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PGFPlots：坐标内的数学运算会产生错误吗？

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PGFPlots：坐标内的数学运算会产生错误吗？

如何在 PGFPlots 坐标内使用数学运算符？

当我写类似的内容时\draw (axis cs: 0,0) -- (axis cs: sqrt(3),1)，出现以下错误：

¡ \pgfplots@evalute@tikz@coord@system@interface@ 的参数有一个额外的}。

但是，\draw (0,0) -> ({sqrt(3)},1)只要sqrt(3)将其写在括号内，在 Tikz 中就可以正常工作{}。

如果我将括号添加到 PGFPlots 代码中，会出现以下错误：

¡ 软件包 PGF 数学错误：无法将输入 '-{sqrt(3)}/2' 解析为浮点数，抱歉。无法读取的部分位于 'sqrt(3)/2' 附近。

梅威瑟：

\documentclass{memoir}

\usepackage{pgfplots}
\pgfplotsset{
    compat=1.8,
    standard/.style={
        axis x line=center,
        axis y line=center,
        axis z line=center,
        xlabel={$x$},
        ylabel={$y$},
        zlabel={$z$},
        axis line style=help lines,
        every axis/.append style={font=\tiny},
    }
}

\begin{document}

{\centering
\begin{tikzpicture}[scale=1]

    \begin{axis}[
        standard,
        hide z axis,
        xmin=-1.25, xmax=1.25,
        ymin=-1.25, ymax=1.25,
        zmin=0, zmax=2.5,
        xtick={-1,1},
        xticklabels={$-2r$,$2r$},
        ytick=\empty,
    ]
        % Draw Circle
        \draw[blue] (axis cs: 0,0,0)
            ellipse [
            x radius=1, y radius=1];

        % Draw Square
        \draw[green]    (axis cs: -0.86602540378,0.5,0) --
                    (axis cs: -0.86602540378,-0.5,0) --
                    (axis cs: -0.86602540378,-0.5,1) --
                    (axis cs: -0.86602540378,0.5,1) --
                    (axis cs: -0.86602540378,0.5,0);
%   I can't use math operators within PGFPlots apparently?
%       \draw[green]    (axis cs: -sqrt(3)/2,1/2,0) --
%                   (axis cs: -sqrt(3)/2,-1/2,0) --
%                   (axis cs: -sqrt(3)/2,-1/2,1) --
%                   (axis cs: -sqrt(3)/2,1/2,1) --
%                   (axis cs: -sqrt(3)/2,1/2,0);
    \end{axis}

% However, I am able to do so in regular Tikz, as long as I put functions like sqrt() in braces...
    \draw[green]    (-{sqrt(3)/2},1/2,0) ->
                (-{sqrt(3)/2},-1/2,0) ->
                (-{sqrt(3)/2},-1/2,1) ->
                (-{sqrt(3)/2},1/2,1) ->
                (-{sqrt(3)/2},1/2,0);
\end{tikzpicture}
}

\end{document}

答案1

看来您无法在 PGFPlots 函数中编写任意表达式。解析器正在寻找单个浮点数，而不是必须求值才能获得单个浮点数的表达式。您可以做的是将结果保存在宏中，然后每次调用该宏。您必须对and表达式-sqrt(3)/2执行相同的操作，否则只需将其替换为and 。1/2-1/20.5-0.5

固定代码：

\documentclass{memoir}

\usepackage{pgfplots}
\pgfplotsset{
    compat=1.8,
    standard/.style={
        axis x line=center,
        axis y line=center,
        axis z line=center,
        xlabel={$x$},
        ylabel={$y$},
        zlabel={$z$},
        axis line style=help lines,
        every axis/.append style={font=\tiny},
    }
}

\begin{document}

{\centering
\begin{tikzpicture}[scale=1]

    \begin{axis}[
        standard,
        hide z axis,
        xmin=-1.25, xmax=1.25,
        ymin=-1.25, ymax=1.25,
        zmin=0, zmax=2.5,
        xtick={-1,1},
        xticklabels={$-2r$,$2r$},
        ytick=\empty,
    ]
        % Draw Circle
        \draw[blue] (axis cs: 0,0,0)
            ellipse [
            x radius=1, y radius=1];

        % Draw Square
        \draw[green]    (axis cs: -0.86602540378,0.5,0) --
                    (axis cs: -0.86602540378,-0.5,0) --
                    (axis cs: -0.86602540378,-0.5,1) --
                    (axis cs: -0.86602540378,0.5,1) --
                    (axis cs: -0.86602540378,0.5,0);
%   I can't use math operators within PGFPlots apparently?
      \pgfmathsetmacro\foo{-sqrt(3)/2}
      \draw[green]    (axis cs: \foo,0.5,0) --
                  (axis cs: \foo,-0.5,0) --
                  (axis cs:\foo,-0.5,1) --
                  (axis cs:\foo,0.5,1) --
                  (axis cs:\foo,0.5,0);
    \end{axis}

% However, I am able to do so in regular Tikz, as long as I put functions like sqrt() in braces...
    \draw[green]    (-{sqrt(3)/2},1/2,0) ->
                (-{sqrt(3)/2},-1/2,0) ->
                (-{sqrt(3)/2},-1/2,1) ->
                (-{sqrt(3)/2},1/2,1) ->
                (-{sqrt(3)/2},1/2,0);
\end{tikzpicture}
}

\end{document}

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