Feature request: is coordinate defined

[daleif]

Often when I do trace images for people, I make a lot of helper coordinates. It is useful to be able to mark them. But I do not want to maintain a list of my defined coordinates, nor do I want to number my coordinates sequentially. At one point I came up with the MWE below. It works for normal coordinates, but not for things like node positions.

So I'm requesting a feature one can use to determine whether a coordinate/.. is defined.

\documentclass[tikz]{standalone}
\makeatletter
% handy macro, then the loop does not need to be over only defined nodes
\def\ifcoordinatedefined#1#2#3{%
  \@ifundefined{pgf@sh@ns@#1}{#3}{#2}%
}
\makeatother

\newcommand\ShowPoints[1][0,1,...,200]{
  \begingroup
  \foreach\n in{#1}{
    \ifcoordinatedefined{T\n}{
      \fill[green,opacity=0.75] (T\n) circle (1pt) node[below,red] {\tiny\n};
    }{}
  }
  \endgroup
}

\begin{document}

\begin{tikzpicture}
  \draw (0,0) circle (1);
  \coordinate (T0) at (0,0);
  \coordinate (T1) at (1,0);
  \node[draw,anchor=west] (A) at (T1) {Test};

  % this works
  \ShowPoints

  % this does not work
  \ifcoordinatedefined{A.north east}{
    \fill[magenta] (A.north east) circle (2pt);
  }{}
  
  
\end{tikzpicture}

\end{document}

[hmenke]

A.north east is not a coordinate but a node anchor.

[daleif]

Thanks, I did not know the proper terminology for A.north east

[hmenke]

You can use this bit to check for the existence of an anchor.

pgf/tex/generic/pgf/modules/pgfmoduleshapes.code.tex

Lines 487 to 499 in fcb8edb

	% Invoke an anchor
	\def\pgf@sh@reanchor#1#2{%
	\pgfutil@ifundefined{pgf@anchor@#1@#2}%
	{%
	\pgfutil@ifundefined{pgf@anchor@generic@#2}{%
	\pgfmathsetcounter{pgf@counta}{#2}%
	\csname pgf@anchor@#1@border\endcsname{\pgfqpointpolar{\the\c@pgf@counta}{1pt}}%
	}{%
	\csname pgf@anchor@generic@#2\endcsname{#1}%
	}%
	}%
	{\csname pgf@anchor@#1@#2\endcsname}%
	}%

The problem is that A.180 is a valid anchor, so if you try to reference a non-existent anchor, like A.weest, PGF will try to evaluate weest as a math function which will fail.

pgf/tex/generic/pgf/modules/pgfmoduleshapes.code.tex

Line 492 in fcb8edb

\pgfmathsetcounter{pgf@counta}{#2}%

There is no way to check whether something is a valid math expression or not prior to evaluation.

[daleif]

Then it seems best to at least just check for existence on coordinates. Then users will just have to make aliases for anchors if they want to use the test.

Wouldn't that be doable?

(I have yet to actually need to test on an anchor)

[hmenke]

I have a possible patch ready here hmenke@98b9d6b but I'm wondering whether this is so useful.

[daleif]

can that test coordinates as well? I don't quite understand it.

[hmenke]

This is just the same definition as you had for \ifcoordinatedefined but renamed to \pgfifnamedshapeundefined and with true and false branch flipped to match the new name.

[hmenke]

I figured that this is not really worth providing in the PGF core, because it ca be implemented by a user in literally a single line.