expansion behavior in tikz/pgf

Question

For a long time, I thougt I understood how LaTeX expands commands. But apparently, I don't.

\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \edef\r{1 and 2}
  \draw (0,0) ellipse (2 and 4); % this works
  \draw (0,0) ellipse (\r); % this causes the error
\end{tikzpicture}
\end{document}

When attempting to compile the above code, I get the following error:

! Package PGF Math Error: Unknown operator `a' or `an' (in '1 and 2').

Can someone explain to me why the above error happens, and what can be done to circumvent this problem?

It has to see an explicit and operator while parsing the content of \r then expands. It does not first expand and then checks the content. — percusse, Nov 19 '16 at 15:11
How come it works in Marks answer here: http://tex.stackexchange.com/questions/339828/pgf-3d-pie-chart-revival? — carsten, Nov 19 '16 at 15:12
I wasn't aware the expansion behavior is different between the two - why is that? — carsten, Nov 19 '16 at 15:18
Only the developers can answer that I think. But the syntax for ellipse is the deprecated one, they recommend using [x radius=2,y radius=4] which is convenient in this case too since you can set the keys instead of a macro def — percusse, Nov 19 '16 at 15:19
Doing \edef\r overwrites an existing macro within the current scope. \def followed by a single letter (or common word) macro name is never wise. — cfr, Nov 20 '16 at 01:31
Note, from \path (or equivalent) to the terminating semicolon, tikz does its own parsing. To insert a latex command inside a path, use \pgfextra{...}. To insert a tikz command inside an option one should put it inside braces. — John Kormylo, Nov 20 '16 at 17:55

score 3 · Accepted Answer · answered Apr 19 '17 at 03:30

This issue is so omnipresent that PGF actually develops a canonical way to bypass it.

\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \edef\r{1 and 2}
  \draw (0,0) ellipse (2 and 4); % this works
  \edef\pgfmarshal{\noexpand\draw (0,0) ellipse (\r);}
  \pgfmarshal
\end{tikzpicture}
\end{document}

(there are 232 \pgf@marshal in the whole package.)

expansion behavior in tikz/pgf

1 Answers1