General ⇒ What does \@gobble do?

[youthdoo]

In macro2e, page 6, there's 3.2 Expanding/Gobbling Arguments, and in which you can see a macro called \@gobble. It's description says Removes (gobbles) its argument. (long) but I still don't know what it does.
I'm asking because I'm recently going through this list of macros and I have trouble understanding this one.

[Stefan Kottwitz]

Hi,

welcome to the forum!

\@gobble takes an argument and does nothing. \@gobbletwo takes two arguments and does nothing. Sounds meaningless, but for example it's a way to disable a command that takes arguments. You can override or disable simple commands like \let\thepage\relax. (\relax also does "nothing") Now, you don't have page numbers displayed anymore. But if you do the same with macros with arguments, the arguments would still be there and appear in your text. Then you can use \gobble, \@gobble or \@gobbletwo.

Quick examples:

Here, we disable page numbering:

Code: Select all

\documentclass{article}
\pagenumbering{gobble}
\begin{document}
First page
\newpage
Second page
\newpage
Third page
\end{document}

Here, we disable the \chapter macro because we don't want to have a separate chapter for the list of figures:

Code: Select all

\documentclass{book}
\makeatletter
\let\chapter\@gobbletwo
\makeatother
\begin{document}
text
\listoffigures
more Text
\end{document}

Stefan

[MjK]

Here, we disable the \chapter macro because we don't want to have a separate chapter for the list of figures:

Code: Select all

\documentclass{book}
\makeatletter
\let\chapter\@gobbletwo
\makeatother
\begin{document}
text
\listoffigures
more Text
\end{document}

IMHO not the best example, at least not without any clarification. Usually \chapter is a command with an optional and a mandatory argument, but this does not fit \@gobbletwo, because it eats two mandatory arguments. So why does it work in the example? It works, because \listoffigures does use \chapter*{\listfigurename}. So in the example, the asterisk * is the first argument and {\listfigurename} is the second. However, the shown code would fail using also a normal chapter:

Code: Select all

\documentclass{book}
\makeatletter
\let\chapter\@gobbletwo
\makeatother
\begin{document}
\listoffigures
\chapter{Test}
With text.
\end{document}

Here it eats also the W of the word With after \chapter{Test} because now, \chapter is a command with two mandatory arguments (and both are not used). An example like

Code: Select all

\documentclass{book}
\makeatletter
\let\chapter\@gobbletwo
\makeatother
\begin{document}
\listoffigures
\chapter[Toas]{Test}
With text.
\end{document}

Would also fail. So a IMHO somehow better example would be:

Code: Select all

\documentclass{book}

\begin{document}
text
\begingroup
\makeatletter
\let\chapter\@gobbletwo
\listoffigures
\endgroup
more Text
\end{document}

But also only with the explanation above.