[Tex/LaTex] Setting \pagestyle{fancy} seems into create an error

conditionalsfancyhdrheader-footer

Calling fancy page style is somehow apparently causing the following condition to test true.

\documentclass{article}
\usepackage{fancyhdr}
\pagestyle{fancy}
\ifcsname chapter\endcsname%
    \renewcommand{\chapter}{}
\fi
\pagestyle{empty}
\begin{document}
testing
\end{document}

If I place \pagestyle{fancy} after the conditional, \ifcsname chapter....\fi, everything works. This is an article class, so this conditional should be false. If I call \pagestyle{plain}, it doesn't matter where I place it.

Why does this order matter?

Best Answer

The page style fancy (\ps@fancy) contains:

\@ifundefined{chapter}{...}{...}

That is the LaTeX test for undefined:

\def\@ifundefined#1{%
  \expandafter\ifx\csname#1\endcsname\relax
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi}

\csname has the side effect that it defines an undefined command with the meaning \relax:

\show\chapter
\pagestyle{fancy}% \@ifundefined{chapter}{...}{...}
\show\chapter

The meaning of \chapter changes:

> \chapter=undefined.
l.3 \show\chapter

? 
> \chapter=\relax.
l.5 \show\chapter

Now \chapter is defined for e-TeX's \ifcsname and \renewcommand{\chapter}{} is called that again used the LaTeX idea of undefined: A command is undefined if it is undefined or has the meaning of \relax. As result \renewcommand complains that `\chapter is not defined.

e-TeX's \ifcsname does not have the side effect and lets the command untouchted. (Also it is internally optimized in the sense that it does not create a hash table entry.)

The test needs to be a little longer:

\ifcsname chapter\endcsname
  \ifx\chapter\relax
  \else
    \renewcommand\chapter{}
  \fi
\fi

A defined \chapter might confuse packages and macros about the capabilities of the class (article does not have chapters). If the purpose of the mysterious \renewcommand\chapter{} is to undefine chapter, then it will fail, because \chapter remains defined with the meaning of an empty macro. A command can be undefined by assigning it to an undefined macro:

\ifcsname chapter\endcsname
  \let\chapter\UnDeFiNeD
\fi

Related Solutions

[Tex/LaTex] How to make pages after the first come out correctly with fancyhdr

If you don't set a proper head height, fancyhdr reports the following:

Package Fancyhdr Warning: \headheight is too small (XX.Xpt): 
 Make it at least YY.YYYYpt.
 We now make it that large for the rest of the document.
 This may cause the page layout to be inconsistent, however.

As such, the newly-set \headheight will be the same throughout the document. To avoid this, I've created the following minimal document that sets a consistent head height, adding a vertical space (gap) on the first page to simulate a larger header:

enter image description here

\documentclass{article}

\usepackage{fancyhdr,datetime,geometry,lipsum,graphicx}

%%%%%%%%%%%%%%%%%%date in format Sunday, October 25, 2011
\usdate
\def\theday {\dayofweekname{\day}{\month}{\year}}
\def\mydate {\theday , \today}

%%%%%%%%%%return address%%%%%%%%%%
\def\myname         {Frank Thorne}
\def\mydeptname     {Department of Mathematics}
\def\myaffiliation  {University of South Carolina}
\def\mystreet       {1523 Greene Street}
\def\mycitystatezip {Columbia,~SC \quad 29208}
\def\myphone        {\textit{Phone:} (803)404-4057 (home)}
\def\office         {(803)777-4224}
\def\fax            {\textit{Fax:}(803)777-3783}
\def\email          {thorne@math.sc.edu}
\def\url            {\texttt{http://www.math.sc.edu/~thornef}} % NOTE: use $\sim$ for tilde
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%margins%%%%%%%%%%%%%%%%%%
\geometry{
  margin = 1in,
  headheight = 13.5pt
}
\setlength{\parindent}{0pt} %controls how the indentation of new paragraphs behaves
\setlength{\parskip}{6pt} %controls the space between paragraphs

