[Tex/LaTex] What does \relax do?

macrostex-core

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What is the difference between \relax and {}?

The title suggest the question. On and off, I see macros here in TeX.SE and I see the \relax command frequently.

I was wondering what it does and where/when should I used it?

Are there any precautions that I should take while using it? (Are there side-effects?)

It would be great if you support your answers with a simple example.

Best Answer

Although \relax does nothing by itself, it is a safe command to stop expansion of another command. Some examples:

(plain tex) \hskip 5pt\relax -- in the absence of \relax, the \hskip will keep looking for plus or minus
(latex) at the end of a line, \\ \relax [...] will prevent what is in brackets from being interpreted as a dimension that would add vertical space

(actually, this is pretty well explained by answers to this question.)

Related Solutions

[Tex/LaTex] Macros with # as the last parameter

This trick of catching until the first open brace can be used in many situations.

As other posters said, it allows to catch an optional argument expandably if it is not the last. It is possible in principle to get this to be fully robust with respect to nesting, but not implemented yet.
It can be used to parse the body of a definition provided by the user, to change it to fit your purposes while keeping a natural syntax. (More on parsing def below.)
I've used this trick primarily to parse a token list expandably without losing any brace. For instance,
- to expandably uppercase or lowercase a given string (see this answer)
- to fully expand a token list expandably (almost as well as the luatex primitive \expanded (see this answer)
- to expand tokens selectively, or in the reverse order
- to write a primitive macro expander (i.e. take a file, and expand user-defined macros)
so basically any situation where you need to be careful with braces, but cannot use \futurelet.

This trick only works if there is only one character with catcode 1 (begin group character). Also, we need to be able to put a sentinel at the end of the token list that we are manipulating: otherwise, in the absence of opening brace, we would get a runaway argument.

On parsing a definition: say that you want to give the user an easy way of defining a macro which possibly takes arguments, and always produces a boxed math result. Say that you also want the parameter text to be arbitrary. Either you let the user do everything, or you parse the definition using the trick you ask about.

%non-user-friendly
\def\foo_#1^#2#3#4{\fbox{$\sum_{#1}^{#2} \frac{#3}{#4}$}}

%more user friendly (perhaps)
\boxeddef\foo_#1^#2#3#4{\sum_{#1}^{#2} \frac{#3}{#4}}

To do that:

\documentclass{article}
\makeatletter
\def\boxeddef#1#2#{\boxeddef@aux{#1}{#2}}
\def\boxeddef@aux#1#2#3{\def#1#2{\fbox{$#3$}}}
\makeatother
\begin{document}
%\def\foo#1#2#3#4{\fbox{$\sum_{#1}^{#2} \frac{#3}{#4}$}}
\boxeddef\foo_#1^#2#3#4{\sum_{#1}^{#2} \frac{#3}{#4}}
\[
\foo_{a}^{b}{C}{D}
\]
\end{document}

[Tex/LaTex] What do \@firstoftwo and \@secondoftwo do

You are missing two very important \expandafters. The normally used, "correct" code is:

\def\ifeq#1#2{%
 \ifx#1#2\relax
   \expandafter\@firstoftwo
 \else
   \expandafter\@secondoftwo
 \fi
}

The difference to a macro which uses #3 and #4 is that the if-statement is fully processed before the first or second of the next two arguments is processed. This allows that code to e.g. look ahead using \@ifnextchar. Otherwise the \else or \fi would still be after the #3 or #4, respectively, which would break such macros. Such techniques to close the if-statement first are also important for recursive macro to not accumulate a lot of cascaded if-statements.

The macros are defined \long simply to allow for the most flexible use.

The \@firstofone macro seems to be a no-op, but actually removes the braces around the argument. It is often used in an if-statement where \@gobble is used in the other clause (both again using \expandafter).

\def\foo#1#2{%
 \ifx#1#2%
   \expandafter\@firstofone
 \else
   \expandafter\@gobble
 \fi
}

\foo{\bar}{\baz}{Argument code}

Best Answer

Related Solutions

[Tex/LaTex] Macros with # as the last parameter

[Tex/LaTex] What do \@firstoftwo and \@secondoftwo do

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