobx that allows any kind of index for its arrays.

\documentclass{article}

\usepackage{tikz}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\newarray}{m}
 {
  \prop_new:c { g_daugmented_array_#1_prop }
 }

\DeclareExpandableDocumentCommand{\arrayget}{mm}
 {
  \prop_get:cx { g_daugmented_array_#1_prop } { #2 }
 }

\NewDocumentCommand{\arrayput}{mmm}
 {
  \prop_gput:cxx { g_daugmented_array_#1_prop } { #2 } { #3 }
 }

\cs_generate_variant:Nn \prop_get:Nn { cx }
\cs_generate_variant:Nn \prop_gput:Nnn { cxx }

\DeclareExpandableDocumentCommand{\fpeval}{m}
 {
  \fp_eval:n { #1 }
 }

\NewDocumentCommand{\fpevalset}{mm}
 {
  \cs_new:Npx #1 { \fp_eval:n { #2 } }
 }
\ExplSyntaxOff

\begin{document}

\fpevalset{\np}{8}
\fpevalset\lastn{\np-1} % number of points
\fpevalset{\angleOffset}{floor(360/\np)} % angle offset between dots

\newarray{pntAngles}

\foreach \i in {0,...,\lastn} {
  \arrayput{pntAngles}{\i}{\fpeval{floor(\i*\angleOffset)}}
}

\foreach \i in {0,...,\lastn} {
  \arrayget{pntAngles}{\i}
}

\end{document}