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TikZ: How to draw an arrow in the middle of the line?

How can I draw delta-complex diagrams like ones on this page from Hatcher's book:

screenshot

Source: http://www.math.cornell.edu/~hatcher/AT/AT.pdf#page=111

using TikZ (or any other LaTeX package)? For my documents I usually draw them by hand and scan them in but I'd like to learn how to do it with LaTeX, so it'll be neater, self-contained, editable, copy-paste-able, etc.

I'm sorry, I've tried to figure this out myself but nothing came even close to working. Thanks for your help.

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marked as duplicate by percusse, lockstep, cgnieder, Tom Bombadil, Harish Kumar Oct 6 '12 at 23:24

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That looks great! How can I get the labels though? Sorry again, I'm just starting out with TikZ. –  topology student Oct 6 '12 at 18:33

1 Answer 1

up vote 5 down vote accepted

Here's one possibility using decorations:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings,positioning}

\begin{document}

\begin{tikzpicture}[node distance=2cm]
\coordinate[label=right:$v$] (tv) {};
\coordinate[left=of tv,label=left:$w$] (tw) {};
\coordinate[below=of tw,label=left:$v$] (bv) {};
\coordinate[below=of tv,label=right:$w$] (bw) {};
\begin{scope}[decoration={markings,mark=at position 0.5 with {\arrow{>}}}] 
\draw[postaction={decorate}] (tv) -- node[auto,swap] {$b$} (tw);
\draw[postaction={decorate}] (bv) -- node[auto] {$a$} (tw);
\draw[postaction={decorate}] (bv) -- node[auto,swap] {$b$} (bw);
\draw[postaction={decorate}] (tv) -- node[auto] {$a$} (bw);
\draw[postaction={decorate}] (bv) -- node[auto,swap] {$c$} (tv);
\end{scope}
\node at (barycentric cs:bv=0.5,tv=0.5,bw=0.9) {$L$};
\end{tikzpicture}

\end{document}

enter image description here

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Awesome! Thanks so much for your help! –  topology student Oct 6 '12 at 18:43
    
Ah, thanks for explaining that. Also, how can we make the label L appear in the middle of its triangle? –  topology student Oct 6 '12 at 18:50
    
@topologystudent answer updated. –  Gonzalo Medina Oct 6 '12 at 18:55

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