forest – grow half of tree the other direction

Question

I am trying to change the growth direction midway while growing my tree with forest. Here is an MWE:

\documentclass{standalone}
\usepackage{forest}
\useforestlibrary{edges}
\forestapplylibrarydefaults{edges}
\begin{document}
\begin{forest}
    forked edges,
    for tree={grow'=east}
    [R
      [1 [a] [b]]
      [2 [c] [d]]
      [3 [e] [f]]
      [4 [g] [h]]
      [5 [i] [j]]
      [6 [k] [l]]]
\end{forest}
\end{document}

The result:

What I would want is for the children 4, 5, and 6 of R to move over to the left side. So that I would have 3-3 children on each side. Is there a way to do this without hacking the position of child node 3 using before ... options? I would like the left side to be symmetrical.

Answer 1

Update 2:

A more general solution that wraps this up into a new option called multiple directions, which requires no manual placing of nodes can be found here.

Updated Solution:

After thinking about this some more, I realized that this can be done with no manual tweaking by placing two empty children of R at the root using before computing xy={l=0,s=0} and setting the grow direction of their subtrees. This can be placed into the main for tree using if level=1{<do this>}{<else do this>}

Here is the code:

\documentclass{article}
\usepackage{forest}
\useforestlibrary{edges}
\begin{document}
\begin{forest}
    for tree={forked edge, minimum height=3.7ex, minimum width=1em, anchor=center,
    if level=1{no edge, before computing xy={l=0,s=0}}{}}
    [R, 
      [, for tree={grow'=east}
        [1 [a] [b]]
        [2 [c] [d]]
        [3 [e] [f]]]
      [, for tree={grow=west}
        [4 [g] [h]]
        [5 [i] [j]]
        [6 [k] [l]]]]
\end{forest}
\end{document}

Old Solution:

Unfortunately, forest isn't designed to have subtrees grow in different directions. There has to be some manual tweaking to make it work. You need the 1, 2, 3 nodes to have a different parent than the 4, 5, 6 nodes. So give the R node two empty children, one that grows west and one that grow's east, and set the s sep to 0. Note that you need to set grow'=south for the R node so that the second subtree (4,5,6) is on the left and the first (1,2,3) is on the right.

Then adjust the position of the R node by lowering it into position with y-=3cm. To get the nodes in the left subtree aligned with the nodes on the right, all nodes must be the same height, so add minimum height=3.5ex to each node.

\documentclass{article}
\usepackage{forest}
\useforestlibrary{edges}
\begin{document}
\begin{forest}
    grow'=south, for tree={forked edge, minimum height=3.7ex,anchor=center}
    [R, s sep=0cm, before drawing tree={y-=3cm}
      [, for tree={grow'=east}, no edge
        [1 [a] [b]]
        [2 [c] [d]]
        [3 [e] [f]]]
      [, for tree={grow=west}, no edge
        [4 [g] [h]]
        [5 [i] [j]]
        [6 [k] [l]]]]
\end{forest}
\end{document}