Using a string as a coordinate fails

Question

When I use \pgfresult to get a node label in a \foreach, I have no problem.

However, I can't seem to get the syntax right to create a coordinate with the same method.

\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}

\begin{document}


\begin{tikzpicture}
        \def\pinlabel{{"SCL","SDA"}}
        \foreach \x in {0,1}{
        \node at (\x,0){\pgfmathparse{\pinlabel[\x]}\pgfmathresult};  %<==== works!

        %\node at (\x,0){\pgfmathparse{\pinlabel[\x]}\pgfmathresult} coordinate ({\pgfmathparse{\pinlabel[\x]}\pgfmathresult});  %<====== doesnt't work

    }
\end{tikzpicture}
\end{document}

The commented line fails with: incomplete \iffalse; all text was ignored after line 15.

I've tried doing the parse earlier, and just using the \pgfmathresult alone, but that also fails.

Answer 1

That's an expansion issue. In imprecise and colloquial terms: if the parser sees all the "junk" before the closing brackets, it does not know how to deal with it. But there is also no reason to do the expansion in the brackets. One way, certainly not unique, is

\documentclass{article}
\usepackage{tikz}

\begin{document}


\begin{tikzpicture}
        \def\pinlabel{{"SCL","SDA"}}
        \foreach \X in {0,1}{  
        \path   \pgfextra{\pgfmathparse{\pinlabel[\X]}
            \xdef\tmp{\pgfmathresult}}
        node at (\X,0) {\tmp} coordinate (\tmp);  
    }
    \draw[blue] (SCL) -- ++(1,1); %test
\end{tikzpicture}
\end{document}

---

If you do not necessarily have to do the computation of the pinlabel while you are constructing the path, you can simply perform it beforehand using \pgfmathsetmacro.

\documentclass{article}
\usepackage{tikz}

\begin{document}


\begin{tikzpicture}
  \def\pinlabel{{"SCL","SDA"}}
  \foreach \X in {0,1}{  
    \pgfmathsetmacro\tmp{\pinlabel[\X]}
    \node at (\X,0) {\tmp} coordinate (\tmp);  
  }
  \draw[blue] (SCL) -- ++(1,1); %test
\end{tikzpicture}
\end{document}

or even more efficiently, because the label doesn't change during one loop iteration, directly evaluate it as part of the \foreach statement.

\documentclass{article}
\usepackage{tikz}

\begin{document}


\begin{tikzpicture}
  \def\pinlabel{{"SCL","SDA"}}
  \foreach \X [evaluate=\X as \tmp using {\pinlabel[\X]}] in {0,1}{  
    \node at (\X,0) {\tmp} coordinate (\tmp);  
  }
  \draw[blue] (SCL) -- ++(1,1); %test
\end{tikzpicture}
\end{document}

Answer 2

Array management in TikZ is very clumsy, in my opinion.

Here's an implementation using expl3 that doesn't require \pgfmathparse juggling.

\documentclass{article}
\usepackage{tikz}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\setarray}{mm}
 {% #1 = array name, #2 = items
  \seq_new:c { l_seidman_array_#1_seq }
  \seq_set_from_clist:cn { l_seidman_array_#1_seq } { #2 }
 }
\NewExpandableDocumentCommand{\getitem}{mm}
 {% #1 = array name, #2 = index
  \seq_item:cn { l_seidman_array_#1_seq } { #2 + 1 }
 }
\ExplSyntaxOff

\setarray{pinlabel}{SCL,SDA} % can also be set locally

\begin{document}

\begin{tikzpicture}
\foreach \x in {0,1}{
  \node at (\x,0) {\getitem{pinlabel}{\x}};
  \node at (\x,1) {\getitem{pinlabel}{\x}}
        coordinate (\getitem{pinlabel}{\x});
}
\end{tikzpicture}

\end{document}