Generating a Star Diagram: Tikz Evaluation in LuaLatex

Question

I have run the following code in LuaLatex. I'm doing everything on overleaf, if that matters.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{graphs,graphdrawing,arrows.meta,graphs.standard}
\usegdlibrary{circular}
\begin{document}
\begin{tikzpicture}
    \graph [
    simple necklace layout, node distance=1.5cm,
    nodes={draw,circle}
    ]
    {
        subgraph C_n [n=8]; 
        \foreach \i in {1,...,5}{
            \i -- \directlua{tex.sprint(3+\i)};
        }
    };
\end{tikzpicture}
\vspace{20 pt}
\begin{tikzpicture}
    \graph [
    simple necklace layout, node distance=1.5cm,
    nodes={draw,circle}
    ]
    {
        subgraph C_n [n=8]; 
        \foreach \i in {1,...,5}{
            \i -- \directlua{tex.sprint(mod({3+\i},8))};
        }
    };
\end{tikzpicture}
\end{document}

The two Tikz pictures have the same code except that in the second loop, 3+\i is replaced by mod({3+\i},8)). Because 3 + i % 8 is equal to 3 + i for i = 1,...,4, I should have ended up with essentially the same picture both times. However, here is what I end up with:

How can I fix this code so that the modular arithmetic is evaluated correctly?

Once I have that working, the idea is to extend the loop to {1,...,8} in order to get the full "star".

Of course, I would have to fix the off by one error before getting the loop to actually work, but the issue with modular arithmetic stands — Ben Grossmann, Dec 18 '20 at 02:53
mod({3+\i},8) is not valid Lua. Try math.mod(3+\i,8). Also you fell for the typical LuaTeX gotcha: tex.sprint(<number>) will not do the correct thing. Always use tex.sprint(tostring(<number>)). — Henri Menke, Dec 18 '20 at 09:42
Related: https://tex.stackexchange.com/questions/550673/what-are-some-typical-luatex-gotchas — Henri Menke, Dec 18 '20 at 09:48

score 3 · Accepted Answer · answered Dec 18 '20 at 03:08

I would just use pgf to compute the mod. Note that mod returns numbers starting from 0 so you probably want int(1+mod(2+\i,8)) instead of int(mod(3+\i,8)).

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{graphs,graphdrawing,arrows.meta,graphs.standard}
\usegdlibrary{circular}
\begin{document}
\begin{tikzpicture}
    \graph [
    simple necklace layout, node distance=1.5cm,
    nodes={draw,circle}
    ]
    {
        subgraph C_n [n=8]; 
        \foreach \i in {1,...,5}{
            \i -- \directlua{tex.sprint(3+\i)};
        }
    };
\end{tikzpicture}
\vspace{20 pt}
\begin{tikzpicture}
    \graph [
    simple necklace layout, node distance=1.5cm,
    nodes={draw,circle}
    ]
    {
        subgraph C_n [n=8]; 
        \foreach \i [evaluate=\i as \j using {int(1+mod(2+\i,8))}]in {1,...,5}{
            \i -- \j;
        }
    };
\end{tikzpicture}
\vspace{20 pt}
\begin{tikzpicture}
    \graph [
    simple necklace layout, node distance=1.5cm,
    nodes={draw,circle}
    ]
    {
        subgraph C_n [n=8]; 
        \foreach \i [evaluate=\i as \j using {int(1+mod(2+\i,8))}]in {1,...,8}{
            \i -- \j;
        }
    };
\end{tikzpicture}
\end{document}

For whatever reason, when I tried something like this earlier the expressions weren't evaluated properly; I had decided that the issue was using pgf in the context was the issue, hence the weird approach. In any case, this works! Thanks for the help. — Ben Grossmann, Dec 18 '20 at 03:46

Generating a Star Diagram: Tikz Evaluation in LuaLatex

1 Answers1