%%%%%%%%%%%%%%%%letterhead%%%%%%%%%%%%%%%%
\fancyfoot{}
\renewcommand{\headrulewidth}{0pt}
\renewcommand{\footrulewidth}{0pt}
\pagestyle{fancy}%

\fancypagestyle{firstpage}{
  \fancyhf{}% Clear header/footer
  %%%usc logo
  \fancyhead[L]{\raisebox{\dimexpr-\height+\baselineskip}[0pt][0pt]{\includegraphics[width=2.5in]{example-image}}}
  %%%%%%%%%%return address on right%%%%%%%
  \fancyhead[R]{\leavevmode\footnotesize\itshape%
  \begin{tabular}[t]{l@{}}
    \myname \\ \mydeptname \\
    \myaffiliation \\ \mystreet \\
    \mycitystatezip \\ \email \\[\normalbaselineskip]
    \mydate
  \end{tabular}}
}

\begin{document}

\thispagestyle{firstpage}

\vspace*{8\baselineskip}

\lipsum[1-10]

\bigskip

\hspace{.5\textwidth}%
\begin{tabular}{@{}l}
  Sincerely, \\[2\normalbaselineskip]
  Frank Thorne \\
  \textit{Assistant Professor of Mathematics}
\end{tabular}

\end{document}

I've used geometry to adjust the page layout - it's just easier/more convenient.

[Tex/LaTex] How to set custom fancy pagestyle back to default fancy

Note that with a KOMA-Script class the usage of scrlayer-scrpage is recommended:

\documentclass[11pt, twoside, a4paper, DIV=12, BCOR=16mm, listof=totoc]{scrartcl}
\usepackage[
  automark,
  headsepline,
  markcase=upper
]{scrlayer-scrpage}
\clearpairofpagestyles
\ihead{\leftmark}
\ohead{\rightmark}
\cfoot*{\pagemark}

\usepackage{acronym}
\newpairofpagestyles[scrheadings]{acronyms}{%
  \ihead{}\ohead{Acronyms}}

\usepackage{blindtext}
\begin{document}
\pagestyle{acronyms}
\section{Acronyms}
  \begin{acronym}
    \acro{BLA}{BLA}
  \end{acronym}
  \clearpage
  \pagestyle{scrheadings}
  \blinddocument
\end{document}

It is also possible to use fancyhdr. Then you have to define another page style:

\documentclass[11pt, twoside, a4paper, DIV=12, BCOR=16mm, listof=totoc]{scrartcl}
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancypagestyle{main}{
\fancyhf{}
\fancyhead[LE,RO]{\slshape \rightmark}
\fancyhead[LO,RE]{\slshape \leftmark}
\fancyfoot[C]{\thepage}
\renewcommand*{\headrulewidth}{0.4pt}
\renewcommand*{\footrulewidth}{0pt}
}

\usepackage{acronym}
\fancypagestyle{acronyms}{
  \fancyhf{}
  \fancyhead[LO,RE]{Acronyms}
  \cfoot{\thepage}
}

\usepackage{blindtext}
\begin{document}
  \pagestyle{acronyms}
  \section{Acronyms}
  \begin{acronym}
    \acro{BLA}{BLA}
  \end{acronym}
  \clearpage
  \pagestyle{main}
  \blinddocument
\end{document}

Warning: Do not ignore any warning regarding the old font commands like \rm, \sl. Note that fancyhdr uses this commands in its default header and footer definitions.

Starting with the current prerelease of the next KOMA-Script Version (3.20) KOMA Script does not define this old commands, see Bekannte Probleme und Änderungen in der zukünftigen KOMA-Script-Version (German). So

\documentclass{scrartcl}[2015/11/06]
\usepackage{fancyhdr}
\pagestyle{fancy}
\begin{document}
Text
\end{document}

will result in errors. You can avoid the errors if you either use \fancyhf{} and define your own header and footer using \fancyhead and \fancyfoot without the usage of old font commands or you use a compatibility option.

Best Answer

Related Solutions

[Tex/LaTex] How to make pages after the first come out correctly with fancyhdr

[Tex/LaTex] How to set custom fancy pagestyle back to default fancy

Related Question