[Tex/LaTex] Expanding all arguments of a command before appending it to another macro

expansion

\documentclass{article}
\makeatletter
\usepackage{ltxcmds}

\newcounter{mycounter}
\setcounter{mycounter}{7}

\newcommand{\mysndc}[3]{#1 #2 #3}

\newcommand{\mycommand}{}

\newcommand{\mytrdc}[1]{\ltx@GlobalAppendToMacro{\mycommand}{%
\mysndc{\arabic{mycounter}}{#1}{\thepage}}}

\makeatother
\begin{document}
x

\mytrdc{8}

\newpage
\setcounter{mycounter}{4}

\mycommand
\end{document}

When \mycommand is called, it is \mysndc{\arabic{mycounter}}{8}{\thepage} resulting in 4 8 2. What I would like it to be is \mysndc{7}{8}{2} resulting in 7 8 1. To achieve this, the arguments of \mysndc must be expanded before \mysndc is appended to \mycommand. \expandafter should be useful, but diverse placements of it (also more than one \expandafter) did not yield the desired result, e.g.

\newcommand{\mytrdc}[1]{\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\ltx@GlobalAppendToMacro{\mycommand\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter}{\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\mysndc\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter{\expandafter\expandafter\expandafter\arabic{mycounter}\expandafter\expandafter\expandafter}\expandafter\expandafter\expandafter{\expandafter#1\expandafter}\expandafter{\thepage}}}

. Where must I place how many \expandafters to expand the arguments of \mysndc before appending it to \mycommand and why exactly? I found numerous similar questions and answers to them here at tex.SE, but still was not able to get it right. If possible, I prefer a solution with expandafters, so that I can learn from it, and without using e-TeX/etextools/etextoolbox/LaTeX3/…

Best Answer

The easiest way to tackle this is probably to force expansion of everything using \edef, as that was we avoid needing to shuffle arguments. (\mytrdc is not expandable in any case, so we can use an assignment.) One possible approach using \edef:

\documentclass{article}

\usepackage{ltxcmds}

\newcounter{mycounter}
\setcounter{mycounter}{7}

\newcommand{\mysndc}[3]{#1 #2 #3}

\newcommand{\mycommand}{}
\makeatletter
\newcommand{\mytrdc}[1]{%
  \begingroup
  \edef\x{%
    \endgroup
    \noexpand\ltx@GlobalAppendToMacro{\noexpand\mycommand}{%
    \noexpand\mysndc{\arabic{mycounter}}{#1}{\thepage}}}%
  \x}

\makeatother
\begin{document}
x

\mytrdc{8}

\newpage
\setcounter{mycounter}{4}

\show\mycommand
\end{document}

The idea is simple: create a temporary, \edefed macro \x which contains the expanded material required plus the necessary 'set up', then execute it. Everything is done in a group so we don't mess up any other meaning of \x.

Related Solutions

[Tex/LaTex] Expanding macro arguments in newcommand

The main problem with datetime is that it is oriented towards printing dates rather than manipulating them.

A "completely expandable" version would be

\makeatletter
\newcommand{\xnewdate}[4]{\@namedef{date@#1}{/#2/#3/#4/}}
\def\xgetdateday#1{\expandafter\expandafter\expandafter\xget@I\csname date@#1\endcsname}
\def\xgetdatemonth#1{\expandafter\expandafter\expandafter\xget@II\csname date@#1\endcsname}
\def\xgetdateyear#1{\expandafter\expandafter\expandafter\xget@III\csname date@#1\endcsname}
\def\xget@I/#1/#2/#3/{#1}
\def\xget@II/#1/#2/#3/{#2}
\def\xget@III/#1/#2/#3/{#3}
\def\xgetdatemonthname#1{%
\ifcase\number\xgetdatemonth{#1}\relax
\or January%
\or February%
\or March%
\or April%
\or May%
\or June%
\or July%
\or August%
\or September%
\or October%
\or November%
\or December%
\fi}
\makeatother

\typeout{\xgetdateday{somedate} \xgetdatemonthname{somedate} \xgetdateyear{somedate}}

[Tex/LaTex] Expanding arguments before macro call

The 'classical' approach is to use \expandafter

\documentclass{article}
\begin{document}

\def\x#1#2#3#4{%
  \def\arga{#2}%
  \def\argb{#3}%
  \def\argc{#4}%
  \expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter#1%
    \expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter
      {\expandafter\expandafter\expandafter\arga\expandafter\expandafter\expandafter}%
        \expandafter\expandafter\expandafter{\expandafter\argb\expandafter}\expandafter
          {\argc}}

\def\y#1#2#3{\detokenize{#1#2#3}}

\x\y{arg1}{arg2}{arg3}

\end{document}

where we need so many of them to expand arg3 then arg2 and finally arg1. (This is what is effectively wrapped up in expl3's \exp_args:Nooo).

The rule of the number of \expandafters we need is 2ⁿ – 1, where n is how many tokens we want to expand. So for one token somewhere ahead, we need just one \expandafter in each place to be 'skipped', to expand two tokens (second one then the first one) we need three \expandafters, for three tokens (as in the current case) we need seven \expandafters, and so one. This is easiest to see if you write/print out a short second and cross off the commands as TeX would read them: you'll find everything works out.

With e-TeX available, we can use an \edef and \unexpanded:

\documentclass{article}
\begin{document}

\def\x#1#2#3#4{%
  \def\arga{#2}%
  \def\argb{#3}%
  \def\argc{#4}%
  \begingroup
    \edef\x{%
      \endgroup
      \noexpand#1
        {\unexpanded\expandafter{\arga}}%
        {\unexpanded\expandafter{\argb}}%
        {\unexpanded\expandafter{\argc}}%
    }%
  \x
}    

\def\y#1#2#3{\detokenize{#1#2#3}}

\x\y{arg1}{arg2}{arg3}

\end{document}

(You can do the same without e-TeX using a series of toks, but that gets a bit confusing so I'd not normally do it.)

The question says no expl3, but for contrast the approach using a minimium of the functions it provides would read

\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn
\def\x#1#2#3#4{
  \def\arga{#2}
  \def\argb{#3}
  \def\argc{#4}
  \exp_args:Nooo#1\arga\argb\argc
}    
\ExplSyntaxOff
\def\y#1#2#3{\detokenize{#1#2#3}}

\x\y{arg1}{arg2}{arg3}

\end{document}

which is I hope a lot more readable. (I'd probably want to use \exp_args:NVVV as we are using 'value stored in a variable', but that function is not pre-defined so I've avoided it here.)

Best Answer

Related Solutions

[Tex/LaTex] Expanding macro arguments in newcommand

[Tex/LaTex] Expanding arguments before macro call

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