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I'm trying to draw two arrows between nodes as follows:

A ------> B
  <--/---

The arrows should be parallel, in opposite directions but not on top of each other. (I can bend the arrows but that's not what I'm after.)

What's the cleanest way to do this in TikZ? And can I have just one of the arrows "struck out" as in the example?

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3 Answers 3

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Here is an idea that might answer both your questions. It uses yshift to move the start and end points of the two paths up/down by 5pt. The result could be further improved by shifting the start/end points along the x axis in order for the paths to really be in contact with the nodes' circles.

A custom TikZ style is used to add a strike-through marking at the middle of the second path. This could be parameterized further to allow moving the marking to any position on the path etc.

I cannot tell whether this is the most elegant solution though.

\documentclass{minimal}

\usepackage{tikz}

\usetikzlibrary{decorations,decorations.markings}

\tikzset{
  strike through/.style={
    postaction=decorate,
    decoration={
      markings,
      mark=at position 0.5 with {
        \draw[-] (-5pt,-5pt) -- (5pt, 5pt);
      }
    }
  }
}

\begin{document}
\begin{tikzpicture}
  \node[draw,circle] (foo) at (0,0) { foo };
  \node[draw,circle] (bar) at (4,0) { bar };

  \draw[>=latex,->]                ([yshift= 5pt] foo.east) -- ([yshift= 5pt] bar.west);
  \draw[>=latex,<-,strike through] ([yshift=-5pt] foo.east) -- ([yshift=-5pt] bar.west);
\end{tikzpicture}
\end{document}

Here is how it looks like:

Screenshot of the result

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  • 8
    Note that you can apply simple shifts on coordinates by using the yshift, xshift or shift options on the coordinate: ([yshift=2pt]foo.east) instead of ($ (foo.east) + (0, 2pt) $). This is often not known. It does not require the calc library.
    – Martin Scharrer
    May 12, 2011 at 16:16
  • Interesting, I didn't know that syntax. I tried yshift in the arguments to \draw as well as after -- (also tried edge instead of --) but neither of them worked. Thanks for the hint.
    – Jannis Pohlmann
    May 12, 2011 at 16:21
  • 1
    @Jannis: If you operate on named coordinates instead of direct ones, you further need transform canvas.
    – Andrey Vihrov
    May 12, 2011 at 16:22
  • @Andrey: Ah, alright.
    – Jannis Pohlmann
    May 12, 2011 at 16:24
  • 1
    @Andrey: It seems to be an issue if you use the node name without an anchor, because then the border anchor is taken which depends on the position of the target coordinate. That's basically the difference between (A) and (A.center). It seems to work without transform canvas if you use an anchor like (A.west).
    – Martin Scharrer
    May 12, 2011 at 18:12
4

As I think the TikZ answer here is a bit outdated I'll provide a solution that is more automated using pics and with no need for loading any libraries.

The strike through can be drawn as a simple \draw[-] (-135:0.5) -- (45:0.5); with a /.pic and be called upon within the drawing by \pic[options]{strike through} or mid path by pic[options]{strike trhough} (the - option is because pics inherit the path options, so that overwrites any possible arrow tips):

\tikzset{strike through/.pic={\draw[-] (-135:.5) -- (45:.5);}}
\draw[->] (foo) -- pic{strike through} (bar);

enter image description here

For the double arrows, I'm not so fond of shifting because sometimes the shifting values can be tricky. Using bent edges one can achieve a very good result with somewhat little code:

\draw[->] (foo) edge[bend right,draw=none] coordinate[at start](foo-b) coordinate[at end](bar-b) (bar)
            edge[bend left,draw=none] coordinate[at start](foo-t) coordinate[at end](bar-t) (bar)
            (foo-t) -- (bar-t);
\draw[->] (bar-b) -- (foo-b);

enter image description here

Well maybe not so little... But that's very powerful, as no matter where foo and bar are placed the arrows will be perfectly separated! Also how much the arrows are separated can be controlled by the bend angle key. So, let's get down to business: with that idea in mind, is possible to make a style and/or a pic to draw everything leaving our drawing area as clean as possible and the drawing with as much flexibility as TikZ can offer!

