Différences

Ci-dessous, les différences entre deux révisions de la page.

--- 2_programmation:macros:cette_commande_est_elle_definie [2018/05/24 22:02]
joseph.wright
+++ 2_programmation:macros:cette_commande_est_elle_definie [2021/01/07 18:07]
jejust Ancienne révision (2020/12/09 20:57) restaurée
@@ Ligne 1: / Ligne 1: @@
----
+====== Is this command defined? ======
-section: Macro programming
-subsection: ''Generic'' macros and techniques
-permalink: /FAQ-isdef.html
-date: 2014-06-10
----
-# Is this command defined?
 Macro sets from the earliest days of TeX programming may be
 observed to test whether commands exist by using
-`\ifx` `\``_command_` `\undefined` &lsaquo;_stuff_&rsaquo; &hellip;
+''\ifx\<command>\undefined'' <//stuff//> ...
-(which of course actually tests that the command _doesn't_
+(which of course actually tests that the command //doesn't//
 exist).  LaTeX programmers can make use of the internal command
-  `ifundefined{cmd name}{action1}{action2}`
+''\@ifundefined{cmd name}{action1}{action2}''
-which executes `action1` if the command is undefined, and
+which executes ''action1'' if the command is undefined, and
-`action2` if it is defined
+''action2'' if it is defined
-(_cmd name_ is the command name only, omitting the `\` character).
+(//cmd name// is the command name only, omitting the ''\'' character).
-The `ifundefined` command is based on the sequence
+The ''\@ifundefined'' command is based on the sequence
-```latex
+<code latex>
 \expandafter \ifx \csname cmd name\endcsname \relax
-```
+</code>
-which relies on the way `\csname` works: if the command doesn't
+which relies on the way ''\csname'' works: if the command doesn't
-exist, it simply creates it as an alias for `\relax`.
+exist, it simply creates it as an alias for ''\relax''.
 So: what is wrong with these techniques?
-Using `\undefined` blithely assumes that the command is indeed not
+Using ''\undefined'' blithely assumes that the command is indeed not
 defined.  This isn't entirely safe; one could make the name more
 improbable, but that may simply make it more difficult to spot a
 problem when things go wrong.  LaTeX programmers who use the
-technique will typically employ `undefined`, adding a single
+technique will typically employ ''\@undefined'', adding a single
 level of obscurity.
-The `ifundefined` mechanism has the unfortunate property of
+The original ''\@ifundefined'' mechanism had the unfortunate property of
 polluting the name space: each test that turns out undefined adds a
-name to the set TeX is holding, and often all those `\relax`
+name to the set TeX is holding, and often all those ''\relax''
-names serve no purpose whatever.  Even so (sadly) there are places in
+names serve no purpose whatever.
-the code of LaTeX where the existence of the `\relax` is relied
-upon, after the test, so we can't get away from `ifundefined`
-altogether.
 David Kastrup offers the (rather tricky)
-```latex
+<code latex>
 {\expandafter}\expandafter\ifx \csname cmd name\endcsname\relax ...
-```
+</code>
-which ''creates'' the `\relax`-command inside the group of the first
+which "creates" the ''\relax''-command inside the group of the first
-`\expandafter`, therefore forgets it again once the test is done.
+''\expandafter'', therefore forgets it again once the test is done.
 The test is about as good as you can do with macros.
-The [&epsilon;-TeX system]({% post_url FAQ-etex %}) system comes to our help here: it
+The [[FAQ-etex|ε-TeX system]] system comes to our help here: it
 defines two new primitives:
--  `\ifdefined`, which tests whether a thing is defined (the
+  * ''\ifdefined'', which tests whether a thing is defined (the negative of comparing with ''\undefined'', as it were), and
-    negative of comparing with `\undefined`, as it were), and
+  * ''\ifcsname cmd name\endcsname'', which does the negative of ''\@ifundefined'' without the ''\relax''-command side-effect.
--  `\ifcsname` `cmd name``\endcsname`, which does the
-    negative of `ifundefined` without the `\relax`-command
-    side-effect.
-So, in an &epsilon;-TeX-based system, the following two conditional clauses do
+So, in an <latex>$\epsilon$-\TeX{}</latex>-based system, the following two conditional clauses do
 the same thing:
-```latex
+<code latex>
 \ifdefined\foo
   \message{\string\foo\space is defined}%
@@ Ligne 74: / Ligne 65: @@
   \message{no command \string\foo}%
 \fi
-```
+</code>
-However, after using the LaTeX
+However, after using the original LaTeX
-`ifundefined{foo}`&hellip;, the conditionals will detect the
+''\@ifundefined{foo}''..., the conditionals will detect the
-command as ''existing'' (since it has been `\let` to `\relax`);
+command as "existing" (since it has been ''\let'' to ''\relax'');
 so it is important not to mix mechanisms for detecting the state of a
 command.
-Since most distributions nowadays use &epsilon;-TeX as their base executable
+In the 2018 <latex>\LaTeX{}</latex> release, the definition of ''\@ifundefined'' was adapted
-for most packages, these two primitives may be expected appear widely
+to use the $\epsilon$-TeX ''\ifcsname'' and now tests for a command being undefined or ''\relax''
-in new macro packages.
+without the side effect of defining undefined commands to ''\relax''.
+-----
+//Source:// [[faquk>FAQ-isdef|Is this command defined?]]
+{{htmlmetatags>metatag-keywords=(LaTeX,programmation LaTeX,tester si une macro est définie,savoir si une commande est définie)
+metatag-og:title=(Is this command defined?)
+metatag-og:site_name=(FAQ LaTeX francophone)
+}}