Autovivification

It is important to remember that autovivification happens when an undefined value is dereferenced. An assignment is not necessary. The debugger session below illustrates autovivification of a hash just from examining it:

DB<1> x \%h

0 HASH(0x2f1a248)

empty hash

DB<2> x $h{1}{2}{3}{4}

0 undef

DB<3> x \%h

0 HASH(0x2f1a248)

1 => HASH(0x2f1a260)

2 => HASH(0x29a3c68)

3 => HASH(0x2dc3038)

empty hash

DB<4>

The debugger session below illustrates autovivification of a hash from assigning to an inner hash:

DB<1> $h{A}{B}{C}{D}=1

DB<2> x \%h

0 HASH(0x83c71ac)

'A' => HASH(0x837d50c)

'B' => HASH(0x83c71e8)

'C' => HASH(0x83c7218)

'D' => 1

DB<3>

Hashes several layers deep were created automatically without any declarations. Autovivification can prevent excessive typing. If Perl did not support autovivification, the structure above would have to be created as follows:

DB<1> %h = (A => {B => {C => {D => 1}}})

DB<2> x \%h

0 HASH(0x83caba4)

'A' => HASH(0x83cfc28)

'B' => HASH(0x83cab74)

'C' => HASH(0x83b6110)

'D' => 1

DB<3>

Perl 5.6.1 and newer support autovivification of file and directory handles.{{cite web|url=http://perldoc.perl.org/perl561delta.html#File-and-directory-handles-can-be-autovivified|title=perl561delta - what's new for perl v5.6.1|website=Perl Programming Documentation }} Calling open() on an undefined variable will set it to a filehandle. According to perl561delta, "[t]his largely eliminates the need for typeglobs when opening filehandles that must be passed around, as in the following example:

for my $file ( qw(this.conf that.conf) ) {

my $fin = open_or_throw('<', $file);

process_conf($fin);

# no close() needed

}

use Carp;

sub open_or_throw {

my ($mode, $filename) = @_;

open my $h, $mode, $filename

or croak "Could not open '$filename': $!";

return $h;

}

The C++ Standard Library's associative containers (std::unordered_map and std::map) use operator[] to get the value associated to a key. If there is nothing associated to this key, it will construct it and value initialize

{{Citation | url = http://en.cppreference.com/w/cpp/language/value_initialization| contribution = Value initialization | title = C++ reference | type = wiki}}{{Rs|date=August 2024|reason=Wikipedia:Reliable_sources/Noticeboard/Archive_440#cppreference.com|certain=y}}{{Fv|date=August 2024|reason=nothing in the source about 'unordered', 'map', or 'operator'}}

the value. For simple types like int or float, the value initialization will be zero initialization.

std::unordered_map> a;

a["the answer"].push_back(42); // Autovivifies the a["the answer"] vector and then appends to it.

Another example of counting occurrences of strings:

std::unordered_map counts;

while (auto& s = GetNextString()) {

counts[s]++; // creates counts[s] if it doesn't exist and set to zero, then increment.

}

A similar trick can be achieved with the insert() method, which returns an iterator to the element associated to the key, even if it already exists.

Python's built-in dict class can be subclassed to implement autovivificious dictionaries simply by overriding the __missing__() method that was added to the class in Python v2.5.{{cite web | url = https://docs.python.org/2/library/stdtypes.html#mapping-types-dict | title = Mapping Types — dict | accessdate = 2016-06-13}} There are other ways of implementing the behavior,{{cite web | url = https://stackoverflow.com/a/19829714/355230 | title = What is the best way to implement nested dictionaries in Python? | accessdate = 2016-06-13}}{{cite web | url = https://gist.github.com/hrldcpr/2012250 | title = One-line Tree in Python | accessdate = 2017-12-27}} but the following is one of the simplest and instances of the class print just like normal Python dictionary objects.

>>> class Tree(dict):

... def __missing__(self, key):

... value = self[key] = type(self)()

... return value

>>> # Common names by class, order, genus, and type-species

>>> common_names = Tree()

>>> common_names['Mammalia']['Primates']['Homo']['H. sapiens'] = 'human being'

>>> common_names

{'Mammalia': {'Primates': {'Homo': {'H. sapiens': 'human being'}}}}

>>> # Famous quotes by play, act, scene, and page

>>> quotes = Tree()

>>> quotes['Hamlet'][1][3][3] = 'This above all: to thine own self be true.'

