Horizontal Probability Tree using Forest with Edge Labels

Question

I’m trying to make a horizontal probability tree as pictured below using the Forest package.

Currently, I am able to get a regular forest tree using the following code

\usepackage{forest}
\begin{document}
\begin{forest}
    [A
        [B[D][E]]
        [C[F][G]]
    ]
\end{forest}
\end{document}

I also know that you can add edge labels manually by specifying, for instance:

[D;edge label = {node[midway,above]{$\frac{3}{5}$}}]

However, since I will be making these trees quite frequently, I was hoping to have the edge labels automatically placed either above or below. As well, there may be more than two branches per node, in which the edge label should default to the top.

I’ve seen code like:

[D;$\frac{3}{5}$]

But I’m not too sure how to achieve that.

Answer 1

Please provide compilable code and be sure that your example demonstrates the problem. For example, if you need cases with more than 2 branches, you need to provide an example in which some cases have more than 2 branches. It is also not obvious, because your code is incomplete, whether you have a tree growing in your desired direction or whether that is also a problem for you. Generally, it is all a bit underspecified.

Here's an example based on the code you gave, a modified version of the syntax specified in your desiderata (I don't think it is good to choose a semicolon here, as that is ubiquitous in TikZ's syntax) and your target picture. Adjust to taste.

\documentclass[border=10pt]{standalone}
\usepackage{forest}
\forestset{
  ptree/.style={
    for tree={
      grow'=0,
      parent anchor=children,
      child anchor=parent,
    },
    before typesetting nodes={
      for tree={
        split option={content}{:}{content, my edge label},
      },
    },
  },
  my edge label/.style={
    if={
      > O_= {n'}{1}
    }{
      edge label={node [midway, below, font=\footnotesize] {$\frac#1$} }
    }{
      edge label={node [midway, above, font=\footnotesize] {$\frac#1$} }
    },
  }
}
\begin{document}
\begin{forest}
  ptree
  [A
    [B:12
    [D:{3}{5}][E:{2}{5}]
    ]
    [C:12
      [F:{1}{4}][G:{3}{4}]
    ]
  ]
\end{forest}
\end{document}

Answer 2

Peraphs is it adapt for you this alternative solution?

\documentclass[a4paper,12pt]{article}

\usepackage{tikz-cd}
\usepackage{mathtools}

\begin{document}
\begin{tikzcd}
 &  & D \\
 & B \arrow[ru, "\dfrac35", no head] \arrow[r, "\tfrac25"', no head] & E \\
A \arrow[ru, no head] \arrow[rd, "\dfrac12"', no head] &  & F \\
 & C \arrow[ru, "\dfrac14", no head] \arrow[rd, "\dfrac34"', no head] &  \\
 &  & G
\end{tikzcd}
\end{document}