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by gods
on Aug 24, 1999 at 22:42 UTC ( #253=perlfunc: print w/replies, xml ) Need Help??


See the current Perl documentation for exists.

Here is our local, out-dated (pre-5.6) version:

exists - test whether a hash key is present

exists EXPR

Returns TRUE if the specified hash key exists in its hash array, even if the corresponding value is undefined.

    print "Exists\n" if exists $array{$key};
    print "Defined\n" if defined $array{$key};
    print "True\n" if $array{$key};

A hash element can be TRUE only if it's defined, and defined if it exists, but the reverse doesn't necessarily hold true.

Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash key lookup:

    if (exists $ref->{"A"}{"B"}{$key}) { ... }

Although the last element will not spring into existence just because its existence was tested, intervening ones will. Thus $ref->{"A"} $ref->{"B"} will spring into existence due to the existence test for a $key element. This autovivification may be fixed in a later release.

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[ambrus]: Corion: really? Does that mean the Prima watcher objects behave as handles that stop the watcher when they're destroyed, just like AnyEvent expects?
[ambrus]: "all the bugs" -- hehe, good luck
[ambrus]: Corion: what I don't understand is, if you make a one-shot AnyEvent timer with short timeout, does this code guarantee that the Prima timer can't queue two timeout events before invoking any of them?
[ambrus]: Because that could break AnyEvent code, which is why I added a guard.
[ambrus]: (It might be easier to use the callback scalar as a guard, as in $c and &$c(); $c = ();)
[Corion]: ambrus: Hurr - yes, that might happen, as Prima will queue lots of stuff (Well, Windows will)
[Corion]: But I'm really amazed that it just took this little code (plus the idle handler I have in mind and something else dk suggested)

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