Undefined control sequence \pgfmath@

Question

Why does the following code give me this error message?

Error

ERROR: Undefined control sequence.

--- TeX said --- \pgfmath@dimen@ ...men@@ #1=0.0pt\relax \pgfmath@

l.28 \draw (\getWickersonLeft{c},0) -- (\getWickersonRight{c},5);

Code

\documentclass{article}

\usepackage{tikz}

\pgfkeys{/wickersons/.cd,
  execute style/.style={#1},                           
  execute macro/.style={execute style/.expand once=#1}, 
  left/.code={\gdef\wickersonsleft{#1}},
  right/.code={\gdef\wickersonsright{#1}}
}

\def\getWickersonLeft#1{\csname wickerson#1left\endcsname}
\def\getWickersonRight#1{\csname wickerson#1right\endcsname}

\newcommand*\defWickersonMacros[3]{%
  \expandafter\edef\csname wickerson#1left\endcsname{#2}
  \expandafter\edef\csname wickerson#1right\endcsname{#3}
}

\begin{document}

\begin{tikzpicture}[x=1mm,y=-1mm]%
  \foreach \x/\xvalue in {c/{left=67,right=77}} { 
    \pgfkeys{/wickersons/.cd, execute macro=\xvalue} 
    \expandafter\defWickersonMacros{c}{\wickersonsleft}{\wickersonsright}
  }
  \draw (\getWickersonLeft{c},0) -- (\getWickersonRight{c},5);
\end{tikzpicture}

\end{document}

Answer 1

I would instead use the \pgfkeysvalueof way of defining things. This is a very convoluted way of avoiding the real power of pgfkeys because instead of keys you are using dummy macros. Consider sending all your options to some keys belonging to a family.

Here the problem is that the argument of a macro is used inside another definition so # chars should be doubled.

\documentclass{article}

\usepackage{tikz}

\pgfkeys{/wickersons/.cd,
  execute style/.style={#1},                           
  execute macro/.style={execute style/.expand once=#1}, 
  left/.code={\xdef\wickersonsleft{#1}},
  right/.code={\xdef\wickersonsright{#1}}
}

\edef\getWickersonLeft#1{\csname wickerson#1left\endcsname}
\edef\getWickersonRight#1{\csname wickerson#1right\endcsname}

\newcommand*\defWickersonMacros[3]{%
  \expandafter\xdef\csname wickerson##1left\endcsname{#2}% Double ##
  \expandafter\xdef\csname wickerson##1right\endcsname{#3}% Double ##
}

\begin{document}

\begin{tikzpicture}[x=1mm,y=-1mm]%
  \foreach \x/\xvalue in {c/{left=67,right=77}} { 
    \pgfkeys{/wickersons/.cd, execute macro=\xvalue} 
    \defWickersonMacros{c}{\wickersonsleft}{\wickersonsright}
  }

\draw (\getWickersonLeft{c},0) -- (\getWickersonRight{c},5);
\end{tikzpicture}

\end{document}

The output :)

Answer 2

After percusse has answered your question, I want to give you an alternative on how to process your list with the abilities of PGF keys and TikZ.

Some of the code is inspired by another recent question of yours.

Code

\documentclass[tikz]{standalone}
\makeatletter
\pgfqkeys{/wickersons}{
  add path/.code args={#1}{%
    \begingroup% a group so the initial values don't need to be repeated but can be set in a scope above
      \pgfqkeys{/wickersons/path vars}{#1}% set only the keys that are given in '#1', other wise the current value will be used (probably the .initial one)
      % now expand everything (but the label which may contain unexpandable stuff, we only want to expand that once)
      \edef\pgf@tempa{{\pgfkeysvalueof{/wickersons/path vars/name}/.style={insert path={%
            (\pgfkeysvalueof{/wickersons/path vars/left},0) -- node [every wickersons label/.try] {\unexpanded\expandafter{\wickersons@pathvars@label}}
            (\pgfkeysvalueof{/wickersons/path vars/right},5)}}}}%
      % and get it out of this group
      \pgfmath@smuggleone\pgf@tempa
    \endgroup
    % the 'path vars' now contain all there values before 'add path' was used
    %
    % have you seen the extra pair of braces in \pgf@tempa? Those where necessary, otherwise I had to use
    % \expandafter\tikzset\expandafter{\pgf@tempa}
    % here
    \expandafter\tikzset\pgf@tempa
  },
  path vars/.cd,
   name/.initial=path,% if 'add path' has been used without the 'name' key the style would have been named 'path'
   left/.initial=0,   % if 'add path' has been used without the 'left' key the left point would have started at (0,0)
   right/.initial=0,  % ...
   label/.store in=\wickersons@pathvars@label,% or toks
   label=%            % ...
}
\makeatother
\begin{document}
\begin{tikzpicture}[
 x=1mm,
 y=-1mm,
 ->,
 thick,
 every wickersons label/.style={midway, above, sloped},
 /wickersons/add path/.list={%
   {name=foo, left= 2, right=23, label=horse},%
   {name=bar, left=25, right=29, label=sheep},%
   {name=baz, left=31,           label=\textbf{zebra}}%
 }]

  \draw [every wickersons label/.append style={pos=.2}] [foo];
  \draw [bar];
  \draw [every wickersons label/.append style={pos=.4}] [baz]; % uses right=0
\end{tikzpicture}
\end{document}

Output