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2_programmation:macros:cette_commande_est_elle_definie [2018/06/03 13:15]
d.p.carlisle
2_programmation:macros:cette_commande_est_elle_definie [2020/12/11 21:21]
dbitouze
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---- +====== Is this command defined? ====== 
-title: Is this command defined? +
-category: programming +
-tags: macros +
-permalink: /FAQ-isdef +
----+
  
 Macro sets from the earliest days of TeX programming may be Macro sets from the earliest days of TeX programming may be
 observed to test whether commands exist by using 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 +exist).  <latex>\LaTeX</latex> programmers can make use of the internal command 
-  `\@ifundefined{cmd name}{action1}{action2}` +''\@ifundefined{cmd name}{action1}{action2}'' 
-which executes `action1if the command is undefined, and +which executes ''action1'' if the command is undefined, and 
-`action2if 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 +<code latex>
-```latex+
 \expandafter \ifx \csname cmd name\endcsname \relax \expandafter \ifx \csname cmd name\endcsname \relax
-``` +</code> 
-which relies on the way `\csnameworks: if the command doesn'+which relies on the way ''\csname'' works: if the command doesn'
-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? So: what is wrong with these techniques?
  
-Using `\undefinedblithely 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 defined.  This isn't entirely safe; one could make the name more
 improbable, but that may simply make it more difficult to spot a improbable, but that may simply make it more difficult to spot a
-problem when things go wrong.  LaTeX programmers who use the +problem when things go wrong.  <latex>\LaTeX</latex> programmers who use the 
-technique will typically employ `\@undefined`, adding a single+technique will typically employ ''\@undefined'', adding a single
 level of obscurity. level of obscurity.
  
-The original `\@ifundefinedmechanism had 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 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.  names serve no purpose whatever. 
  
 David Kastrup offers the (rather tricky) David Kastrup offers the (rather tricky)
-```latex+ 
 +<code latex>
 {\expandafter}\expandafter\ifx \csname cmd name\endcsname\relax ... {\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 test is about as good as you can do with macros.
  
-The [&epsilon;-TeX system](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: 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 `\@ifundefinedwithout 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: the same thing:
-```latex+ 
 +<code latex>
 \ifdefined\foo \ifdefined\foo
   \message{\string\foo\space is defined}%   \message{\string\foo\space is defined}%
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   \message{no command \string\foo}%   \message{no command \string\foo}%
 \fi \fi
-``` +</code> 
-However, after using the original LaTeX +However, after using the original <latex>\LaTeX</latex> 
-`\@ifundefined{foo}`&hellip;, the conditionals will detect the +''\@ifundefined{foo}''..., the conditionals will detect the 
-command as "existing" (since it has been `\letto `\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 so it is important not to mix mechanisms for detecting the state of a
 command. command.
  
-In the 2016 LaTeX release, the definition of `\@ifundefinedwas adapted +In the 2018 <latex>\LaTeX</latex> release, the definition of ''\@ifundefined'' was adapted 
-to use the &epsilon;-TeX `\ifcsnameand now tests for a command being undefined or `\relax` +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`.+without the side effect of defining undefined commands to ''\relax''. 
 + 
 +----- 
 +//Source:// μfaquk>FAQ-isdef&Is this command defined?¢ 
 + 
 +metatag-keywords=(LaTeX,programmation LaTeX,tester si une macro est définie,savoir si une commande est définie) 
 + 
 + 
  
2_programmation/macros/cette_commande_est_elle_definie.txt · Dernière modification: 2021/09/27 21:41 de bdumont
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