Current Perl documentation can be found at perldoc.perl.org.
Here is our local, out-dated (pre-5.6) version:
Hashes are pairs of scalars: the first is the key, the second is the value.
The key will be coerced to a string, although the value can be any kind of
scalar: string, number, or reference. If a key $key
is present in the array, exists($key) will return true. The value for a given key can be undef, in which case $array{$key}
will be
undef while $exists{$key}
will return true. This corresponds to ($key
, undef) being in the hash.
Pictures help... here's the %ary
table:
keys values +------+------+ | a | 3 | | x | 7 | | d | 0 | | e | 2 | +------+------+
And these conditions hold
$ary{'a'} is true $ary{'d'} is false defined $ary{'d'} is true defined $ary{'a'} is true exists $ary{'a'} is true (perl5 only) grep ($_ eq 'a', keys %ary) is true
If you now say
undef $ary{'a'}
your table now reads:
keys values +------+------+ | a | undef| | x | 7 | | d | 0 | | e | 2 | +------+------+
and these conditions now hold; changes in caps:
$ary{'a'} is FALSE $ary{'d'} is false defined $ary{'d'} is true defined $ary{'a'} is FALSE exists $ary{'a'} is true (perl5 only) grep ($_ eq 'a', keys %ary) is true
Notice the last two: you have an undef value, but a defined key!
Now, consider this:
delete $ary{'a'}
your table now reads:
keys values +------+------+ | x | 7 | | d | 0 | | e | 2 | +------+------+
and these conditions now hold; changes in caps:
$ary{'a'} is false $ary{'d'} is false defined $ary{'d'} is true defined $ary{'a'} is false exists $ary{'a'} is FALSE (perl5 only) grep ($_ eq 'a', keys %ary) is FALSE
See, the whole entry is gone!