无法改变循环中的松散程度

Question

如果我尝试在 $Z_2$ 上的循环中添加松散标志，则以下代码无法编译。如何更改循环的松散程度？请注意，删除松散标志后，代码可以编译。

\documentclass[]{beamer}

\usetheme{Madrid}

\usepackage{amsfonts}
\usepackage{amsmath,amssymb,bm}
\usepackage{mathtools}
\usepackage{multirow}
\usepackage{float}
\usepackage{pgfplots}
\usepgfplotslibrary{fillbetween}


\usepackage{xcolor}
\usepackage{caption}
\usepackage{subcaption}

\usepackage{tikz}
\usetikzlibrary{automata,arrows.meta, positioning}

\tikzset{
    ->, % makes the edges directed
    >=stealth, % makes the arrow heads bold
    node distance=3cm, % specifies the minimum distance between two nodes. Change if necessary.
    every state/.style={thick, fill=gray!10}, % sets the properties for each ’state’ node
    initial text=$ $, % sets the text that appears on the start arrow
}


\begin{document}

\begin{frame}
  \begin{tikzpicture}
    \node[] (z1) at (0,0) {  };
    \node[right = 3cm of z1] (z2) {};
    \draw (z1) edge node[midway,above] {$a$} (z2);
    \draw (z2) edge[loop right,looseness=33] node[above,outer sep=4pt,pos=0.2] {$a$} ();
  \end{tikzpicture}
\end{frame}


\end{document}

我收到的错误信息是：

! Package pgf Error: No shape named `' is known.

See the pgf package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.39 \end{frame}

Answer 1

指定将使用 的内部样式looseness = 33覆盖该集合。to pathlooploop right

如果我们改变loop它似乎可以正常工作。

或者，只需再次指定起始坐标作为目标。

如果您希望所有循环都具有这种松散性，请调整every loop样式：

every loop/.append style={looseness=33}

通常，用于循环的节点较大且大多为圆形，此时默认松散度和/或最小距离似乎恰到好处。

代码

\documentclass[tikz]{standalone}
\usetikzlibrary{arrows.meta, positioning}
\tikzset{
  loop/.code={% overwrite default, what will it break?
    \let\tikztotarget\tikztostart,
    \pgfkeysalso{looseness=8,min distance=+5mm,every loop}}}
\begin{document}
\begin{tikzpicture}[>=Stealth, node distance=3cm]
\node[draw]                    (z1) {};
\node[draw, right = 3cm of z1] (z2) {};
\path (z1) edge node[midway,above] {$a$} (z2)
      (z2) edge[loop right, looseness=33]
                node[above, outer sep=4pt, pos=0.2] {$a$} ()
%           edge[help lines, loop right] () % compare with default
;
\end{tikzpicture}
\end{document}

输出