How to determine which register a count uses

Question

Suppose there is

\newcount\c@mycountera
\newcount\c@mycounterb
\newcount\c@mycounterc

Each \newcount uses \alloc@ and that has \wlog{\string#5=\string#2\the\allocationnumber}. Thus to determine which register mycountera uses I can look into the .log file for the (e.g.) \c@mycountera=\count123.

But how can I determine the number of the used register inside of LaTeX automatically? (Something like \registernumber{\c@mycountera} expanding to 123.) After \newcount\c@mycountera it is \the\allocationnumber, but when there is an unknown number of counts created between \newcount\c@mycountera and the time when I need the information this does not help. (I cannot use \xdef\myallocationnumber{\the\allocationnumber} immediatly after \newcount\c@mycountera, because it is defined inside of a package.)

(I suspect that the answer is usable with e.g. \dimen, \box,... too.)

Answer 1

You can use the fact that \meaning will print something like \countXXX for a register:

\def\registernumber#1{%
  \expandafter\registernumberauxi\meaning#1\halt\stop
}
\def\registernumberauxi#1#2\stop{%
  \ifx\halt#1%
    \expandafter\gobbletwo
  \else
    \expandafter\registernumberauxii
  \fi
    {#1}{#2}%
}
\def\registernumberauxii#1#2{%
  \ifnum`#1<48\else\ifnum`#1>57\else#1\fi\fi
  \registernumberauxi#2\stop
}
\def\gobbletwo#1#2{}
\def\halt{\halt}
\newcount\mycount
\registernumber\mycount
\bye

I've used a simple loop to remove the type of register from the front of the number here. There are perhaps better ways to do that, but if proves the principal. The loop uses the fact that \char48 is 0 and \char57 is 9, so anything outside of this range is not a number.

Answer 2

Here is one that works also for box registers:

\makeatletter
\def\showregister#1{\expandafter\extract@number\meaning#1\showregister}
\def\extract@number#1{%
  \ifx#1\showregister
     \expandafter\@gobbletwo
  \else
     \if\string"#1#1%
       \expandafter\expandafter\expandafter\@firstoftwo
     \else
       \ifnum`#1<`0 \else \ifnum`#1>`9 \else #1\fi\fi
       \expandafter\expandafter\expandafter\@secondoftwo
     \fi
  \fi
  \extract@rest\extract@number}
\def\extract@rest#1\showregister{#1}

\def\showcounter#1{\expandafter\showregister\csname c@#1\endcsname}
\makeatother

\newcount\AAA
\showregister\AAA

\newdimen\AAB
\showregister\AAB

\newlength\AAC
\showregister\AAC

\newsavebox\AAD
\showregister\AAD

\newcount{AAE}
\showcounter{AAE} % or \expandafter\showregister\csname c@AAE\endcsname

Box register numbers will be in hexadecimal form, preceded by "

The output should be (if starting without any package)

79
102
41
"1A
80

(Moved additional code to this other answer)

Here's a LaTeX3 version:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\showcounter} { m }
  { \exp_args:Nc \showregister { c@#1 } }
\DeclareExpandableDocumentCommand{\showregister} { m }
  {
   \tl_map_function:fN { \cs_meaning:N #1 } \showreg_scan:n
  }
\cs_generate_variant:Nn \tl_map_function:nN { f }
\cs_new:Npn \showreg_scan:n #1
  {
   \bool_if:nT
     {
      (  \int_compare_p:n { `#1 >= `0 } && \int_compare_p:n { `#1 <= `9 } )
      ||
      \int_compare_p:n { `#1 = `" }
      ||
      ( \int_compare_p:n { `#1 >= `A } && \int_compare_p:n { `#1 <= `F } )
     }
     { #1 }
  }
\ExplSyntaxOff

\def\test#1{\edef\x{\showregister#1}\message{\x\space = \meaning#1}}

\test\fboxsep

\newbox\mybox

\test\mybox

The output on the terminal will be

36 = \dimen36
"23 = \char"23

Answer 3

This is too long for a comment. As per the OP the counter number is to be used for checking the order of packages loaded. This is highly unlikely that it can be a full proof technique.

LaTeX2e, stores all the counters allocated in an \@elt list called \cl@@ckpt.

The following MWE will display all counters allocated and the name of the counter i.e., the part after c@. As you will observe by running the example, hyperTexies like Heiko will fiddle the system. Mostly though, it will predict order of loading:

% See all the counters in LaTeX2e
% and their allocation numbers
\documentclass{book}
\usepackage{graphicx}
\usepackage{longtable}
\usepackage{verse}
\usepackage{hyperref}
\usepackage{caption}
\newcounter{ZZZ}
\begin{document}
\makeatletter
\def\Z{%
   \def\@elt##1{%
       \expandafter\meaning
       \csname c@##1\endcsname
       \space = ##1,\par
    }%
   \cl@@ckpt%
}
All counter in article class plus one!
Expand list.

\Z

\section{Test}
\ifcsname c@section@level\endcsname 
   hyperef package has been loaded
    \else 
   not loaded
\fi
%\expandafter\let\csname c@section@level\endcsname\undefined 
\the\count118

\the\c@section@level

\end{document}  

A better way to check for file order is to check the @filelist.

\documentclass{book}
\usepackage{graphicx}
\usepackage{longtable}
\usepackage{verse}
\usepackage{hyperref}
\usepackage{caption}
\newcounter{ctr}
\makeatletter
\let\afilelist\@filelist
\begin{document}
% Just to see files
\@for\next:=\afilelist\do{%
 \next, \par
}
% Command factory
\stepcounter{ctr}
\@for\next:=\afilelist\do{%
      \expandafter\edef\csname @\next\endcsname{\thectr}
     \stepcounter{ctr}
}

% Define macro to check order
\def\checkpkgorder#1#2{%
   \edef\X{\csname @#1\endcsname}
    \edef\Y{\csname @#2\endcsname}
    \expandafter\ifnum\X\expandafter<\Y
      Yes loaded earlier \X, \Y
     \else
      No loaded later \X, \Y
   \fi
}

\checkpkgorder{book.cls}{ifxetex.sty}
\end{document}