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FAQ 4.61 What's the difference between "delete" and "undef" with hashes? #4 433658

This is an excerpt from the latest version perlfaq4.pod, which
comes with the standard Perl distribution. These postings aim to 
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

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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 $hash{'a'}                is true (Perl 5 only)
            grep ($_ 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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PerlFAQ
3/7/2008 8:03:01 PM
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