FAQ 4.61: What's the difference between "delete" and "undef" with hashes?

From: PerlFAQ Server (comdog_at_panix.com)
Date: 11/15/04


Date: Mon, 15 Nov 2004 11:03:02 +0000 (UTC)

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--------------------------------------------------------------------

4.61: 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 $hash{'a'} is true (Perl5 only)
            grep ($_ 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 $hash{'a'} is true (Perl5 only)
            grep ($_ 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 $hash{'a'} is FALSE (Perl5 only)
            grep ($_ eq 'a', keys %hash) is FALSE

    See, the whole entry is gone!

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