17

In the following MWE I've used \renewcommand to redefine a command defined through \def:

\documentclass{article}

\def\mymacro{Hello}
\renewcommand{\mymacro}{Hello world}

\begin{document}
\mymacro
\end{document}

and it works fine, as far as I can see.

What I would like to know is:

  1. Are there cases in which it doesn't work?
  2. Is it a good practice or I should I use \def to redefine commands defined through \def?

EDIT

I've already taken a look at What is the difference between \def and \newcommand?. What I mean is that sometimes I need to hack some commands defined through \def and I would like to know whether I'm allowed to do that with \renewcommand or not.

Community
  • 1
Sterry
  • 865
  • The general rule (in LaTeX) is not to use \def unless you need to. Use \(re)newcommand where possible. – Ian Thompson Nov 16 '13 at 09:16
  • @percusse Thanks for the link. But see my edit. – Sterry Nov 16 '13 at 09:29
  • 1
    renewcommand is only a wrapper around \def. – Marco Daniel Nov 16 '13 at 10:17
  • Short answers: (1) No; (2) Yes. – egreg Nov 16 '13 at 12:01
  • 1
    if a command created with \def is "context sensitive", it can't be redefined with \renewcommand. but for a simple definition like the one you show, \renewcommand works just fine, and is probably preferable. – barbara beeton Nov 16 '13 at 13:32
  • @barbarabeeton Could you please extend your comment into an answer, showing a case in which it doesn't work? I'll be happy to accept it. – Sterry Nov 16 '13 at 14:48
  • 4
    All that \renewcommand requires is 1) that the command already have a meaning, 2) that it is not \relax, and 3) that it doesn't start with \end. It doesn't care how the command acquired it's meaning. It could be via \def, \let or even \chardef, \countdef, etc. What @barbarabeeton is saying is that \renewcommand can't always duplicate the argument syntax of the original command. – Dan Nov 16 '13 at 22:53

1 Answers1

22

if a command created with \def is "context sensitive" (or "delimited"), it can't be redefined with \renewcommand. but for a simple definition like the one you show, \renewcommand works just fine, and is considered preferable in LaTeX.

the following pattern is used internally in amsmath to check the presence or class of symbols in order to "do the right thing" when defining symbol commands that are more flexible than similar ones provided by plain tex: 

\def\@tempa#1#2\@nil{<...>}

the string \@nil looks like a command, but it really isn't. instead, it is used as a "sentinel" to delimit the scope of what is being tested.

use of a sentinel in this matter avoids the need for braces when using the defined command. the "LaTeX way" is to use braces, but in some contexts, the proliferation of braces makes input more difficult to read (and debug when errors occur), and the sentinel method can be used to advantage.

edit: as noted by @Dan, this doesn't mean that a command can't be redefined using \renewcommand, but that it's impossible to replicate the usage syntax of the original when doing so.

barbara beeton
  • 88,848