The \show command is extremely useful for figuring out what's going on with a particular macro. Similarly, using \the can tell me the value of a counter. I'd like to know if there's something similar for lengths and skips. At the moment, I end up doing something like \rule{1pt}{\unknownlength} but that's a fairly crude method.
TeX Core – Is There a \show for Lengths?
debugginglengthstex-core
Related Solutions
You need the layouts package.
\documentclass{article}
\usepackage{layouts}
\begin{document}
\drawlist
\begin{enumerate}
\item foo
\currentlist
\begin{figure}
\listdesign
\end{figure}
\end{enumerate}
\end{document}
As already stated the \setcounter{section}{..} must only include material which expands to a number. The string manipulation macro requires assignments internally, I guess, which makes it not fully expandable, so it causes an error.
I would recommend the xstring package instead. Its macros can also store the returned string into a (then expandable!) macro:
\documentclass{article}
\usepackage{xstring}
\begin{document}
\StrChar{\jobname}{5}[\mysubstring]%
% Or: (useful if the number would be two digits long
%\StrMid{\jobname}{5}{5}[\mysubstring]%
\setcounter{section}{\mysubstring}%
\addtocounter{section}{-1}% because it is incremented in \section
\section{Foo}
\end{document}
Note the egreg's answer is very fine and does not require a most likely big package to be loaded. I just wanted to show a LaTeX-highlevel solution for people which prefer that.
Best Answer
To show registers (which include dimensions, skips and counts) you want
\showthe.