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Como gravar a saída \StrGobbleLeft em um arquivo externo?

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Como gravar a saída \StrGobbleLeft em um arquivo externo?

Não aprendi como a expansão do TeX funciona nos bastidores. Resumindo, como gravar \StrGobbleLefta saída em um arquivo externo?

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents*}{rawdata.txt}
beamerthemeAnnArbor.sty
beamerthemeAntibes.sty
beamerthemeBergen.sty
beamerthemeBerkeley.sty
beamerthemeBerlin.sty
beamerthemeBoadilla.sty
beamerthemeboxes.sty
beamerthemeCambridgeUS.sty
beamerthemeCopenhagen.sty
beamerthemeDarmstadt.sty
beamerthemedefault.sty
beamerthemeDresden.sty
beamerthemeEastLansing.sty
beamerthemeFrankfurt.sty
beamerthemeGoettingen.sty
beamerthemeHannover.sty
beamerthemeIlmenau.sty
beamerthemeJuanLesPins.sty
beamerthemeLuebeck.sty
beamerthemeMadrid.sty
beamerthemeMalmoe.sty
beamerthemeMarburg.sty
beamerthemeMontpellier.sty
beamerthemePaloAlto.sty
beamerthemePittsburgh.sty
beamerthemeRochester.sty
beamerthemeSingapore.sty
beamerthemeSzeged.sty
beamerthemeWarsaw.sty
beamerthemebars.sty
beamerthemeclassic.sty
beamerthemecompatibility.sty
beamerthemelined.sty
beamerthemeplain.sty
beamerthemeshadow.sty
beamerthemesidebar.sty
beamerthemesplit.sty
beamerthemetree.sty
\end{filecontents*}

\usepackage{xstring}

\begin{document}
% writer
\newwrite\writer
\immediate\openout\writer=list.txt\relax
% reader
\newread\reader
\openin\reader=rawdata.txt\relax
\makeatletter
\loop
    \read\reader to \x
    \unless\ifeof\reader
        \filename@parse{\x}
        \immediate\write\writer{\StrGobbleLeft{\filename@base}{11}}
\repeat
\makeatother
\closein\reader
\immediate\closeout\writer
\end{document}

Responder1

\StrGobbleLeftnão é expansível, portanto não funcionará internamente \write. Mas o pacote xstringé tão gentil que fornece um argumento opcional no final. Se presente, é necessário um nome de macro que armazena o resultado.

\StrGobbleLeft{\filename@base}{11}[\Result]
\immediate\write\writer{\Result}

Responder2

Você precisa de uma macro que funcione via expansão se quiser colocá-la na gravação, por exemplo:

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents*}{rawdata.txt}
beamerthemeAnnArbor.sty
beamerthemeAntibes.sty
beamerthemeBergen.sty
beamerthemeBerkeley.sty
beamerthemeBerlin.sty
beamerthemeBoadilla.sty
beamerthemeboxes.sty
beamerthemeCambridgeUS.sty
beamerthemeCopenhagen.sty
beamerthemeDarmstadt.sty
beamerthemedefault.sty
beamerthemeDresden.sty
beamerthemeEastLansing.sty
beamerthemeFrankfurt.sty
beamerthemeGoettingen.sty
beamerthemeHannover.sty
beamerthemeIlmenau.sty
beamerthemeJuanLesPins.sty
beamerthemeLuebeck.sty
beamerthemeMadrid.sty
beamerthemeMalmoe.sty
beamerthemeMarburg.sty
beamerthemeMontpellier.sty
beamerthemePaloAlto.sty
beamerthemePittsburgh.sty
beamerthemeRochester.sty
beamerthemeSingapore.sty
beamerthemeSzeged.sty
beamerthemeWarsaw.sty
beamerthemebars.sty
beamerthemeclassic.sty
beamerthemecompatibility.sty
beamerthemelined.sty
beamerthemeplain.sty
beamerthemeshadow.sty
beamerthemesidebar.sty
beamerthemesplit.sty
beamerthemetree.sty
\end{filecontents*}

\usepackage{xstring}

\begin{document}
% writer
\newwrite\writer
\immediate\openout\writer=list.txt\relax
% reader
\newread\reader
\openin\reader=rawdata.txt\relax
\makeatletter
\loop
    \read\reader to \x
    \unless\ifeof\reader
        \filename@parse{\x}%
        \def\zaptheme beamertheme{}%
        \immediate\write\writer{\expandafter\zaptheme \filename@base}
\repeat
\makeatother
\closein\reader
\immediate\closeout\writer
\end{document}

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