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回答原始问题

tag-icon • tag-icon tikz-pgf calculations
回答原始问题

我想画一个图，但不喜欢它包含魔法常数，所以我尝试将其参数化。然而，似乎和的出现(1 + \N) / 2必须\N - 2以某种方式转义。正确的做法是什么？

\begin{tikzpicture}
    \def \N {6}

    \node (p) at ((1 + \N) / 2, -2) {};

    \foreach \i in {1, ..., \N - 2, \N} {
        \node (p\i) at (\i, 0) {};
    }

    \path foreach \i in {1, ..., \N - 2, \N} {
        (p\i) -- ($(p.center)!0.5!(p.south)$)
    };

    \node at ($(p\N - 2)!0.5!(p\N)$) {${\cdots}$};

\end{tikzpicture}

答案1

回答原始问题

括号是语法字符，需要在数学表达式中加以保护。此外，在\node问题的两个命令中，还有一个额外的右括号需要删除。

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\def\N{4}
\node at ({(1 + \N)/ 2}, 0) {};
\end{tikzpicture}
\end{document}

\foreach通过 e-TeX 计算数字来解决循环问题：

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\def\N{4}
\foreach \i in {1, ..., \numexpr\N - 2\relax} {
  % \typeout{i=\i}
  \node at ({(1 + \i)/ 2}, 0) {};
}
\end{tikzpicture}
\end{document}

或者可以先计算出数字：

\pgfmathtruncatemacro\M{\N - 2}
\foreach \i in {1, ..., \M} {}

回答更新的问题

MWE 的修正评论：

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}
    \def \N {6}

    % Term put in curly braces (argument braces), because
    % the outmost '(' and ')' serve as syntax characters.
    \node (p) at ({(1 + \N) / 2}, -2) {};

    % Workaround via e-TeX
    \foreach \i in {1, ..., \numexpr\N - 2\relax, \N} {
        \node (p\i) at (\i, 0) {};
    }

    % Another workaround by performing the calculation beforehand
    \pgfmathtruncatemacro\M{\N - 2}
    \path foreach \i in {1, ..., \M, \N} {
        (p\i) -- ($(p.center)!0.5!(p.south)$)
    };

    % Inside an expandable name, pgf/TikZ math cannot be used.
    % Therefore, e-TeX performs the expandable calculation.
    \node at ($(p\the\numexpr\N - 2\relax)!0.5!(p\N)$) {${\cdots}$};

\end{tikzpicture}
\end{document}

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