% Copyright 2019 by Till Tantau
%
% This file may be distributed and/or modified
%
% 1. under the LaTeX Project Public License and/or
% 2. under the GNU Free Documentation License.
%
% See the file doc/generic/pgf/licenses/LICENSE for more details.


\section{Making Trees Grow}
\label{section-trees}

\subsection{Introduction to the  Child Operation}

\emph{Trees} are a common way of visualizing hierarchical structures. A simple
tree looks like this:
%
\begin{codeexample}[]
\begin{tikzpicture}
  \node {root}
    child {node {left}}
    child {node {right}
      child {node {child}}
      child {node {child}}
    };
\end{tikzpicture}
\end{codeexample}

Admittedly, in reality trees are more likely to grow \emph{upward} and not
downward as above. You can tell whether the author of a paper is a
mathematician or a computer scientist by looking at the direction their trees
grow. A computer scientist's trees will grow downward while a mathematician's
tree will grow upward. Naturally, the \emph{correct} way is the mathematician's
way, which can be specified as follows:
%
\begin{codeexample}[]
\begin{tikzpicture}
  \node {root} [grow'=up]
    child {node {left}}
    child {node {right}
      child {node {child}}
      child {node {child}}
    };
\end{tikzpicture}
\end{codeexample}

In \tikzname, there are two ways of specifying trees: Using either the |graph|
path operation, which is covered in Section~\ref{section-library-graphs}, or
using the |child| path operation, which is covered in the present section. Both
methods have their advantages.

In \tikzname, trees are specified by adding \emph{children} to a node on a path
using the |child| operation:

\begin{pathoperation}{child}{\opt{\oarg{options}}%
        \opt{|foreach|\meta{variables}|in|\marg{values}}\opt{\marg{child path}}}
    This operation should directly follow a completed |node| operation or
    another |child| operation, although it is permissible that the first
    |child| operation is preceded by options (we will come to that).

    When a |node| operation like |node {X}| is followed by |child|, \tikzname\
    starts counting the number of child nodes that follow the original
    |node {X}|. For this, it scans the input and stores away each |child| and
    its arguments until it reaches a path operation that is not a |child|. Note
    that this will fix the character codes of all text inside the child
    arguments, which means, in essence, that you cannot use verbatim text
    inside the nodes inside a |child|. Sorry.

    Once the children have been collected and counted, \tikzname\ starts
    generating the child nodes. For each child of a parent node \tikzname\
    computes an appropriate position where the child is placed. For each child,
    the coordinate system is transformed so that the origin is at this
    position. Then the \meta{child path} is drawn. Typically, the child path
    just consists of a |node| specification, which results in a node being
    drawn at the child's position. Finally, an edge is drawn from the first
    node in the \meta{child path} to the parent node.

    The optional |foreach| part (note that there is no backslash before
    |foreach|) allows you to specify multiple children in a single |child|
    command. The idea is the following: A |\foreach| statement is (internally)
    used to iterate over the list of \meta{values}. For each value in this
    list, a new |child| is added to the node. The syntax for \meta{variables}
    and for \meta{values} is the same as for the |\foreach| statement, see
    Section~\ref{section-foreach}. For example, when you say
    %
\begin{codeexample}[code only]
node {root} child [red] foreach \name in {1,2} {node {\name}}
\end{codeexample}
    %
    the effect will be the same as if you had said
    %
\begin{codeexample}[code only]
node {root} child[red] {node {1}} child[red] {node {2}}
\end{codeexample}
    %
    When you write
    %
\begin{codeexample}[code only]
node {root} child[\pos] foreach \name/\pos in {1/left,2/right} {node[\pos] {\name}}
\end{codeexample}
    %
    the effect will be the same as for
    %
\begin{codeexample}[code only]
node {root} child[left] {node[left] {1}} child[right] {node[right] {2}}
\end{codeexample}

    You can nest things as in the following example:
    %
\begin{codeexample}[]
\begin{tikzpicture}
  [level distance=4mm,level/.style={sibling distance=8mm/#1}]
  \coordinate
    child foreach \x in {0,1}
      {child foreach \y in {0,1}
        {child foreach \z in {0,1}}};
\end{tikzpicture}
\end{codeexample}

