What is the meanging of "\expandafter\PDR@getfileinfo\reserved@a\relax? ? \relax\relax"

Question

Needing help of explainning the meaning of the following code for me, the code is in the file "PostDocRep.cls" that begins with

    \NeedsTeXFormat{LaTeX2e}[1995/12/01]
    \ProvidesClass{PostDocRep}
      [2009/09/23 v0.1e PostDocRep
       document class]
    ........
\def\PDR@getfileinfo#1 #2 #3\relax#4\relax{%
  \def\PDRfiledate{#1}%
  \def\PDRfileversion{#2}%
  \def\PDRfileinfo{#3}}%
\expandafter\ifx\csname ver@PostDocRep.cls\endcsname\relax
  \edef\reserved@a{\csname ver@ctextemp_PostDocRep.cls\endcsname}
\else
  \edef\reserved@a{\csname ver@PostDocRep.cls\endcsname}
\fi
\expandafter\PDR@getfileinfo\reserved@a\relax? ? \relax\relax

I do not know what is the result of statement

\expandafter\ifx\csname ver@PostDocRep.cls\endcsname\relax

if it ture or false and which branch it selects?

\edef\reserved@a{\csname ver@ctextemp_PostDocRep.cls\endcsname}

or

  \edef\reserved@a{\csname ver@PostDocRep.cls\endcsname}

Moreover, I have no idea of how the statement

\expandafter\PDR@getfileinfo\reserved@a\relax? ? \relax\relax

execute? why there are so many \relax,

this is the third question you have asked where you just pick one line out of context and ask what it means. I'm not sure it's a useful way to learn latex. — David Carlisle, Apr 09 '22 at 22:22
the code depends on ver@ctextemp_PostDocRep.cls which is presumably defined somewhere in code you have not shown. — David Carlisle, Apr 09 '22 at 22:33
\expandafter\ifx\csname ver@PostDocRep.cls\endcsname\relax should never be true as it follows \ProvidesClass{PostDocRep} that defines that command so it is hard to guess why this test is there without seeing the full context — David Carlisle, Apr 09 '22 at 22:36
Why are you using a decade old document class that someone gave you but you don't understand? — Teepeemm, Apr 09 '22 at 22:43
@DavidCarlisle can I upload the wholefile, and the name of the whole class file is PostDocRep.cls the begins with \NeedsTeXFormat{LaTeX2e}[1995/12/01] \ProvidesClass{PostDocRep} [2009/09/23 v0.1e PostDocRep document class] ........ meanwhile, there is no ver@ctextemp_PostDocRep.cls at all, only the file PostDocRep.cls — Peter Yang, Apr 09 '22 at 23:27
well as I say that branch chould never be used as the test is pointless the command will always be defined in latex. the code probably predates latex. But anyway I already posted an answer. — David Carlisle, Apr 09 '22 at 23:34
If you really want to learn LaTeX programming refer to package writing - Where do I start LaTeX programming? - TeX - LaTeX Stack Exchange. Otherwise, I think asking people to do work for you every time is not going to be very well-received — user202729, Apr 10 '22 at 04:09

David Carlisle · Answer 1 · 2022-04-09T23:07:10.947

\reserved@a will be 2009/09/23 v0.1e PostDocRep document class

so \expandafter\PDR@getfileinfo\reserved@a\relax

is

\PDR@getfileinfo 2009/09/23 v0.1e PostDocRep document class\relax

which will do

  \def\PDRfiledate{2009/09/23}%
  \def\PDRfileversion{v0.1e}%
  \def\PDRfileinfo{PostDocRep document class}%

The trailing ? and \relax ensure that even if for some reason \reserved@a did not have the right form there would still be some space characters and \relax so that \PDR@getfileinfo will always see the arguments and not have a runaway argument error looking for a string delimited by a missing \relax

What is the meanging of "\expandafter\PDR@getfileinfo\reserved@a\relax? ? \relax\relax"

1 Answers1