TikZ Path – Labeling Paths and Using –cycle Effectively in TikZ

nodestikz-pgf

\draw (0,0) -- node{a} (2,0) -- node{b}  (2,1)  -- node{c}  (0,1)  -- cycle;

I can't find a simple way to make a label on the last edge. What I want is something like

\draw (0,0) -- node{a} (2,0) -- node{b}  (2,1)  -- node{c}  (0,1)  -- node{d} cycle;

but it doesn't work.

\draw (0,0) coordinate(A) -- node{a} (2,0) -- node{b}  (2,1)  -- 
                             node{c}  (0,1) coordinate(D) -- cycle;
\draw (A) -- node{d} (D);

This works, but seems too complicated for such a simple task and I think I'm missing something.

Best Answer

The problem is that the cycle option short circuits the system for putting nodes on paths. The usual path operations have a system whereby they record their path component so that TikZ can figure out where to put nodes afterwards. The -- (lineto) component is no exception to this. The problem is that the cycle interrupts the lineto before it can record the necessary information.

A fix would be to put this information back in. I don't know how robust this fix is, but it works for your example.

\documentclass{article}
%\url{http://tex.stackexchange.com/q/97602/86}
\usepackage{tikz}

\makeatletter
\def\tikz@@close cycle{%
  \edef\tikz@timer@start{\noexpand\pgfqpoint{\the\tikz@lastx}{\the\tikz@lasty}}
  \tikz@flush@moveto%
  \tikz@path@close{\expandafter\pgfpoint\pgfsyssoftpath@lastmoveto}%
  \edef\tikz@timer@end{\noexpand\pgfpoint\pgfsyssoftpath@lastmoveto}%
  \def\pgfstrokehook{}%
  \let\tikz@timer=\tikz@timer@line%
  \tikz@scan@next@command%
}
\makeatother


\begin{document}
\begin{tikzpicture}
\draw (0,0) -- node{a} (2,0) -- node{b}  (2,1)  -- node[pos=.3]{c}  (0,1)  -- node[pos=.3]{d} cycle;
\end{tikzpicture}
\end{document}

closed path with labels

Related Solutions

TikZ-PGF – Using the Cycle Option When Drawing Between Nodes

When you specify a bare node as a coordinate, as in (a), then TikZ tries to be helpful and interpret that as the most appropriate target on a. Most of the time, this is exactly what you want: \tikz \draw[->] (0,0) node (a) {A} (1,0) node (b) {B} (a) -- (b); typesets A → B, with the arrow starting a little to the right of the A and ending a little to the left of the B.

However, when doing a "cycle", you don't want that behaviour. You want the lines to join up. Even empty nodes have size, unless you explicitly tell them not to. So you need to override this "helpfulness" which is done by specifying which target you mean. Thus:

\node (a) at (0,0) {};
\node (b) at (1,0) {};
\node (c) at (0.5, -1) {};
\draw (a.center) -- (b.center) -- (c.center) -- cycle;

does what you want it to.

There are other ways to do this, in particular as your nodes are empty you could force them to have zero size. Indeed, \coordinate is actually a shorthand for \node[coordinate] and the special shape coordinate has empty size. So your first example, which works, is actually a special case of the second where the nodes are fiddled to be "point masses".

Draw Inside a TikZ Node Using Node Style – How to Draw Inside TikZ Nodes

All TikZ node shapes are defined using lower-level PGF code. This is described in the pgfmanual in section 75.5 Declaring New Shapes, on page 625 of the v2.1 manual.

You can use the existing code of the circle shape as a base and add the lines to it. The code can be found in the file ${TEXMF}/tex/generic/pgf/modules/pgfmoduleshapes.code.tex.

Here the way I would do it:

Shape declaration:

\pgfdeclareshape{oplus}
%
% Shaped like '\oplus' math symbol. Based on 'circle' shape
%
{%
  % All anchors are taken from the 'circle' shape:
  \inheritsavedanchors[from={circle}]%
  \inheritanchor[from={circle}]{center}%
  \inheritanchor[from={circle}]{mid}%
  \inheritanchor[from={circle}]{base}%
  \inheritanchor[from={circle}]{north}%
  \inheritanchor[from={circle}]{south}%
  \inheritanchor[from={circle}]{west}%
  \inheritanchor[from={circle}]{east}%
  \inheritanchor[from={circle}]{mid west}%
  \inheritanchor[from={circle}]{mid east}%
  \inheritanchor[from={circle}]{base west}%
  \inheritanchor[from={circle}]{base east}%
  \inheritanchor[from={circle}]{north west}%
  \inheritanchor[from={circle}]{south west}%
  \inheritanchor[from={circle}]{north east}%
  \inheritanchor[from={circle}]{south east}%
  \inheritanchorborder[from={circle}]%
  %
  % Only the background path is different
  %
  \backgroundpath{%
    % First the existing 'circle' code:
    \pgfutil@tempdima=\radius%
    \pgfmathsetlength{\pgf@xb}{\pgfkeysvalueof{/pgf/outer xsep}}%
    \pgfmathsetlength{\pgf@yb}{\pgfkeysvalueof{/pgf/outer ysep}}%
    \ifdim\pgf@xb<\pgf@yb%
      \advance\pgfutil@tempdima by-\pgf@yb%
    \else%
      \advance\pgfutil@tempdima by-\pgf@xb%
    \fi%
    \pgfpathcircle{\centerpoint}{\pgfutil@tempdima}%
    %
    % Now the | and -- lines:
    \pgfmoveto{\pgfpointadd{\centerpoint}{\pgfpoint{0pt}{\pgfutil@tempdima}}}%
    \pgflineto{\pgfpointadd{\centerpoint}{\pgfpoint{0pt}{-\pgfutil@tempdima}}}%
    \pgfmoveto{\pgfpointadd{\centerpoint}{\pgfpoint{\pgfutil@tempdima}{0pt}}}%
    \pgflineto{\pgfpointadd{\centerpoint}{\pgfpoint{-\pgfutil@tempdima}{0pt}}}%
  }%
}

Usage example:

\documentclass{standalone}

\usepackage{tikz}
\usepackage{pgfshape_oplus}

\begin{document}
\begin{tikzpicture}
  \node [draw=blue,shape=oplus] {Test};
\end{tikzpicture}
\end{document}

Result:

Result

You might also want to draw the additional lines in the \foregroundpath instead. For this see the forbidden sign shape in the file pgflibraryshapes.symbols.code.tex is an good example.