4

I am drawing a simple graph with tikz. This works OK but when I have two edges going in opposite directions between a pair of nodes I get one arrow with two heads (for example between nodes B and D below). How can I get two arrows? Here is a MWE:

\documentclass[]{article}
\usepackage{tikz}  
\usetikzlibrary{arrows.meta,positioning}
\usepackage{tkz-graph}
\usepackage{graphicx}  
\begin{document}  
\section{}
\begin{center}
    \begin{tikzpicture}[auto,node distance=2cm,thick,main node/.style={circle,fill=blue!20,draw,font=\sffamily\Large\bfseries}, scale=2]
    \node[main node, label=$0$] (A) at (0,5) {A};
    \node[main node, label=$\infty$] (B) at (2,6.5) {B};
    \node[main node, label=$\infty$] (E) at (4,5) {E};
    \node[main node, label=$\infty$] (C) at (1,3) {C};
    \node[main node, label=$\infty$] (D) at (3,3) {D};

    \path [->] (A) edge node {$-1$} (B);
    \path [->] (B) edge node {$2$} (E);
    \path [->] (A) edge node {$4$} (C);
    \path [->] (B) edge node {$3$} (C);
    \path [->] (B) edge node {$2$} (D);
    \path [->] (D) edge node {$1$} (B);
    \path [->] (D) edge node {$5$} (C);
    \path [->] (E) edge node {$-3$} (D);
    \end{tikzpicture}
\end{center}
\end{document}

2 Answers 2

6

Here is a proposal using the calc library to shift the paths by some amount away from the center. Of course, if you do that very often, you may write a macro for that. Or you just bend the paths, as illustrated in the arrows between the B and D nodes.

\documentclass[]{article}
\usepackage{tikz}  
\usetikzlibrary{arrows.meta,positioning,calc}
\usepackage{tkz-graph}
\usepackage{graphicx}  
\begin{document}  
\section{}
\begin{center}
    \begin{tikzpicture}[auto,node distance=2cm,thick,main node/.style={circle,fill=blue!20,draw,font=\sffamily\Large\bfseries}, scale=2]
    \node[main node, label=$0$] (A) at (0,5) {A};
    \node[main node, label=$\infty$] (B) at (2,6.5) {B};
    \node[main node, label=$\infty$] (E) at (4,5) {E};
    \node[main node, label=below:$\infty$] (C) at (1,3) {C};
    \node[main node, label=below:$\infty$] (D) at (3,3) {D};

    \path [->] (A) edge node {$-1$} (B);
    \path [->] (B) edge node {$2$} (E);
    \path [->] (A) edge node {$4$} (C);
    \path [->] (B) edge node {$3$} (C);
    \path [->] let \p1=($(D)-(B)$),\n1={atan2(\y1,\x1)},\n2={180+\n1} in
     ($ (B.\n1)!2pt!90:(D.\n2) $) edge node {$2$} ($ (D.\n2)!2pt!-90:(B.\n1) $);
    \path [->] let \p1=($(B)-(D)$),\n1={atan2(\y1,\x1)},\n2={180+\n1} in
     ($ (D.\n1)!2pt!90:(B.\n2) $) edge node {$-2$} ($ (B.\n2)!2pt!-90:(D.\n1) $);
    \path [->,bend left=10] (D) edge node {$5$} (C);
    \path [->,bend left=10] (C) edge node {$-5$} (D);
    \path [->] (E) edge node {$-3$} (D);
    \end{tikzpicture}
\end{center}
\end{document}

enter image description here

Off-topic but I just saw AndréC's answer and even though I generally agree with him, I'd not use a graph for that. A very simple loop is easier IMHO and shorter.

