Confusion regarding strings and numbers in tikz

Question

I'm using a macro that uses \StrBetween from xstring

It seems to be working great.

However, when I try to use a value that it returns as a coordinate in a tikz picture, I get a confusing error.

Consider this snippet:

\documentclass[letterpaper,12pt]{article}

\usepackage{xstring}
\usepackage{tikz}

% http://tex.stackexchange.com/a/21566/3068
\newcommand*\GetListMember[2]{\StrBetween[#2,\number\numexpr#2+1]{,#1,},,\par}%

\begin{document}
\pagestyle{empty}
\begin{tikzpicture}
\def\A{\GetListMember{1,2,3}{1}}

% This makes me think the value of \A is 1
\node [draw] at (1,0) {\A};

% Uncommenting the following gives an error that is very confusing to me
% \node [draw] at (\A,1) {\A};
\end{tikzpicture}
\end{document}

It works, but if I uncomment the last commented line, I get this error:

! Undefined control sequence.
<argument> \@@cls 

l.18 \node [draw] at (\A,1)
                            {\A};

I think I need to convert the value of \A to a number, or something, but I don't know how.

p.s. This happens a lot, where an error message contains text that's not in my input file. (In this case \@@cls) Is there a way for me to see the intermediate forms? I feel like I'm debugging Lisp code with no macroexpand-1.

Thanks, as ever. (:

Answer 1

You haven't defined \A to be 1 you've defined it to be \GetListMember{1,2,3}{1} (you get 1 when \A is expanded). TikZ has no idea what to do with \GetListMember when it encounters it in a coordinate. In this case you can use \pgfmathtruncatemacro. To see the difference consider the following (for clarity I've used \A and \B):

\documentclass{article}
\usepackage{xstring}
\usepackage{tikz}

\begin{document}
\def\A{\GetListMember{1,2,3}{1}}
\show\A
\pgfmathtruncatemacro\B{{1,2,3}[0]}
\show\B
\end{document}

When you look at the log file you should see:

> \A=macro:
->\GetListMember {1,2,3}{1}.
l.7 \show\A
> \B=macro:
->1.
l.9 \show\B 

Answer 2

Your problem is expansion. The line

\def\A{\GetListMember{1,2,3}{1}}

defines \A to be \GetListMember{1,2,3}{1}, which is clearly not a number. TeX tries to expand this as part of the internals of \node, but \StrBetween is not 'expandable. You can see that if you try to force expansion using \edef:

\edef\A{\GetListMember{1,2,3}{1}}

which will also give the same error.

What you need is an expandable comma-list item selector. The xstring package does not provide one, but expl3 does:

\documentclass[letterpaper,12pt]{article}

\usepackage{expl3}
\usepackage{tikz}

\begin{document}
\pagestyle{empty}
\begin{tikzpicture}
\ExplSyntaxOn
\edef\A{\clist_item:nn{1,2,3}{1}}
\ExplSyntaxOff

\node [draw] at (1,0) {\A};
\node [draw] at (\A,1) {\A};
\end{tikzpicture}
\end{document}

Probably you'll want to wrap up \clist_item:nn in a user-level macro.