Tikz foreach path between nodes starts at wrong position

Question

This picture

has been generated by following code. In red circles the edges start/end not on the correct side of the node.

\documentclass[tikz]{standalone}

% ==================================================
% GENREAL OBLIQUE CRYSTALLOGRAPHIC COORDINATE SYSTEM
% ==================================================
\makeatletter 
\tikzdeclarecoordinatesystem{general}
{%
  {%
    \pgf@xa=0pt% point
    \pgf@ya=0pt%
    \pgf@xb=0pt% sum
    \tikz@bary@dolist#1,=,%
    \pgfmathparse{1}%
    % modified from copy
    % /usr/local/texlive/2018/texmf-dist/tex/generic/pgf/frontendlayer/tikz/tikz.code.tex
    \global\pgf@x=\pgfmathresult\pgf@xa%
    \global\pgf@y=\pgfmathresult\pgf@ya%
  }%
}%
\makeatother 

\begin{document}
\begin{tikzpicture}

    \coordinate (A) at (0,-2);
    \coordinate (B) at (3,0);

    \path foreach \na in {0,...,2} { foreach \nb in {0,...,2} {
    % Wyckoff letter 'a'
      (general cs:A=\na,B=\nb)
        node (a\na\nb) {2}
        % node     {a}
        node[yshift=0.5cm] {a\na\nb}
    % Wyckoff letter 'b'
      \ifnum \nb<2
       (general cs:A=\na,B=\nb+0.5)
       node (b\na\nb) {2}
        % node {b}
        node[yshift=0.5cm] {b\na\nb}
      \fi
    }
    };

    % PROBLEM
    \foreach \nb [evaluate=\nb as \nbnext using \nb+1] in {0,...,2} {
      \ifnum \nb<2
        \draw (a0\nb) -- (b0\nb) -- (a0\nbnext);
      \fi
    };

    % NO PROBLEM:
    % \draw (a00) -- (b00) -- (a01);
    % \draw (a01) -- (b01) -- (a02);

\end{tikzpicture}
\end{document}

There is no problem when I use the commented code

\draw (a00) -- (b00) -- (a01);
\draw (a01) -- (b01) -- (a02);

I think my foreach loop produces the exact same code as I would write manually and I am completely suprised that tikz struggles here.

Update due to answer by Schrödinger's cat:

As pointed out simple integer arithmetic still results in a fixed point width number, e.g. 1+1=2.0. For instance, this is implicitly shown in section 95.3.1 Basic artihmetic functions of the pgf/tikz documentation of v3.1.5b:

81.0 \pgfmathparse{add(75,6)} \pgfmathresult

It can be verified by printing a0\nbext as the content of a node.

In this cirumstance this has the consequence that .0 is interpreted as anchor specification. The pgf/tikz documentation (Section 17.2.1 Syntax of the Node Command) already says that periods should not occur in node names:

Assigns a name to the node for later reference. Since this is a “high-level” name (drivers never know of it), you can use spaces, number, letters, or whatever you like when naming a node. Thus, you can name a node just 1 or perhaps start of chart or even y_1. Your node name should not contain any punctuation like a dot, a comma, or a colon since these are used to detect what kind of coordinate you mean when you reference a node.

Related questions:

• TikZ: Variable arithmetic result in node name
• node names and nested loops in TikZ

Answer 1

It is the usual problem that, if you do not explicitly say you want integers, TikZ adds .0, which gets interpreted as an anchor, the east anchor in this case. All I did was to replace

[evaluate=\nb as \nbnext using \nb+1]

by

[evaluate=\nb as \nbnext using {int(\nb+1)}] 

to obtain

\documentclass[tikz]{standalone}

% ==================================================
% GENREAL OBLIQUE CRYSTALLOGRAPHIC COORDINATE SYSTEM
% ==================================================
\makeatletter 
\tikzdeclarecoordinatesystem{general}
{%
  {%
    \pgf@xa=0pt% point
    \pgf@ya=0pt%
    \pgf@xb=0pt% sum
    \tikz@bary@dolist#1,=,%
    \pgfmathparse{1}%
    % modified from copy
    % /usr/local/texlive/2018/texmf-dist/tex/generic/pgf/frontendlayer/tikz/tikz.code.tex
    \global\pgf@x=\pgfmathresult\pgf@xa%
    \global\pgf@y=\pgfmathresult\pgf@ya%
  }%
}%
\makeatother 

\begin{document}
\begin{tikzpicture}

    \coordinate (A) at (0,-2);
    \coordinate (B) at (3,0);

    \path foreach \na in {0,...,2} { foreach \nb in {0,...,2} {
    % Wyckoff letter 'a'
      (general cs:A=\na,B=\nb)
        node (a\na\nb) {2}
        % node     {a}
        node[yshift=0.5cm] {a\na\nb}
    % Wyckoff letter 'b'
      \ifnum \nb<2
       (general cs:A=\na,B=\nb+0.5)
       node (b\na\nb) {2}
        % node {b}
        node[yshift=0.5cm] {b\na\nb}
      \fi
    }
    };

    % PROBLEM
    \foreach \nb [evaluate=\nb as \nbnext using {int(\nb+1)}] in {0,...,2} {
      \ifnum \nb<2
        \draw (a0\nb) -- (b0\nb) -- (a0\nbnext);
      \fi
    };

    % NO PROBLEM:
    % \draw (a00) -- (b00) -- (a01);
    % \draw (a01) -- (b01) -- (a02);

\end{tikzpicture}
\end{document}