    The details and options for this operation are described in the rest of
    this present section.
\end{pathoperation}


\subsection{Child Paths and Child Nodes}

For each |child| of a root node, its \meta{child path} is inserted at a
specific location in the picture (the placement rules are discussed in
Section~\ref{section-tree-placement}). The first node in the \meta{child path},
if it exists, is special and called the \emph{child node}. If there is no first
node in the \meta{child path}, that is, if the \meta{child path} is missing
(including the curly braces) or if it does not start with |node| or with
|coordinate|, then an empty child node of shape |coordinate| is automatically
added.

Consider the example |\node {x} child {node {y}} child;|. For the first child,
the \meta{child path} has the child node |node {y}|. For the second child, no
child node is specified and, thus, it is just |coordinate|.

As for any normal node, you can give the child node a name, shift it around, or
use options to influence how it is rendered.
%
\begin{codeexample}[preamble={\usetikzlibrary{shapes.geometric}}]
\begin{tikzpicture}[sibling distance=15mm]
  \node[rectangle,draw] {root}
    child {node[circle,draw,yshift=-5mm] (left node) {left}}
    child {node[ellipse,draw] (right node) {right}};
  \draw[dashed,->] (left node) -- (right node);
\end{tikzpicture}
\end{codeexample}

In many cases, the \meta{child path} will just consist of a specification of a
child node and, possibly, children of this child node. However, the node
specification may be followed by arbitrary other material that will be added to
the picture, transformed to the child's coordinate system. For your
convenience, a move-to |(0,0)| operation is inserted automatically at the
beginning of the path. Here is an example:
%
\begin{codeexample}[]
\begin{tikzpicture}
  \node {root}
    child {[fill] circle (2pt)}
    child {[fill] circle (2pt)};
\end{tikzpicture}
\end{codeexample}

At the end of the \meta{child path} you may add a special path operation called
|edge from parent|. If this operation is not given by yourself somewhere on the
path, it will be automatically added at the end. This option causes a
connecting edge from the parent node to the child node to be added to the path.
By giving options to this operation you can influence how the edge is rendered.
Also, nodes following the |edge from parent| operation will be placed on this
edge, see Section~\ref{section-edge-from-parent} for details.

To sum up:
%
\begin{enumerate}
    \item The child path starts with a node specification. If it is not there,
        it is added automatically.
    \item The child path ends with a |edge from parent| operation, possibly
        followed by nodes to be put on this edge. If the operation is not given
        at the end, it is added automatically.
\end{enumerate}


\subsection{Naming Child Nodes}

Child nodes can be named like any other node using either the |name| option or
the special syntax in which the name of the node is placed in round parentheses
between the |node| operation and the node's text.

If you do not assign a name to a child node, \tikzname\ will automatically
assign a name as follows: Assume that the name of the parent node is, say,
|parent|. (If you did not assign a name to the parent, \tikzname\ will do so
itself, but that name will not be user-accessible.) The first child of |parent|
will be named |parent-1|, the second child is named |parent-2|, and so on.

This naming convention works recursively. If the second child |parent-2| has
children, then the first of these children will be called |parent-2-1| and the
second |parent-2-2| and so on.

If you assign a name to a child node yourself, no name is generated
automatically (the node does not have two names). However, ``counting
continues'', which means that the third child of |parent| is called |parent-3|
independently of whether you have assigned names to the first and/or second
child of |parent|.

