xstring macros inside foreach

Question

I don't really understand what is the problem here, but I guess it has to do with expansion and fragile macros since when I do the same outside of \path it works.

Code:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{arrows,calc}
\usepackage{xstring}

\newcommand{\aw}{10pt}
\newcommand{\bbw}{20pt}
\newcommand{\cw}{30pt}

\newcommand{\width}[1]{%
  \StrBefore{#1}{'}[\notprimed]
  \IfEndWith{#1}{'}
  { \csname \notprimed w\endcsname }
  { \csname #1w\endcsname }
}

\begin{document}

\foreach \name in {a,bb,c,a'}
{ ++(\width{\name},0pt) node {\name} };

\begin{tikzpicture}
  % \draw (0pt,0pt) node {s}
  %   ++(\aw,0pt) node {a}
  %   ++(\bbw,0pt) node {bb}
  %   ++(\cw,0pt) node {c}
  %   ++(\aw,0pt) node {a'};

  \draw (0pt,0pt) node {s}
    \foreach \name in {a,bb,c,a'}
    { ++(\width{\name},0pt) node {\name} };
\end{tikzpicture}


\end{document}

I think it has to do with xstring also because when I remove the \StrBefore and \IfEndWith from the definition of \width it also works.

Any ideas?

Answer 1

You need to move the usage of the xstring macros outside of the coordinates as they are not fully expandable and do not result simply in \csname #1w\endcsname.

With \pgfextra{<stuff>} or \pgfextra <stuff> \endpgfextra you can do <stuff> that does not belong into a path.

It seems to me, you just want to strip a trailing ' from, so you could simply do:

\makeatletter
\newcommand{\width}[1]{%
    \StrSubstitute{#1}{'}{}[\qrr@width]%
    \expandafter\let\expandafter\widthresult\csname\qrr@width w\endcsname}
\makeatother

and use it in

  \foreach \name in {a,bb,c,a'}{
    \pgfextra{\width{\name}}
    ++ (\widthresult,0pt) node {\name}
  }

---

The following solution strips everything after the first ' and works (due its simplicity) even inside a coordinate.

Reference

• What do \makeatletter and \makeatother do?
• Macro delimiter (one of many)

Code

\documentclass{article}
\usepackage{tikz}
\newcommand{\aw}{10pt}
\newcommand{\bbw}{20pt}
\newcommand{\cw}{30pt}
\makeatletter
\def\strip@upquote#1'#2\@strip@upquote{#1}
\newcommand{\width}[1]{\csname\expandafter\strip@upquote#1'\@strip@upquote w\endcsname}
\makeatother
\begin{document}
\begin{tikzpicture}
  \draw (0pt,0pt) node {s}
  \foreach \name in {a,bb,c,a'}{
    ++ (\width{\name},0pt) node {\name}
  };
\end{tikzpicture}
\end{document}