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Explaining Autovivication

by perly-gates (Initiate)
on Jun 11, 2008 at 22:29 UTC ( #691557=perlquestion: print w/replies, xml ) Need Help??
perly-gates has asked for the wisdom of the Perl Monks concerning the following question:

I am pretty new to PERL, however I have been programming in other languages for several years (Java, C++). I was wondering what AUTOVIVICATION means?

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Re: Explaining Autovivication
by Joost (Canon) on Jun 12, 2008 at 00:49 UTC
    autovivication basically means that datastructures are automatically expanded at first use.

    For instance, if you've got a hash and access a not previously instantiated element and treat it like a hash reference, a hash reference will be created for you:

    #!/usr/bin/perl -w use strict; my %hash = (); $hash{a}->{b} = 1; print $hash{a}->{b};
    This means you need to do less explicit typing/instantiating, at the cost of possibly accidentally creating structures that aren't there when you mistype.

    Note that perl will still throw an error when you for example treat a hash reference as an array reference, so there's still some checking going on.

Re: Explaining Autovivication
by pc88mxer (Vicar) on Jun 11, 2008 at 23:03 UTC
    Wikipedia has a pretty good explanation: Autovivification.

    Another example:

    $hash->{$x}->{$y} = 1; # with autovivification ( $hash->{$x} ||= {} )->{$y} = 1; # without
Re: Explaining Autovivication
by almut (Canon) on Jun 12, 2008 at 01:59 UTC

    Just another example of where Perl automagically creates data structures as needed:

    #!/usr/bin/perl use strict; use warnings; my $href; # not initialized for my $key (keys %$href) { # ... do something } use Data::Dumper; print Dumper $href; # --> $VAR1 = {};

    which shows that $href has been autovivified to hold a reference to an (empty) hash, simply because you were using the scalar in a way (keys %$href) that would not have worked without such a data structure...

Re: Explaining Autovivication
by planetscape (Chancellor) on Jun 12, 2008 at 10:53 UTC
Re: Explaining Autovivication
by throop (Chaplain) on Jun 12, 2008 at 04:20 UTC
    Autovivification saves a lot of annoying effort. So if $quick is originally empty
    $quick->{brown}->{fox} ->{$jumped} = 1 / $Lazy_dog;
    creates the intermediate hashes so $quick can point to a hash-of-hashes-of-hashes.

    But autovivification can bite you / surprise you, too. If you are testing a value (rather than setting it) autovivification can still add entries to your hash. That is, new keys. E.g., if you start with

    $stateCityPopulation = {Illinois => {Springfield => 116482}, Massachusets => {Springfield => 154082}, Missouri => {Springfield => 150797}}};
    and then you
    foreach my $state (qw(Illinois Massachusets Ohio Missouri)){ my $pop = $stateCityPopulation->{$state}->{Springfield} || next; $springfielders += $pop}
    Now when you look at keys %$stateCityPopulation, 'Ohio' is in there. It's holding an empty array. It was created when you were trying to access the value for $stateCityPopulation->{Ohio}->{Springfield} . Even though you got undef back as a value, the itermediate array got created 'by side effect.'

    If you're debugging code, and you find an array has a bunch of unexpected keys, which hold empty hashes, then autovivification is to blame. If you'd changed the code in the above to

    $pop = ($stateCityPopulation->{$state} && $stateCityPopulation->{$state}->{Springfield}) || next;
    You'd have skipped the autovivification (at the expense of messier code.)

    BTW, if you're debugging a rogue autovivification, the 'w' switch in the debugger is very handy.


Re: Explaining Autovivication
by ikegami (Pope) on Jun 12, 2008 at 08:41 UTC
    When a scalar ($foo, $foo[$i], $foo{$k}) is undefined and it is used as a reference (@$foo, $foo->[$i], ${$foo}[$i], %$foo, $foo->{$k}, ${$foo}{$k}, $$foo), two things can happen.

    One possibility is that you get a warning. This occurs when you are trying to read from the dereferenced structure. (Technically, when the reference is used as an rvalue).

    >perl -wle"my @x = @$ref; print $ref" Use of uninitialized value in array dereference at -e line 1. Use of uninitialized value in print at -e line 1.

    The other possibility is that the reference is autovivified. That means the appropriate structure (array, hash or scalar) is constructed and a reference to it is stored in the variable. This occurs when you are trying to write to the dereferenced structure. (Technically, when the reference is used as an lvalue).

    >perl -wle"@$ref = my @x; print $ref" ARRAY(0x225228)

    Note then creating an alias to the dereferenced structure is an lvalue, and results in autovivification.

    >perl -wle"sub {}->(@$ref); print $ref" ARRAY(0x225230) >perl -wle"for (@$ref) {} print $ref" ARRAY(0x225228)

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