pgfmath in graph library giving compilation errors

Question

I want to represent a graph where the labels for each node are obtained computing a modulo computation. The first and easy way to do it is as follow :

\documentclass[tikz]{standalone}
\usetikzlibrary{graphs,graphs.standard}

\begin{document}
\begin{tikzpicture}
    \def \n {3}
    \graph[clockwise=\n,radius=3cm] {
        \foreach \x [evaluate=\x as \xn using {int(mod(\x+1,\n))}] in {0,...,\the\numexpr\n-1} {
            \foreach \y [evaluate=\y as \yn using {int(mod(\y+1,\n))}] in {\x,...,\the\numexpr\n-1} {
                \x / {$a_\x, a_\xn$} --
                \y / {$a_\y, a_\yn$};
            };
        };
    };
\end{tikzpicture}

\end{document}

Then I found out about pgfmathtruncatemacro and wrote

\documentclass[tikz]{standalone}
\usetikzlibrary{graphs,graphs.standard}

\begin{document}
\begin{tikzpicture}
    \def \n {3}
    \graph[clockwise=\n,radius=3cm] {
        \foreach \x in {0,...,\the\numexpr\n-1} {
            \pgfmathtruncatemacro{\xn}{mod(\x+1,\n)}%
            \foreach \y in {\x,...,\the\numexpr\n-1} {
                \pgfmathtruncatemacro{\yn}{mod(\y+1,\n)}%
                \x / {$a_\x, a_\xn$} --
                \y / {$a_\y, a_\yn$};
            };
        };
    };
\end{tikzpicture}

\end{document}

And strangely it gives me these errors :

! Undefined control sequence. \pgfmathsetcount ... \pgfmath@onquick #2\pgfmath@ {\afterassignment \pgfmath... l.16 } ; The control sequence at the end of the top line of your error message was never \def'ed. If you have misspelled it (e.g., \hobx'), typeI' and the correct spelling (e.g., `I\hbox'). Otherwise just continue, and I'll forget about whatever was undefined.

! Undefined control sequence. \tikz@lib@graph@handle@node@cont ...kzgraphnodeas \tikzgraphnodeas@default ... l.16 } ; The control sequence at the end of the top line of your error message was never \def'ed. If you have misspelled it (e.g., \hobx'), typeI' and the correct spelling (e.g., `I\hbox'). Otherwise just continue, and I'll forget about whatever was undefined.

! Undefined control sequence. \tikz@lib@graph@typesetter ->\tikzgraphnodetext l.16 } ; The control sequence at the end of the top line of your error message was never \def'ed. If you have misspelled it (e.g., \hobx'), typeI' and the correct spelling (e.g., `I\hbox'). Otherwise just continue, and I'll forget about whatever was undefined.

) ! Incomplete \iffalse; all text was ignored after line 16. \fi

And I really can't understand why it wouldn't work.

[EDIT] I wrote my second code based on this answer. The part where I write \n - 1 without \the\numexpr is not needed when working in Overleaf so it's NOT the problem here. And I don't want to have \pgfmathtruncatemacro in the labelling of the node since the answer I linked works perfectly for me too without needing it. My problem is that it looks buggy when using it in the \graph environment.

[EDIT 2] Added \the\numexpr since it would not compile on every installation of TL and since it was not this that made the program not work.

Answer 1

The \graph macro changes the syntax: you can't use TeX code directly. You can use the /utils/exec key to execute some arbitrary TeX code.

Here is a solution with the final version of TeXLive 2018 (used by Overleaf, cf. TeX Live upgrade Sep.2019):

\documentclass[tikz]{standalone}
\usetikzlibrary{graphs,graphs.standard}
\begin{document}
\begin{tikzpicture}
    \def \n {3}
    \graph[clockwise=\n,radius=3cm] {
        \foreach \x in {0,...,\n-1} {
            [/utils/exec={\pgfmathtruncatemacro{\xn}{mod(\x+1,\n)}}]%
            \foreach \y in {\x,...,\n-1} {
                [/utils/exec={\pgfmathtruncatemacro{\yn}{mod(\y+1,\n)}}]%
                \x / {$a_\x, a_\xn$} --
                \y / {$a_\y, a_\yn$};
            };
        };
    };
\end{tikzpicture}
\end{document}

And, here is a solution with TeXLive 2019:

\documentclass[tikz]{standalone}
\usetikzlibrary{graphs,graphs.standard}
\begin{document}
\begin{tikzpicture}
  \def\n{3}
  \graph[clockwise=\n,radius=3cm] {
    \foreach[parse=true] \x in {0,...,\n-1} {
      [/utils/exec={\pgfmathtruncatemacro{\xn}{mod(\x+1,\n)}}]
      \foreach[parse=true] \y in {\x,...,\n-1} {
        [/utils/exec={\pgfmathtruncatemacro{\yn}{mod(\y+1,\n)}}]
        \x / {$a_\x, a_\xn$} --
        \y / {$a_\y, a_\yn$};
      }
    }
  };
\end{tikzpicture}
\end{document}

Here is a solution with TeXLive 2019 using \pgfmathsetmacro to compute \xmax (without the parse option that still does not work very well):

\documentclass[tikz]{standalone}
\usetikzlibrary{graphs,graphs.standard}
\begin{document}
\begin{tikzpicture}
  \def\n{3}
  \graph[clockwise=\n,radius=3cm] {
    [/utils/exec={\pgfmathsetmacro\xmax{int(\n-1)}}]
    \foreach \x in {0,...,\xmax} {
      [/utils/exec={\pgfmathtruncatemacro{\xn}{mod(\x+1,\n)}}]
      \foreach \y in {\x,...,\xmax} {
        [/utils/exec={\pgfmathtruncatemacro{\yn}{mod(\y+1,\n)}}]
        \x / {$a_\x, a_\xn$} --
        \y / {$a_\y, a_\yn$};
      }
    }
  };
\end{tikzpicture}
\end{document}

Answer 2

This is a compilable version of your first example. I do not see any benefit in using \pgfmathtruncatemacro since you'd also have to wrap it into \pgfextra and so on if you want to use it in a path.

\documentclass[tikz]{standalone}
\usetikzlibrary{graphs,graphs.standard}

\begin{document}
\begin{tikzpicture}
    \def \n {3}
    \graph[clockwise=\n,radius=3cm] {
        \foreach \x [evaluate=\x as \xn using {int(mod(\x+1,\n))}] in
        {0,...,\the\numexpr\n-1} {
            \foreach \y [evaluate=\y as \yn using {int(mod(\y+1,\n))}] in
            {\x,...,\the\numexpr\n-1} {
                \x / {$a_\x, a_\xn$} --
                \y / {$a_\y, a_\yn$}
            }
        }
    };
\end{tikzpicture}

\end{document}

And here is a version where \pgfmathtruncatemacro gets used. Luckily in this application the result is not needed for the path but only for typesetting. So you can add \pgfmathtruncatemacro just to the stuff you want to typeset.

\documentclass[tikz]{standalone}
\usetikzlibrary{graphs,graphs.standard}

\begin{document}
\begin{tikzpicture}
    \def \n {3}
    \graph[clockwise=\n,radius=3cm] {
        \foreach \x  in {0,...,\the\numexpr\n-1} {
            \foreach \y in {\x,...,\the\numexpr\n-1} {               
                \x / {$\pgfmathtruncatemacro{\xn}{int(mod(\x+1,\n))}a_\x, a_\xn$} --
                \y / {$\pgfmathtruncatemacro{\yn}{int(mod(\y+1,\n))}a_\y, a_\yn$}
            }
        }
    };
\end{tikzpicture}

\end{document}