Here is an example:
%
\begin{codeexample}[]
\begin{tikzpicture}[sibling distance=15mm]
  \node (root) {root}
    child
    child {
      child {coordinate (special)}
      child
    };
  \node at (root-1) {root-1};
  \node at (root-2) {root-2};
  \node at (special) {special};
  \node at (root-2-2) {root-2-2};
\end{tikzpicture}
\end{codeexample}


\subsection{Specifying Options for Trees and Children}
\label{section-tree-options}

Each |child| may have its own \meta{options}, which apply to ``the whole
child'', including all of its grandchildren. Here is an example:
%
\begin{codeexample}[]
\begin{tikzpicture}
  [thick,level 1/.style={sibling distance=15mm},
         level 2/.style={sibling distance=10mm}]
  \coordinate
    child[red]   {child child}
    child[green] {child child[blue]};
\end{tikzpicture}
\end{codeexample}

The options of the root node have no effect on the children since the options
of a node are always ``local'' to that node. Because of this, the edges in the
following tree are black, not red.
%
\begin{codeexample}[]
\begin{tikzpicture}[thick]
  \node [red] {root}
    child
    child;
\end{tikzpicture}
\end{codeexample}
%
This raises the problem of how to set options for \emph{all} children.
Naturally, you could always set options for the whole path as in
|\path [red] node {root} child child;| but this is bothersome in some
situations. Instead, it is easier to give the options \emph{before the first
child} as follows:
%
\begin{codeexample}[]
\begin{tikzpicture}[thick]
  \node [red] {root}
    [green] % option applies to all children
    child
    child;
\end{tikzpicture}
\end{codeexample}

Here is the set of rules:
%
\begin{enumerate}
    \item Options for the whole tree are given before the root node.
    \item Options for the root node are given directly to the |node| operation
        of the root.
    \item Options for all children can be given between the root node and the
        first child.
    \item Options applying to a specific child path are given as options to the
        |child| operation.
    \item Options applying to the node of a child, but not to the whole child
        path, are given as options to the |node| command inside the \meta{child
        path}.
\end{enumerate}
%
\begin{codeexample}[code only]
\begin{tikzpicture}
  \scoped
    [...]              % Options apply to the whole tree
    \node[...] {root}  % Options apply to the root node only
       [...]           % Options apply to all children
       child[...]      % Options apply to this child and all its children
       {
         node[...] {}  % Options apply to the child node only
         ...
       }
       child[...]      % Options apply to this child and all its children
    ;
\end{tikzpicture}
\end{codeexample}

There are additional styles that influence how children are rendered:
%
\begin{stylekey}{/tikz/every child (initially \normalfont empty)}
    This style is used at the beginning of each child, as if you had given the
    style's contents as options to the |child| operation.
\end{stylekey}

\begin{stylekey}{/tikz/every child node (initially \normalfont empty)}
    This style is used at the beginning of each child node in addition to the
    |every node| style.
\end{stylekey}

\begin{stylekey}{/tikz/level=\meta{number} (initially \normalfont empty)}
    This style is executed at the beginning of each set of children, where
    \meta{number} is the current level in the current tree. For example, when
    you say |\node {x} child child;|, then |level=1| is used before the first
    |child|. The style or code of this key will be passed \meta{number} as its
    first parameter. If this first |child| has children itself, then |level=2|
    would be used for them.
    %
\begin{codeexample}[]
\begin{tikzpicture}[level/.style={sibling distance=20mm/#1}]
  \node {root}
    child { child child }
    child { child child child };
\end{tikzpicture}
\end{codeexample}
    %
\end{stylekey}

\begin{stylekey}{/tikz/level \meta{number} (initially \normalfont empty)}
    This style is used in addition to the |level| style. So, when you say
    |\node {x} child child;|, then the following key list is executed:
    |level=1,level 1|.
    %
\begin{codeexample}[]
\begin{tikzpicture}
  [level 1/.style={sibling distance=20mm},
   level 2/.style={sibling distance=5mm}]
  \node {root}
    child { child child }
    child { child child child };
\end{tikzpicture}
\end{codeexample}
    %
\end{stylekey}


\subsection{Placing Child Nodes}
\label{section-tree-placement}

\subsubsection{Basic Idea}

Perhaps the most difficult part in drawing a tree is the correct layout of the
children. Typically, the children have different sizes and it is not easy to
arrange them in such a manner that not too much space is wasted, the children
do not overlap, and they are either evenly spaced or their centers are evenly
distributed. Calculating good positions is especially difficult since a good
position for the first child may depend on the size of the last child.

