Différences

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

--- 2_programmation:macros:cette_commande_est_elle_definie [2018/05/25 08:55]
joseph.wright
+++ 2_programmation:macros:cette_commande_est_elle_definie [2021/01/07 18:23]
jejust Traduction partielle en français.
@@ Ligne 1: / Ligne 1: @@
----
+====== Comment tester si une commande est définie ? ======
-section: Macro programming
-subsection: ''Generic'' macros and techniques
-permalink: /FAQ-isdef
-date: 2014-06-10
----
-# Is this command defined?
+===== Méthodes traditionnelles =====
-Macro sets from the earliest days of TeX programming may be
+==== En TeX ====
-observed to test whether commands exist by using
-`\ifx` `\``_command_` `\undefined` &lsaquo;_stuff_&rsaquo; &hellip;
-(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}`
-which executes `action1` if the command is undefined, and
-`action2` if it is defined
-(_cmd name_ is the command name only, omitting the `\` character).
-The `ifundefined` command is based on the sequence
+Voici ce qu'on trouve dans les anciennes macros écrites en <latex>\TeX{}</latex>
-```latex
+pour tester l'existence d'une commande ''<//commande//>'':
+''\ifx\<//commande//>\undefined<//code à exécuter//>''
+(ceci exécute le code si la commande **n'**existe **pas**, bien sûr).
+==== En LaTeX ====
+Quand on programme en <latex>\LaTeX{}</latex>, on peut directement utiliser
+''\@ifundefined{<//cmd name//>}{<//action1//>}{<//action2//>}'',
+qui exécute ''<//action1//>'' si la commande **n'**est **pas** définie,
+et ''<//action2//>'' dans le cas contraire
+(''<//cmd name//>'' est le nom de la commande tout nu, **sans son antislash** ''\'').
+La macro ''\@ifundefined'' utilise ce mécanisme:
+<code latex>
 \expandafter \ifx \csname cmd name\endcsname \relax
-```
+</code>
-which relies on the way `\csname` works: if the command doesn't
+qui repose sur le fonctionnement de ''\csname'':
-exist, it simply creates it as an alias for `\relax`.
+si la commande n'existe pas, il la crée comme alias de ''\relax''.
-So: what is wrong with these techniques?
+===== Qu'est-ce qui ne va pas avec ces méthodes ? =====
-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)
+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](FAQ-etex.md) 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 ε-<latex>\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 74: @@
   \message{no command \string\foo}%
 \fi
-```
+</code>
-However, after using the LaTeX
+However, after using the original LaTeX ''\@ifundefined{foo}...'',
-`ifundefined{foo}`&hellip;, the conditionals will detect the
+the conditionals will detect the command as "existing"
-command as ''existing'' (since it has been `\let` to `\relax`);
+(since it has been ''\let'' to ''\relax'') ; so it is important
-so it is important not to mix mechanisms for detecting the state of a
+not to mix mechanisms for detecting the state of a command.
-command.
+In the 2018 <latex>\LaTeX{}</latex> release, the definition of ''\@ifundefined'' was adapted
+to use the $\epsilon$-TeX ''\ifcsname'' and now tests for a command being undefined or ''\relax''
+without the side effect of defining undefined commands to ''\relax''.
-Since most distributions nowadays use &epsilon;-TeX as their base executable
+-----
-for most packages, these two primitives may be expected appear widely
+//Source:// [[faquk>FAQ-isdef|Is this command defined?]]
-in new macro packages.
+{{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)
+}}