Using forest together with \input

Question

I'm trying to put some externally produced code into a forest environment. For the tikzpicture environment it's possible like this:

\documentclass[border=10pt]{standalone}
\usepackage{tikz}
\usepackage{filecontents}

\begin{filecontents}{file.txt}
  \node[draw, circle] {};
\end{filecontents}

\begin{document}
  \begin{tikzpicture}
    \input{file.txt}
  \end{tikzpicture}
\end{document}

Is it possible to achieve a similar result with the forest environment?

The same approach with a forest environment yields an error:

\documentclass[border=10pt]{standalone}
\usepackage{forest}

\begin{document}
  \begin{forest}%
   \input{file.txt}%
  \end{forest}
\end{document}

(the filecontents-package ist not used because it adds a newline to the end of a file (how-can-i-input-a-file-without-getting-the-implicit-newline.)

Answer 1

You can do it with the help of catchfile:

\begin{filecontents*}{\jobname-code.tex}
  % https://tex.stackexchange.com/a/295354/
  for tree={
    grow'=east,
    math content,
    edge={->,>=latex},
    if={isodd(n_children())}{
      calign primary child/.pgfmath={(n_children()+1)/2},
      calign=child edge,
    }{}
  },
  forked edges
  [T>T^0
    [T>A_f
      [C_a(T-A_f) <\sigma <C_a (T-A_s)
          [  {\zeta_s^n=\zeta_s^0-\frac{\zeta_s^0}{\zeta^0}(\zeta^0-\zeta^n)} ]
          [  {\zeta_T^n=\zeta_T^0-\frac{\zeta_T^0}{\zeta^0}(\zeta^0-\zeta^n)} ]
          [{\zeta^n=\frac{\zeta^0}{2}(cos \big (\alpha_A(T-A_s-\frac{\sigma}{C_a})\big )+1)} ]
      ]
      [\sigma<C_a(T-A_s)
        [  {\zeta^n=\zeta^{n-1}} ]
        [  {\zeta_s^n=\zeta_s^{n-1}} ]
        [  {\zeta_T^n=\zeta_T^{n-1}} ]
      ]
    ]
    [A_s<T<A_f
      [\sigma<C_a(T-A_s)
            [  {\zeta_s^n=\zeta_s^0-\frac{\zeta_s^0}{\zeta^0}(\zeta^0-\zeta^n)} ]
            [  {\zeta_T^n=\zeta_T^0-\frac{\zeta_T^0}{\zeta^0}(\zeta^0-\zeta^n)} ]
            [{\zeta^n=\frac{\zeta^0}{2}(cos \big (\alpha_A(T-A_s-\frac{\sigma}{C_a})\big )+1)}, calign with current]
        [\sigma>C_a(T-A_s)
            [  {\zeta^n=\zeta^{n-1}} ]
            [  {\zeta_s^n=\zeta_s^{n-1}} ]
            [  {\zeta_T^n=\zeta_T^{n-1}} ]
        ]
      ]
    ]
    [T<A_s
      [  {\zeta^n=\zeta^{n-1}} ]
      [  {\zeta_s^n=\zeta_s^{n-1}} ]
      [  {\zeta_T^n=\zeta_T^{n-1}} ]
    ]
  ]
\end{filecontents*}

\documentclass[border=10pt]{standalone}
\usepackage[edges]{forest}
\usepackage{catchfile}

\newcommand{\inputforest}[2][]{%
  \begingroup
  \CatchFileDef{\temp}{#2}{}%
  \edef\temp{\endgroup
    \unexpanded{\begin{forest}#1}%
    \unexpanded\expandafter{\temp}%
    \unexpanded{\end{forest}}%
  }\temp
}

\begin{document}

\inputforest{\jobname-code}

\end{document}

Code courtesy of cfr. If you call it as \inputforest[<some forest code>]{file}, <some forest code> would be placed immediately before the file’s contents.

A different approach is with action character; thanks to cfr for suggesting it.

\documentclass[border=10pt]{standalone}
\usepackage[edges]{forest}
\usepackage{catchfile}

\newcommand{\inputforest}[2][@]{%
  \begingroup
  \bracketset{action character=#1}
  \CatchFileDef{\temp}{#2}{}%
  \begin{forest}#1\temp\end{forest}
  \endgroup
}

\begin{document}

\inputforest[!]{\jobname-code}

\end{document}

Here the optional character is the “action character”: it should be one that doesn't appear in the forest code, default @.