In basic \tikzname, when you do not make use of the graph drawing facilities
explained in Part~\ref{part-gd}, a comparatively simple approach is taken to
placing the children. In order to compute a child's position, all that is taken
into account is the number of the current child in the list of children and the
number of children in this list. Thus, if a node has five children, then there
is a fixed position for the first child, a position for the second child, and
so on. These positions \emph{do not depend on the size of the children} and,
hence, children can easily overlap. However, since you can use options to shift
individual children a bit, this is not as great a problem as it may seem.

Although the placement of the children only depends on their number in the list
of children and the total number of children, everything else about the
placement is highly configurable. You can change the distance between children
(appropriately called the |sibling distance|) and the distance between levels
of the tree. These distances may change from level to level. The direction in
which the tree grows can be changed globally and for parts of the tree. You can
even specify your own ``growth function'' to arrange children on a circle or
along special lines or curves.


\subsubsection{Default Growth Function}

The default growth function works as follows: Assume that we are given a node
and five children. These children will be placed on a line with their centers
(or, more generally, with their anchors) spaced apart by the current
|sibling distance|. The line is orthogonal to the current \emph{direction of
growth}, which is set with the |grow| and |grow'| option (the latter option
reverses the ordering of the children). The distance from the line to the
parent node is given by the |level distance|.
%
{\catcode`\|=12
\begin{codeexample}[]
\begin{tikzpicture}[sibling distance=15mm, level distance=15mm]
  \path [help lines]
    node (root) {root}
    [grow=-10]
    child {node {1}}
    child {node {2}}
    child {node {3}}
    child {node {4}};

  \draw[|<->|,thick] (root-1.center)
    -- node[above,sloped] {sibling distance} (root-2.center);

  \draw[|<->|,thick] (root.center)
    -- node[above,sloped] {level distance} +(-10:\tikzleveldistance);
\end{tikzpicture}
\end{codeexample}
}

\begin{key}{/tikz/level distance=\meta{distance} (initially 15mm)}
    This key determines the distance between different levels of the tree, more
    precisely, between the parent and the line on which its children are
    arranged. When given to a single child, this will set the distance for this
    child only.
    %
\begin{codeexample}[]
\begin{tikzpicture}
  \node {root}
    [level distance=20mm]
    child
    child {
      [level distance=5mm]
      child
      child
      child
    }
    child[level distance=10mm];
\end{tikzpicture}
\end{codeexample}

\begin{codeexample}[]
\begin{tikzpicture}
  [level 1/.style={level distance=10mm},
   level 2/.style={level distance=5mm}]
  \node {root}
    child
    child {
      child
      child[level distance=10mm]
      child
    }
    child;
\end{tikzpicture}
\end{codeexample}
    %
\end{key}

\begin{key}{/tikz/sibling distance=\meta{distance} (initially 15mm)}
    This key specifies the distance between the anchors of the children of a
    parent node.
    %
\begin{codeexample}[]
\begin{tikzpicture}
  [level distance=4mm,
   level 1/.style={sibling distance=8mm},
   level 2/.style={sibling distance=4mm},
   level 3/.style={sibling distance=2mm}]
  \coordinate
     child {
       child {child child}
       child {child child}
     }
     child {
       child {child child}
       child {child child}
     };
\end{tikzpicture}
\end{codeexample}

\begin{codeexample}[]
\begin{tikzpicture}
  [level distance=10mm,
   every node/.style={fill=red!60,circle,inner sep=1pt},
   level 1/.style={sibling distance=20mm,nodes={fill=red!45}},
   level 2/.style={sibling distance=10mm,nodes={fill=red!30}},
   level 3/.style={sibling distance=5mm,nodes={fill=red!25}}]
  \node {31}
     child {node {30}
       child {node {20}
         child {node {5}}
         child {node {4}}
       }
       child {node {10}
         child {node {9}}
         child {node {1}}
       }
     }
     child {node {20}
       child {node {19}
         child {node {1}}
         child[missing]
       }
       child {node {18}}
     };
\end{tikzpicture}
\end{codeexample}
    %
\end{key}

\begin{key}{/tikz/grow=\meta{direction}}
    This key is used to define the \meta{direction} in which the tree will
    grow. The \meta{direction} can either be an angle in degrees or one of the
    following special text strings: |down|, |up|, |left|, |right|, |north|,
    |south|, |east|, |west|, |north east|, |north west|, |south east|, and
    |south west|. All of these have ``their obvious meaning'', so, say,
    |south west| is the same as the angle $-135^\circ$.

