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4.62: What's the difference between "delete" and "undef" with hashes?

Hashes contain 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 $key is
present in %hash, "exists($hash{$key})" will return true. The value for
a given key can be "undef", in which case $hash{$key} will be "undef"
while "exists $hash{$key}" will return true. This corresponds to ($key,
"undef") being in the hash.

Pictures help... here's the %hash table:

keys values
+------+------+
| a | 3 |
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+

And these conditions hold

$hash{'a'} is true
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is true
exists $hash{'a'} is true (Perl 5 only)
grep ($_ eq 'a', keys %hash) is true

If you now say

undef $hash{'a'}

your table now reads:

keys values
+------+------+
| a | undef|
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+

and these conditions now hold; changes in caps:

$hash{'a'} is FALSE
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is FALSE
exists $hash{'a'} is true (Perl 5 only)
grep ($_ eq 'a', keys %hash) is true

Notice the last two: you have an undef value, but a defined key!

Now, consider this:

delete $hash{'a'}

your table now reads:

keys values
+------+------+
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+

and these conditions now hold; changes in caps:

$hash{'a'} is false
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is false
exists $hash{'a'} is FALSE (Perl 5 only)
grep ($_ eq 'a', keys %hash) is FALSE

See, the whole entry is gone!



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