Is it possible to use a macro to name a TikZ node?

Question

Is it possible to use a macro to name and reference TikZ nodes. For example:

\begin{tikzpicture}
    \node (0) at (0,0) {X};
    \node (\mymacro{2}{3}) at (1,2) {Y};
    \draw (\mymacro{2}{3}) -- (0);
\end{tikzpicture}

Where \mymacro is (for example):

\newcount\counterA
\def\mymacro#1#2%
{\counterA=#1\relax%
\advance\counterA by#2\relax%
\the\counterA}

The actual calculation that my macro computes uses \loop\ifnum (which I think) means I cannot use the solution in Modular arithmetic on node names in TikZ? without defining a new \pgfmathMyMacro function.

Is this a limitation of TikZ or is there some \edef or \expandafter black magic I could use?

Answer 1

The problem is due to expansion. TikZ names the node via an \edef so your macro for determining the node has to be expandable in an \edef. I don't know the exact rules of what is and isn't expandable, but your macro using \count is not expandable. You can see this by doing:

\def\test#1#2{%
\countA#1
\advance\countA by #2
\the\countA}
\edef\testit{\test{2}{3}}
\show\testit

The result is:

> \testit=macro:
->\countA 2 \advance \countA by 3 0.

Anything using \pgfmathparse is also not expandable, and I'd guess that whatever else you tried was not expandable.

Fortunately, those LaTeX3 gurus are masters of expansion and their integer stuff seems to be expandable. Thus:

\usepackage{expl3}
\ExplSyntaxOn
\def\mymacro#1#2%
{\int_eval:n {#1 + #2}}
\ExplSyntaxOff

\edef\testit{\mymacro{2}{3}}
\show\testit

Result:

> \testit=macro:
->5.

And, indeed, the following works. The node is correctly labelled (5).

\documentclass{article}
%\url{http://tex.stackexchange.com/q/27769/86}
\usepackage{expl3}
\usepackage{tikz}

\ExplSyntaxOn
\newcount\countA
\def\mymacro#1#2%
{\int_eval:n {#1 + #2}}
\ExplSyntaxOff

\begin{document}
\begin{tikzpicture}
    \node (0) at (0,0) {X};
    \node (\mymacro{2}{3}) at (1,2) {Y};
    \draw (\mymacro{2}{3}) -- (0);
    \draw (5) -- +(1,0);
\end{tikzpicture}
\end{document}

Answer 2

This seems to work:

\documentclass{article}
\usepackage{tikz}

\newcommand*{\mymacro}[2]{#1+#2}%

\begin{document}
\begin{tikzpicture}
    \node (0) at (0,0) {X};
    \node (\mymacro{2}{3}) at (1,2) {Y};
    \draw (\mymacro{2}{3}) -- (0);
\end{tikzpicture}
\end{document}

As Andrew mentions, this unfortunately results in the node being named 2+3 and not 5. To fix this, I attempted to use the solution from this question on pgfmath expansion - call a command from within a pgfmath environment as follows:

\newcommand{\mathresult}[1]{%
    \pgfmathparse{#1}\pgfmathsetmacro\mymathresult{\pgfmathresult}%
}
\newcommand{\mymacro}[2]{%
    \mathresult{#1}%
    \pgfmathparse{#2 + \mymathresult}%
    \pgfmathsetmacro\mymathresult{\pgfmathresult}\mathresult%
}%

but this results in a Incomplete \iffalse when it is used. I simplified the above as much as I could but could not eliminate that error message.

See follow up question: Incomplete \iffalse using pgfmath naming a TikZ node which shows that you'll need to be careful in only choosing integer values, as decimal values do not work as labels.