    As a side effect, this option installs the default growth function.

    In addition to setting the direction, this option also has a seemingly
    strange effect: It sets the sibling distance for the current level to
    |0pt|, but leaves the sibling distance for later levels unchanged.

    This somewhat strange behavior has a highly desirable effect: If you give
    this option before the list of children of a node starts, the ``current
    level'' is still the parent level. Each child will be on a later level and,
    hence, the sibling distance will be as specified originally. This will
    cause the children to be neatly aligned in a line orthogonal to the given
    \meta{direction}. However, if you give this option locally to a single
    child, then ``current level'' will be the same as the child's level. The
    zero sibling distance will then cause the child to be placed exactly at a
    point at distance |level distance| in the direction \meta{direction}.
    However, the children of the child will be placed ``normally'' on a line
    orthogonal to the \meta{direction}.

    These placement effects are best demonstrated by some examples:
    %
\begin{codeexample}[]
\tikz \node {root} [grow=right] child child;
\end{codeexample}

\begin{codeexample}[]
\tikz \node {root} [grow=south west] child child;
\end{codeexample}

\begin{codeexample}[]
\begin{tikzpicture}[level distance=10mm,sibling distance=5mm]
  \node {root}
    [grow=down]
    child
    child
    child[grow=right] {
      child child child
    };
\end{tikzpicture}
\end{codeexample}

\begin{codeexample}[]
\begin{tikzpicture}[level distance=2em]
  \node {C}
    child[grow=up]    {node {H}}
    child[grow=left]  {node {H}}
    child[grow=down]  {node {H}}
    child[grow=right] {node {C}
        child[grow=up]    {node {H}}
        child[grow=right] {node {H}}
        child[grow=down]  {node {H}}
      edge from parent[double]
        coordinate (wrong)
    };
  \draw[<-,red] ([yshift=-2mm]wrong) -- +(0,-1)
    node[below]{This is wrong!};
\end{tikzpicture}
\end{codeexample}

\begin{codeexample}[]
\begin{tikzpicture}
  \node[rectangle,draw] (a) at (0,0) {start node};
  \node[rectangle,draw] (b) at (2,1) {end};

  \draw (a) -- (b)
    node[coordinate,midway] {}
      child[grow=100,<-] {node[above] {the middle is here}};
\end{tikzpicture}
\end{codeexample}
    %
\end{key}

\begin{key}{/tikz/grow'=\meta{direction}}
    This key has the same effect as |grow|, only the children are arranged in
    the opposite order.
\end{key}


\subsubsection{Missing Children}

Sometimes one or more of the children of a node are ``missing''. Such a missing
child will count as a child with respect to the total number of children and
also with respect to the current child count, but it will not be rendered.

\begin{key}{/tikz/missing=\meta{true or false} (default true)}
    If this option is given to a child, the current child counter is increased,
    but the child is otherwise ignored. In particular, the normal contents of
    the child is completely ignored.
    %
\begin{codeexample}[]
\begin{tikzpicture}[level distance=10mm,sibling distance=5mm]
  \node {root} [grow=down]
    child          { node {1} }
    child          { node {2} }
    child          { node {3} }
    child[missing] { node {4} }
    child          { node {5} }
    child          { node {6} };
\end{tikzpicture}
\end{codeexample}
    %
\end{key}


\subsubsection{Custom Growth Functions}

\begin{key}{/tikz/growth parent anchor=\meta{anchor} (initially center)}
    This key allows you to specify which anchor of the parent node is to be
    used for computing the children's position. For example, when there is only
    one child and the |level distance| is |2cm|, then the child node will be
    placed two centimeters below the \meta{anchor} of the parent node. ``Being
    placed'' means that the child node's anchor (which is the anchor specified
    using the |anchor=| option in the |node| command of the child) is two
    centimeters below the parent node's \meta{anchor}.

