Pass number string to setcounter

Question

Here's my current MWE: (Note, you must save the file as lec-05.tex)

\documentclass{article}
\usepackage{currfile}
\usepackage{substr}
\def\fulllecnum{
  \BehindSubString{lec-}{\scantokens\expandafter{\currfilebase\noexpand}}
}
\newcommand\lecnum{
  \IfBehindSubStringEmpty{lec-0}{\scantokens\expandafter{\currfilebase\noexpand}}{
    \BehindSubString{lec-}{\scantokens\expandafter{\currfilebase\noexpand}}
  }{
    \BehindSubString{lec-0}{\scantokens\expandafter{\currfilebase\noexpand}}
  }
}
\setcounter{section}{\lecnum}
\begin{document}
  \section{HI}
\end{document}

Here's the error:

! Missing number, treated as zero.
<to be read again> 
                   \let 
l.17 \setcounter{section}{\lecnum}
?

How can I convert \lecnum to an integer and then pass it to \setcounter{section}{}?

You need to process and store the results of xstring macros first, before you can pass that to some other function; fundamentally the xstring extractions aren't expandable. — Werner, Nov 13 '22 at 05:48
Perhaps start here: Why isn't everything expandable?. For xstring, use the optional final argument to store the result of some string test/extraction. Then you can use that result somewhere else. — Werner, Nov 13 '22 at 07:12
you can split the argument using expansion but why split? it would seem more natural and easier to start from the number. \newcommand\lec[1]{\setcounter{section}{#1}\input{lec-#1}} — David Carlisle, Nov 13 '22 at 08:45

score 0 · Accepted Answer · edited Nov 13 '22 at 10:48

Thanks to @David Carlisle who pointed this out.

\makeatletter
\def\@lecnum{}
\newcommand\lec[1]{%
  \ifnum #1<10 %
    \def\@lecnum{0#1}%
  \else
    \def\@lecnum{#1}%
  \fi
%
  \setcounter{section}{#1}%
  \renewcommand\thesubsection{#1.\arabic{subsection}}%
%
  \input{lectures/lec-\@lecnum.tex}%
}
\makeatother

I have files in the lectures folder named lec-01.tex, lec-02.tex, .... Then, I can just call this command like \lec{1} and it sets the section number to 1 and inputs the lectures/lec-01.tex file.

I was doing it the complicated way, first getting the lecture number from performing regex on the file name, then trying to convert that string to a number to set the section number.

TeX language is not “free-form”. Watch out for spaces, endlines and blank lines in definitions. — egreg, Nov 13 '22 at 09:59

Pass number string to setcounter

1 Answers1