我的 tikz 代码有什么问题？

Question

由于某种原因，下面的代码给了我错误

Package pgfplots Error: Sorry, I could not read the plot coordinates near ') '. Please check for format mistakes.

但我只是不明白为什么或如何修复它。似乎我无法在 y 坐标中指定数学表达式？！

\documentclass{standalone}

\usepackage{pgfplots}
\pgfplotsset{compat=newest}

\begin{document}
\begin{tikzpicture}[line/.style={black, thick},
    declare function={foo(\r,\x) = (\x)^(\r); },
    ]
    \begin{axis}[
      width=8cm, height=8cm, %
      xmin=0, xmax=1, %
      ymode=log, axis on top]

      \addplot [line, domain=0:1, samples=200]{foo(2, x)};

      \foreach \r in {1,2,3,4} %
      {
        \addplot [line, domain=0:1, samples=200]{foo(\r, x)};
        \addplot [draw=black]
        coordinates {
          (2^(-\r), 1)
          (2^(-\r), foo(\r, x))
          %(2^(-\r), x^(3)) %% Same error
          % (2^(-\r), 0.002) %% This works
        };
      }
    \end{axis}
  \end{tikzpicture}
\end{document}

Answer 1

您需要添加括号。否则解析器会感到困惑，不知道括号何时界定函数的坐标或参数。

\documentclass{standalone}

\usepackage{pgfplots}
\pgfplotsset{compat=newest}

\begin{document}
\begin{tikzpicture}[line/.style={black, thick},
    declare function={foo(\r,\x) = (\x)^(\r); },
    ]
    \begin{axis}[
      width=8cm, height=8cm, %
      xmin=0, xmax=1, %
      ymode=log, axis on top]

      \addplot [line, domain=0:1, samples=200]{foo(2, x)};

      \foreach \r in {1,2,3,4} %
      {
        \addplot [line, domain=0:1, samples=200]{foo(\r, x)};
        \addplot [draw=black]
        coordinates {
          (2^(-\r), 1)
          (2^(-\r),{ foo(\r, x)})
         ({2^(-\r)},{ x^(3)}) %% 
           ({2^(-\r)}, 0.002) %% This works
        };
      }
    \end{axis}
  \end{tikzpicture}
\end{document}