    In the following example, the two red lines both have length |1cm|.
    %
\begin{codeexample}[]
\begin{tikzpicture}[level distance=1cm]
  \node [rectangle,draw] (a) at (0,0) {root}
  [growth parent anchor=south] child;

  \node [rectangle,draw] (b) at (2,0) {root}
  [growth parent anchor=north east] child;

  \draw [red,thick,dashed] (a.south) -- (a-1);
  \draw [red,thick,dashed] (b.north east) -- (b-1);
\end{tikzpicture}
\end{codeexample}

    In the next example, the top and bottom nodes are aligned at the top and
    the bottom, respectively.
    %
\begin{codeexample}[]
\begin{tikzpicture}
  [level distance=2cm,growth parent anchor=north,
   every node/.style={anchor=north,rectangle,draw}
   every child node/.style={anchor=south}]

  \node at (0,0) {root} child {node {small}};

  \node at (2,0) {big root} child {node {\large big}};
\end{tikzpicture}
\end{codeexample}
    %
\end{key}

\begin{key}{/tikz/growth function=\meta{macro name} (initially \normalfont an internal function)}
    This rather low-level option allows you to set a new growth function. The
    \meta{macro name} must be the name of a macro without parameters. This
    macro will be called for each child of a node. The initial function is an
    internal function that corresponds to downward growth.

    The effect of executing the macro should be the following: It should
    transform the coordinate system in such a way that the origin becomes the
    place where the current child should be anchored. When the macro is called,
    the current coordinate system will be set up such that the anchor of the
    parent node is in the origin. Thus, in each call, the \meta{macro name}
    must essentially do a shift to the child's origin. When the macro is
    called, the \TeX\ counter |\tikznumberofchildren| will be set to the total
    number of children of the parent node and the counter
    |\tikznumberofcurrentchild| will be set to the number of the current child.

    The macro may, in addition to shifting the coordinate system, also
    transform the coordinate system further. For example, it could be rotated
    or scaled.

    Additional growth functions are defined in the library, see
    Section~\ref{section-tree-library}.
\end{key}


\subsection{Edges From the Parent Node}
\label{section-edge-from-parent}

Every child node is connected to its parent node via a special kind of edge
called the |edge from parent|. This edge is added to the \meta{child path} when
the following path operation is encountered:

\begin{pathoperation}{edge from parent}{\opt{\oarg{options}}}
    This path operation can only be used inside \meta{child paths} and should
    be given at the end, possibly followed by \meta{node specifications} like
    |node {a}|. If a \meta{child path} does not contain this operation, it will
    be added at the end of the \meta{child path} automatically.

    By default, this operation does the following:
    %
    \begin{enumerate}
        \item The following style is executed:
            %
            \begin{stylekey}{/tikz/edge from parent (initially draw)}
                This style is inserted right before the |edge from parent path|
                and before the \meta{options} are inserted.
                %
\begin{codeexample}[]
\begin{tikzpicture}
  [edge from parent/.style={draw,red,thick}]
  \node {root}
    child {node {left} edge from parent[dashed]}
    child {node {right}
      child {node {child}}
      child {node {child} edge from parent[draw=none]}
    };
\end{tikzpicture}
\end{codeexample}
            \end{stylekey}
        \item Next, the \meta{options} are executed.
        \item Next, the text stored in the following key is inserted:
            %
            \begin{key}{/tikz/edge from parent path=\meta{path} (initially \normalfont code shown below)}
                This option allows you to set the |edge from parent path| to a
                new path. Initially, this path is the following:
                %
\begin{codeexample}[code only]
(\tikzparentnode\tikzparentanchor) -- (\tikzchildnode\tikzchildanchor)
\end{codeexample}
                %
                The |\tikzparentnode| is a macro that will expand to the name
                of the parent node. This works even when you have not assigned
                a name to the parent node, in this case an internal name is
                automatically generated. The |\tikzchildnode| is a macro that
                expands to the name of the child node. The two |...anchor|
                macros are empty by default. So, what is essentially inserted
                is just the path segment
                |(\tikzparentnode) -- (\tikzchildnode)|; which is exactly an
                edge from the parent to the child.

