Uniform choice of start position and end position for lines between nodes

Question

I am drawing a figure which has many nodes and many lines connecting the nodes. I want every line connecting two nodes to start from "north" and to end to "south". So, essentially, my code is something like

\begin{tikzpicture}
 \node (a) at (0,0) {};
 \node (b) at (0,1) {};
 \draw (a.north) -- (b.south);
\end{tikzpicture}

with many nodes definitions and many draws. I want to define once and for all that all line should start from "north" and end to "south", so I tried something like

 every path/.style={parent anchor=north, child anchor=south}

but it doesn't work. The question is: is there a way to specify in the template that any line should start from "north" and end to "south" so avoiding the need to write this in every draw?

Answer 1

You can use a custom to path

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}[ns/.style={
    to path={(\tikztostart.north) -- (\tikztotarget.south)\tikztonodes (\tikztotarget)}
}]
 \node[draw] (a) at (0,0) {};
 \node[draw] (b) at (1,1) {};
 \node[draw] (c) at (2,-1){};
 \draw (a) to [ns] (b) to[ns] (c) to[ns] (a);
\end{tikzpicture}
\end{document}

Answer 2

You could use macros. I should add that using one letter macro names is a bad idea. Also, I'm surprised I didn't have to use \pgfextra.

\documentclass{standalone}
\usepackage{tikz}

\newcommand{\n}[1]{(#1.north)}
\newcommand{\s}[1]{(#1.south)}

\begin{document}

\begin{tikzpicture}
 \node (a) at (0,0) {};
 \node (b) at (0,1) {};
 \draw \n{a} -- \s{b};
\end{tikzpicture}

\end{document}

Answer 3

You could use a loop to draw the lines. To specify different colours for the lines, you can have a third loop variable, and leave that empty where you want the default colour (black, unless you say otherwise).

\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
 \node (a) at (0,0) {};
 \node (b) at (0,1) {};
 \node (c) at (0.2,0) {};
 \node (d) at (0.2,1) {};
 \node (e) at (0.4,0) {};
 \node (f) at (0.4,1) {};

\foreach \i/\j/\clr in {
   a/b/,
   c/d/red,
   e/f/}
  \draw [\clr] (\i.north) -- (\j.south);
\end{tikzpicture}
\end{document}