We first create a style double arrows={#1-#2} that insert the edges and place coordinates like (#1-t) and (#1-b) for the top and bottom part of node #1 and the same for node #2. Later on we can use this style to create the coordinates so we draw the path manually (that's useful for placing pics and nodes mid path).

\tikzset{double arrows/.style args={#1-#2}{%
           insert path={(#1) edge[bend right,draw=none] coordinate[at start](#1-b) coordinate[at end](#2-b) (#2)
                             edge[bend left,draw=none] coordinate[at start](#1-t) coordinate[at end](#2-t) (#2)
 }}}
\draw[double arrows={foo-bar},->] (foo-t) -- node[above]{$F(x)$} (bar-t);
\draw[->] (bar-b) -- pic{strike through} (foo-b);

Finally, we can make everything very automatic with a \pic[options]{double arrows={foo-bar}}. In order to make everything more tikz-like some styles were created, a every double arrows that's loaded by the double arrows pic, if modified all double arrows pics are modified along with it. Also the bend angle can be given to an specific pic by the double arrows split style (default is 10). Furthermore, a shorten key was also introduced so that the arrows do not touch the nodes (default is 2mm). A complete MWE:

\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{positioning}

\tikzset{strike through/.pic={\draw (-135:.5) -- (45:.5);},
        shorten/.style={shorten <=#1, shorten >=#1},
        every double arrows/.style={-latex, shorten=2mm},
        double arrows split/.style={bend angle=#1},
        double arrows split/.default={10},
        double arrows/.style args={#1-#2}{%
                        insert path={(#1) edge[bend right,draw=none] coordinate[at start](#1-b) coordinate[at end](#2-b) (#2)%
                                          edge[bend left,draw=none] coordinate[at start](#1-t) coordinate[at end](#2-t) (#2)
                                    }
        },
        pics/double arrows/.style args={#1-#2}{%
                        code={%
                              \begin{scope}[double arrows split]
                              \draw[double arrows={#1-#2}, every double arrows] (#1-t) -- (#2-t);%
                              \draw[every double arrows] (#2-b) -- (#1-b);%
                              \end{scope}
                              }
        }%
}

\begin{document}
\begin{tikzpicture}[every node/.style={draw, circle}]
  \node (foo) at (0,0) {foo};
  \node[right=3cm of foo] (bar) {bar};
  \node[below right=1cm and 2cm of foo] (fow) {fow};
  \node[above right=1cm and 1cm of foo] (bow) {bow};

  \begin{scope}[bend angle=20]
    \draw[double arrows={foo-bar}, shorten=4pt, -latex] (foo-t) -- node[above, draw=none, rectangle]{$F(x)$} (bar-t);
    \draw[-latex, shorten=4pt] (bar-b) -- pic[-,red,thick]{strike through} (foo-b);
  \end{scope}

  \pic{double arrows={foo-bow}};
  \pic[every double arrows/.style={->, shorten=0mm}]{double arrows={foo-fow}};
  \pic[red]{double arrows={bar-bow}};
  \pic{double arrows={fow-bar}};
\end{tikzpicture}
\end{document}

enter image description here

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With PSTricks. Unfortunately, no option to change the radius of \circlenode to be uniform.

enter image description here

\documentclass[pstricks,border=12pt]{standalone}
\usepackage{pst-node}
\psset{arrows=->,arrowinset=0}
\begin{document}
\begin{pspicture}[showgrid](5,2)
    \rput(1,1){\circlenode{Left}{left}}
    \rput(4,1){\circlenode{Right}{right}}
    \ncline[offset=5pt]{Left}{Right}
    \ncline[offset=5pt]{Right}{Left}
    \ncput{\psline[arrows=-](5pt;45)(5pt;-135)}
\end{pspicture}
\end{document}
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