将非数字数组的元素打印为节点标签

Question

我想检索传递给 TikZ 图片作为参数的非数字数组的成员，以将其用作节点标签。 \foreach在迭代数组时很容易做到这一点，但我无法对单个元素做到这一点。以下是我两次失败的尝试：

\documentclass[tikz]{standalone}

\begin{document}

\tikzset{
   pics/mypic/.style n args={1}{
      code={
         \foreach \a [count=\i] in {#1}{
            \node at (\i, 1) {$\a$};
         }

         % Attempt 1: Doesn't work
         % \node at (1, 2) {$#1[0]$};

         % Attempt 2: Doesn't compile
         % Error: Undefined control sequence. ... Missing $ inserted. ...
         \node at (2, 2) {$\pgfmathparse{{#1}[0]}\pgfmathresult$};
      }
   }
}

\tikz{\pic {mypic={\mathtt{a},\mathtt{b},\mathtt{c}};}}
% \tikz{\pic {mypic={a,b,c};}}   % It's the same for this one.
% \tikz{\pic {mypic={0,1,2};}}   % Attempt 2 works with this one,
% \tikz{\pic {mypic={0,a,2};}}   % but not with this one ("Unknown function `a').

\end{document}

输出：

我希望能够以某种方式绘制一个带有标签的节点#1[0]。关于第二次尝试，我注意到它对文本数组也会产生同样的错误。它对全数字数组工作正常，但混合数组也会出现问题，即使访问的元素是数字。

Answer 1

在这里，我使用listofitems将所需数据读入数组\mydata。OP建议对我的原始方法进行有用的编辑，方法是将所有listofitems处理都包含在代码中\tikzset。

请注意，2in \mydata[2]（调用单个数组元素）可以是任何可扩展的计算，其结果为整数。此外，负值表示“反向索引”，即相对于数组的末尾，而不是开头。

\documentclass[tikz]{standalone}
\usepackage{listofitems}

\begin{document}

\tikzset{
   pics/mypic/.style n args={1}{
      code={
         \foreach \a [count=\i] in {#1}{
            \node at (\i, 1) {$\a$};
         }

         \readlist\mydata{#1}
         \node at (2, 2) {$\mydata[2]$};
      }
   }
}

\tikz{\pic {mypic={\mathtt{a},\mathtt{b},\mathtt{c}};}}
\end{document}

Answer 2

以下命令用于\foreach检索给定索引处的元素，并将其全局分配给给定的宏。

% Does #1 = #2[#3] with 0-based indexing
\newcommand\arrayget[3]{
    \foreach \a [count=\i from 0] in {#2}{
        \ifnum #3=\i
            \xdef#1{\unexpanded\expandafter{\a}}
        \fi
    }
}

供好奇者参考：

• 全局定义\xdef：来源1
• \unexpanded\expandafter{...}展开\a一次：来源2

它应该能够处理任何\foreach可以处理的事情。演示：

\documentclass[tikz]{standalone}

\newcommand\arrayget[3]{   % Does #1 = #2[#3] with 0-based indexing
    \foreach \a [count=\i from 0] in {#2}{
        \ifnum #3=\i
            \xdef#1{\unexpanded\expandafter{\a}}
        \fi
    }
}

\begin{document}

\tikzset{
   pics/mypic/.style n args={1}{
      code={
         \foreach \a [count=\i] in {#1}{ \node at (\i, 1) {$\a$}; }

         \arrayget{\asd}{#1}{0} \node at (1.5, 2) {$\asd$};
         \arrayget{\asd}{#1}{1} \node at (2.0, 2) {$\asd$};
         \arrayget{\qwe}{#1}{2} \node at (2.5, 2) {$\qwe$};
      }
   }
}

\tikz{\pic {mypic={\mathtt{a},\mathtt{b},\mathtt{c}};}}
\tikz{\pic {mypic={a,b,c};}}
\tikz{\pic {mypic={0,1,2};}}
\tikz{\pic {mypic={0,a,2};}}

\end{document}

输出：