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The element is not autovivified if it doesn't exist.</p> <pre><code>print "Exists\n" if exists $hash{$key}; print "Defined\n" if defined $hash{$key}; print "True\n" if $hash{$key}; print "Exists\n" if exists $array[$index]; print "Defined\n" if defined $array[$index]; print "True\n" if $array[$index];</code></pre> <p>A hash or array element can be true only if it's defined, and defined if it exists, but the reverse doesn't necessarily hold true.</p> <p>Given an expression that specifies the name of a subroutine, returns true if the specified subroutine has ever been declared, even if it is undefined. Mentioning a subroutine name for exists or defined does not count as declaring it. Note that a subroutine which does not exist may still be callable: its package may have an <code>AUTOLOAD</code> method that makes it spring into existence the first time that it is called -- see <a href="/5.8.8/perlsub">perlsub</a>.</p> <pre><code>print "Exists\n" if exists &subroutine; print "Defined\n" if defined &subroutine;</code></pre> <p>Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or array key lookup or subroutine name:</p> <pre><code>if (exists $ref->{A}->{B}->{$key}) { } if (exists $hash{A}{B}{$key}) { } if (exists $ref->{A}->{B}->[$ix]) { } if (exists $hash{A}{B}[$ix]) { } if (exists &{$ref->{A}{B}{$key}}) { }</code></pre> <p>Although the deepest nested array or hash will not spring into existence just because its existence was tested, any intervening ones will. Thus <code>$ref->{"A"}</code> and <code>$ref->{"A"}->{"B"}</code> will spring into existence due to the existence test for the $key element above. This happens anywhere the arrow operator is used, including even:</p> <pre><code>undef $ref; if (exists $ref->{"Some key"}) { } print $ref; # prints HASH(0x80d3d5c)</code></pre> <p>This surprising autovivification in what does not at first--or even second--glance appear to be an lvalue context may be fixed in a future release.</p> <p>See <a href="/5.8.8/perlref#Pseudo-hashes%3A-Using-an-array-as-a-hash">"Pseudo-hashes: Using an array as a hash" in perlref</a> for specifics on how exists() acts when used on a pseudo-hash.</p> <p>Use of a subroutine call, rather than a subroutine name, as an argument to exists() is an error.</p> <pre><code>exists &sub; # OK exists &sub(); # Error</code></pre> </dd> </dl> </div> <div id="footer"> <p>Perldoc Browser is maintained by Dan Book (<a href="https://metacpan.org/author/DBOOK">DBOOK</a>). 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