Tikzmath, ERROR: Missing \endcsname inserted

Question

I'm trying to use tikzmath to draw lines between a set of points, however I've an error ERROR: Missing \endcsname inserted. on the line \ni=\i+1;. Any idea why?

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{math,calc}

\begin{document}

\begin{tikzpicture}
  \tikzmath{
    \n=5;
  }
  \foreach \i in {-\n,...,\n} {
    \foreach \j in {-\n,...,\n} {
      \node[circle,fill,inner sep=1pt] at ($(\i+.3*\j,.5*\j)$) (n-\i-\j) {};
    }
  }
  \foreach \i in {-\n,...,\n-1} {
    \foreach \j in {-\n,...,\n-1} {
      \tikzmath{%
        \ni=\i+1;
        \nj=\j+1;
      };
      \draw[] (n-\i-\j) -- (n-\i-\nj);
    }
  }
\end{tikzpicture}
\end{document}

Answer 1

Due to how \tikzmath works, you can’t use variable names that correspond to already defined commands that point to mathematical symbols.

The standard definition of \ni is by \mathchardef\ni="3233 (in a glorified form in fontmath.ltx) and TeX considers \mathchardef tokens as numbers when it’s looking for them.

The same error happens, for instance, if I use \propto instead of \ni.

Use another name.

With other fixes, as suggested by percusse:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{math,calc}

\begin{document}
\pagestyle{empty}

\begin{tikzpicture}
  \tikzmath{
    \n=5;
  }
  \foreach \i in {-\n,...,\n} {
    \foreach \j in {-\n,...,\n} {
      \node[circle,fill,inner sep=1pt] at ($(\i+.3*\j,.5*\j)$) (\n-\i-\j) {};
    }
  }
  \foreach \i in {-\n,...,\numexpr\n-1} {
    \foreach \j in {-\n,...,\numexpr\n-1} {
      \tikzmath{%
        \nni=\i+1;
        \nj=\j+1;
      };
      \draw[] (\n-\i-\j) -- (\n-\i-\nj);
    }
  }
\end{tikzpicture}
\end{document}

Answer 2

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{math,calc}
\usepackage{verbatim}

\begin{document}
\pagestyle{empty}

\begin{tikzpicture}
\def\n{5}
  \foreach \i in {-\n,...,\n} {
    \foreach \j in {-\n,...,\n} {
      \node[circle,inner sep=1pt] at ($(\i+.3*\j,.5*\j)$) (n-\i-\j) {};
    }
  }
  \foreach \i [evaluate=\i as \ni using int(\i+1)] in {-\n,...,\numexpr\n-1} {
    \foreach \j [evaluate=\j as \nj using int(\j+1)] in {-\n,...,\numexpr\n-1} {
      \draw (n-\i-\j) -- (n-\ni-\nj);
    }
  }
\end{tikzpicture}
\end{document}