in reply to Re: Should this happen?
in thread Should calling 'exists' create a hash key?

Your second example with an array isn't really autovivification. Perl hashes are sparse (elements aren't stored contiguously in memory), whereas Perl arrays aren't sparse (the sequence of elements is stored contiguously in memory). When you do my %h; $h{ 100 } = 2; the one SV* is stored; when you do your example, Perl has to allocate room to store 101 SV*s and store the SV* for the 2 in the 101st (the other 100 contain pointers to sv_undef).

Autovivification is when Perl allocates a reference to a new anonymous hash or anonymous array on the fly; this is just how arrays work. Now if you did $a[100][42] = 3.14159 that would be autovivification in that it would create a new arrayref (again, with room for 43 elements preallocated) and store it into $a[100] and then store 3.14159 into the last element.

The difference from C or Java is that Perl will automatically extend arrays for you (which again isn't autovivification), whereas the other languages you do have to manually allocate or extend them.

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Re^3: Should this happen?
by pg (Canon) on Sep 20, 2005 at 13:21 UTC

    In the case of $a[100][42] = pi, obviously there is autovivification. That has no difference from $a->{"a"}{"a"} = pi.

    Whether $a[100] makes $a[0] through $a[99] autovivified is not a black white thing, and I am not the first person who classifies that as autovivification. When it is valid to look at this from a perl internal point of view as you did, it is at least not less valid to look at this from a logical point of view. Look at this:

    use strict;
use warnings;

my @as;
$as[10] = 0;
for my $a (@as) {
    print ".";
}

    [download]

    this will print 11 dots, not 1 or 12.

    To further pursuit a single correct answer to this question is not very useful, as autovivification is not a clearly defined concept, but rather a commonly used not-clearly-defined terminology that is often expressed through examples.

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      Quoting perldoc perlref:

      This is one of the cases we mentioned earlier in which references could spring into existence when in an lvalue context. Before this statement, $array[$x] may have been undefined. If so, it's automatically defined with a hash reference so that we can look up "{"foo"}" in it. Likewise "$array[$x]->{"foo"}" will automatically get defined with an array reference so that we can look up "[0]" in it. This process is called autovivification.

      Going by the documentation it would seem to have a specific, clear definition: autovivification is the process of automatically creating a new reference (hashref or arrayref) to an anonymous data structure in an lvalue context. Automatically extending an array doesn't create a reference to a new anonymous data structure, it merely increases the size of an existing one.

      But yes, this is degenerating into pedantry . . . :)

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