pgfplots 中的二项树

Question

问题

使用 pgfplots 重组树解决方案（不是纯 TikZ）。

语境

有多个关于树的帖子（这里，这里，这里等等），解决方案以多种方式被拒绝（节点矩阵、树等）。但是我没有看到解决方案pgfplots。

在MWE努力适应这里，我被两种绘制箭头的方法困住了。

1. 第一组从左到右的箭头
2. 第二组箭头从右向左

    \documentclass{standalone}
    \usepackage{pgfplots}
    \usetikzlibrary{arrows.meta} 

    \begin{document}

    \def\mallevel{5} % a number of levels we wish to get

    \tikzset{
        inner sep=0pt, outer sep=2pt, % some node reserve
        malarrow/.style={->, shorten >=0pt, shorten <=-2pt, -{Stealth[length=5pt, width=3pt,     inset=1pt]},
        }, % a style for arrows, -2 is a shift back (an experiment)
        malnode/.style={text=white,draw=blue!50, minimum width=5mm, circle, inner sep=1pt,font=\tiny,fill=blue,opacity=0.5,text opacity=1}, % a style for nodes
    }
    \begin{tikzpicture}
        \begin{axis}[
                xmin = 0,
                xmax = \mallevel,
                ymin = -\mallevel,
                ymax = \mallevel,
                xlabel = $x$,
                yticklabel=\empty,
                y axis line style={draw=none},  
                clip=false,     
            ]

        \foreach \x in {0,...,\mallevel}
        {
            \foreach \y in {0,...,\x}
            {
                \pgfmathparse{-\x/2+\y} % move up by a half of a tree (vertical direction)
                \let\movey=\pgfmathresult 
                \edef\temp{\noexpand
                    \node[malnode] (\x-\y) at (axis cs:\x,\movey) {{\x}-{\y}};
                }
                \temp

                %%%%%%%% draw the arrows    %%%%%%%%
                \ifnum\x>0 
                    \pgfmathparse{int(\x-1)}
                    \let\previousx=\pgfmathresult % previous level (horizontal direction)
                    \ifnum\y>0 
                        \pgfmathparse{int(\y-1)}
                        \let\previousy=\pgfmathresult % previous level (vertical direction)
    %                       \node[malnode] (\previousx-\previousy) at (axis cs:\previousx,\previousy) {check};
    %                       \draw[malarrow] (\previousx-\previousy) -- (\x-\y);
                        \fi 
                    \fi 
                } 
            } 
            \end{axis}
        \end{tikzpicture}

    \end{document}

Answer 1

根据你具体在做什么，最好在 之外做这种事情axis。但无论如何，你主要只需要重复这个\edef\temp{...}\temp技巧。使用evaluate可以简化代码，不需要 等\pgfmathparse。

不知道箭头是否符合您的要求，但如果不是，您应该能够修复它。

\documentclass{standalone}
\usepackage{pgfplots}
\usetikzlibrary{arrows.meta} 

\begin{document}

\def\mallevel{5} % a number of levels we wish to get

\tikzset{
    inner sep=0pt, outer sep=2pt, % some node reserve
    malarrow/.style={->, shorten >=0pt, shorten <=-2pt, -{Stealth[length=5pt, width=3pt,     inset=1pt]},
    }, % a style for arrows, -2 is a shift back (an experiment)
    malnode/.style={text=white,draw=blue!50, minimum width=5mm, circle, inner sep=1pt,font=\tiny,fill=blue,opacity=0.5,text opacity=1}, % a style for nodes
}
\begin{tikzpicture}
    \begin{axis}[
            xmin = 0,
            xmax = \mallevel,
            ymin = -\mallevel,
            ymax = \mallevel,
            xlabel = $x$,
            yticklabel=\empty,
            y axis line style={draw=none},  
            tickwidth=0,
            clip=false,     
        ]

    \foreach \x in {0,...,\mallevel}
    {
        \foreach [evaluate={
                    \movey=-\x/2+\y;
                    \previousy=int(\y-1);
                    \previousx=int(\x-1)}
                    ] \y in {0,...,\x}
        {
            \edef\temp{\noexpand
                \node[malnode] (\x-\y) at (axis cs:\x,\movey) {{\x}-{\y}};
            }
            \temp

            %%%%%%%% draw the arrows    %%%%%%%%
            \ifnum\x>0 
                \ifnum\y>0 
                       \edef\temp{%
                        \noexpand\draw[malarrow] (\previousx-\previousy) -- (\x-\y);
                        \noexpand\draw[malarrow] (\previousx-\previousy) -- (\x-\previousy);
                        }
                        \temp
                    \fi 
                \fi 
            } 
        } 
        \end{axis}
    \end{tikzpicture}

\end{document}