\documentclass[]{article}
\usepackage{tikz}  
\usetikzlibrary{calc}
\begin{document}  
\begin{center}
    \begin{tikzpicture}[auto,node distance=2cm,thick,main node/.style={circle,fill=blue!20,draw,font=\sffamily\Large\bfseries}, scale=2]
    \foreach \X/\Z [count=\Y,evaluate=\Y as \angle using {90-72+72*\Y}] in {B/\infty,A/0,C/\infty,D/\infty,E/\infty}
    {\node[main node,label={\angle:$\Z$}] (\X) at (\angle:2) {\X};}

    \path [->] (A) edge node {$-1$} (B);
    \path [->] (B) edge node {$2$} (E);
    \path [->] (A) edge node {$4$} (C);
    \path [->] (B) edge node {$3$} (C);
    \path [->] let \p1=($(D)-(B)$),\n1={atan2(\y1,\x1)},\n2={180+\n1} in
     ($ (B.\n1)!2pt!90:(D.\n2) $) edge node {$2$} ($ (D.\n2)!2pt!-90:(B.\n1) $);
    \path [->] let \p1=($(B)-(D)$),\n1={atan2(\y1,\x1)},\n2={180+\n1} in
     ($ (D.\n1)!2pt!90:(B.\n2) $) edge node {$-2$} ($ (B.\n2)!2pt!-90:(D.\n1) $);
    \path [->,bend left=10] (D) edge node {$5$} (C);
    \path [->,bend left=10] (C) edge node {$-5$} (D);
    \path [->] (E) edge node {$-3$} (D);
    \end{tikzpicture}
\end{center}
\end{document}
5
  • Thank you, this definitely works. I am surprised there isn't a "less clever" solution though somewhere within a tikz library.
    – Simd
    Commented Oct 25, 2018 at 15:56
  • @Anush An easier way is to just bend the paths. I could add an example if you wish.
    – user121799
    Commented Oct 25, 2018 at 15:58
  • That would be great if you don't mind. This was my first ever tikz diagram so I have much to learn.
    – Simd
    Commented Oct 25, 2018 at 15:59
  • @Anush Done, please have a look.
    – user121799
    Commented Oct 25, 2018 at 16:00
  • That is very nice!
    – Simd
    Commented Oct 25, 2018 at 16:15
4

To build graphs, there is the graphs library.

Here are two slightly different syntaxes that give the same result.

The circular placement option allows you to place the nodes on a circle. As there are 5, to draw a regular pentagon, it is enough that the angle is equal to 360/5=72°, its radius is given by radius=40mm.

The clockwise=5 option gives the same result in abbreviated form.

To place arrows, with the quotes library, simply place the text in quotation marks {B->["3"]C}.

\documentclass[tikz,border=5mm]{standalone}
\usetikzlibrary{arrows.meta}
\usetikzlibrary{graphs}
\usetikzlibrary{quotes}
\begin{document}  

   \tikz[>={Straight Barb[length=6pt,width=8pt]},thick] 
   \graph [circular placement, group polar shift=(-72:0), radius=40mm,nodes={draw,circle,font=\sffamily\Large\bfseries,fill=blue!20}]
   {B[label=$\infty$],E[label={[inner sep=0pt]60:$\infty$}],D[label=-90:$\infty$],C[label=-90:$\infty$],A[label={[inner sep=0pt]120:$0$}],
   {A->["-1"]B->["2"] E->["-3"] D->["5"] C},
   {B->["3"]C},
   {B->[bend left=10,"2"]D},
   {D->[bend left=10,"1"]B},
   {A->["4"]C}
   };


\tikz[>={Straight Barb[length=6pt,width=8pt]},thick] 
\graph [clockwise=5,radius=40mm,nodes={draw,circle,font=\sffamily\Large\bfseries,fill=blue!20}] 
   {B[label=$\infty$],E[label={[inner sep=0pt]60:$\infty$}],D[label=-90:$\infty$],C[label=-90:$\infty$],A[label={[inner sep=0pt]120:$0$}],
   {A->["-1"]B->["2"] E->["-3"] D->["5"] C},
   {B->["3"]C},
   {B->[bend left=10,"2"]D},
   {D->[bend left=10,"1"]B},
   {A->["4"]C}
   }; 

\end{document}

graphe

Translated with www.DeepL.com/Translator

2
  • This is great. It's definitely fussy but the labels of E and A seem a little far from the nodes. Can they easily be made closer?
    – Simd
    Commented Oct 25, 2018 at 18:32
  • 1
    @Anush You can add options to the labels in brackets A[label={{[inner sep=0pt]120:$0$}] Does that work for you? I updated the answer.
    – AndréC
    Commented Oct 25, 2018 at 19:22

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