                You can modify this edge from parent path to achieve all sorts
                of effects. For example, we could replace the straight line by
                a curve as follows:
                %
\begin{codeexample}[]
\begin{tikzpicture}[level distance=15mm, sibling distance=15mm,
  edge from parent path=
  {(\tikzparentnode.south) .. controls +(0,-1) and +(0,1)
                           .. (\tikzchildnode.north)}]
  \node {root}
    child {node {left}}
    child {node {right}
      child {node {child}}
      child {node {child}}
    };
\end{tikzpicture}
\end{codeexample}

                Further useful |edge from parent path|s are defined in the tree
                library, see Section~\ref{section-tree-library}.

                The nodes in a \meta{node specification} following the
                |edge from parent| path command get executed as if the |pos|
                option had been added to all these nodes, see also
                Section~\ref{section-pos-option}.

                As an example, consider the following code:
                %
\begin{codeexample}[code only]
\node (root) {} child {node (child) {} edge to parent node {label}};
\end{codeexample}
                %
                The |edge to parent| operation and the following |node|
                operation will, together, have the same effect as if we had
                said:
                %
\begin{codeexample}[code only]
(root) -- (child) node [pos=0.5] {label}
\end{codeexample}

                Here is a more complicated example:
                %
\begin{codeexample}[]
\begin{tikzpicture}
  \node {root}
    child {
      node {left}
      edge from parent
        node[left] {a}
        node[right] {b}
    }
    child {
      node {right}
        child {
          node {child}
          edge from parent
            node[left] {c}
        }
        child {node {child}}
      edge from parent
        node[near end] {x}
    };
\end{tikzpicture}
\end{codeexample}

                As said before, the anchors in the default
                |edge from parent path| are empty. However, you can set them
                using the following keys:
                %
                \begin{key}{/tikz/child anchor=\meta{anchor} (initially border)}
                    Specifies the anchor where the edge from parent meets the
                    child node by setting the macro |\tikzchildanchor| to
                    |.|\meta{anchor}.

                    If you specify |border| as the \meta{anchor}, then the
                    macro |\tikzchildanchor| is set to the empty string. The
                    effect of this is that the edge from the parent will meet
                    the child on the border at an automatically calculated
                    position.
                    %
\begin{codeexample}[]
\begin{tikzpicture}
  \node {root}
    [child anchor=north]
    child {node {left} edge from parent[dashed]}
    child {node {right}
      child {node {child}}
      child {node {child} edge from parent[draw=none]}
    };
\end{tikzpicture}
\end{codeexample}
                \end{key}

                \begin{key}{/tikz/parent anchor=\meta{anchor} (initially border)}
                    This option works the same way as the |child anchor|, only
                    for the parent.
                \end{key}
            \end{key}
    \end{enumerate}

    All of the above describes the standard functioning of the
    |edge from parent| command. You may, however, sometimes need even more
    fine-grained control (the graph drawing engine needs it, for instance). In
    such cases the following key gives you complete control:
    %
    \begin{key}{/tikz/edge from parent macro=\meta{macro}}
        The \meta{macro} gets expanded each time the |edge from parent| path
        operation is used. This \meta{macro} must take two parameters and must
        expand to some text that is subsequently parsed by the parser. The
        first parameter will be the set of \meta{options} that where passed to
        the |edge from parent| command, the second parameter will be the
        \meta{node specifications} that following the command.

        The standard behavior of drawing a straight line from the parent node
        to the child node could be achieved by setting the \meta{macro} to the
        following:
        %
\begin{codeexample}[code only]
\def\mymacro#1#2{
  [style=edge from parent, #1]
  (\tikzparentnode\tikzparentanchor) -- #2 (\tikzchildnode\tikzchildanchor)
}
\end{codeexample}
        %
        Note that |#2| is placed between |--| and the node to ensure that nodes
        are put ``on top'' of the line.
    \end{key}
\end{pathoperation}


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