>>> quotes

{'Hamlet': {1: {3: {3: 'This above all: to thine own self be true.'}}}}

Ruby hashes can take a block specifying an object to be returned for non-existing indexes. These can be used to implement autovivificious maps.

irb(main):001:0> tree = proc { Hash.new { |hash, key| hash[key] = tree.call } }

=> #

irb(main):002:0> lupin = tree.call

=> {}

irb(main):003:0> lupin["express"][3] = "stand and deliver"

=> "stand and deliver"

irb(main):004:0> lupin

=> {"express"=>{3=>"stand and deliver"}}

Java Map has a method computeIfAbsent{{cite web | url = http://docs.oracle.com/javase/8/docs/api/java/util/Map.html#computeIfAbsent-K-java.util.function.Function- | title = Map (Java Platform SE 8) | accessdate = 2015-05-17}} that can be used to emulate autovivificous maps.

public static Function defaultDict(Map map, Supplier supplier) {

return key -> map.computeIfAbsent(key, k -> supplier.get());

}

public static void main(String[] args) {

Function> dict = defaultDict(new HashMap<>(), ArrayList::new);

dict.apply("foo").add("bar");

}

PHP arrays are natively autovivificious.

$arr = array();

$arr["express"][3] = "stand and deliver";

However, this only applies to assignment, and not array access.

ES6 introduces a new {{mono|Proxy}} class that can be used to implement autovivification. With other features of JavaScript, this can be reduced to a single line of code:

var tree = () => new Proxy({}, { get: (target, name) => name in target ? target[name] : target[name] = tree() });

// Test:

var t = tree();

t.first.second.third = 'text';

console.log(t.first.second.third); // or t['first']['second']['third']

C#, using indexers and C# 4.0 dynamics,

class Tree

{

private IDictionary _dict = new Dictionary();

public dynamic this[string key]

{

get { return _dict.ContainsKey(key) ? _dict[key] : _dict[key] = new Tree(); }

set { _dict[key] = value; }

}

}

// Test:

var t = new Tree();

t["first"]["second"]["third"] = "text";

Console.WriteLine(t["first"]["second"]["third"]);

{{Code|DynamicObject}} can be used for implementing different syntaxes also,

using System;

using System.Collections.Generic;

using System.Dynamic;

class Tree : DynamicObject

{

private IDictionary dict = new Dictionary();

// for t.first.second.third syntax

public override bool TryGetMember(GetMemberBinder binder, out object result)

{

var key = binder.Name;

if (dict.ContainsKey(key))

result = dict[key];

else

dict[key] = result = new Tree();

return true;

}

public override bool TrySetMember(SetMemberBinder binder, object value)

{

dict[binder.Name] = value;

return true;

}

// for t["first"]["second"]["third"] syntax

public override bool TryGetIndex(GetIndexBinder binder, object[] indexes, out object result)

{

var key = indexes[0];

if (dict.ContainsKey(key))

result = dict[key];

else

dict[key] = result = new Tree();

return true;

}

public override bool TrySetIndex(SetIndexBinder binder, object[] indexes, object value)

{

dict[indexes[0]] = value;

return true;

}

}

// Test:

dynamic t = new Tree();

t.first.second.third = "text";

Console.WriteLine(t.first.second.third);

// or,

dynamic t = new Tree();

t["first"]["second"]["third"] = "text";

Console.WriteLine(t["first"]["second"]["third"]);

• Evaluation strategy
• Variable
• Vivification

{{notelist}}

• [http://perldoc.perl.org/perl561delta.html#File-and-directory-handles-can-be-autovivified perl561delta: File and directory handles can be autovivified]
• [http://sysarch.com/perl/autoviv.txt Autovivification in Perl: An In-Depth Tutorial]
• [https://web.archive.org/web/20060115055718/http://moonbase.rydia.net/mental/blog/programming/ruby/autovivification.html Autovivification in Ruby] - emulate Perl's autovivification
• [http://www.eecs.harvard.edu/~cduan/technical/ruby/ycombinator.shtml A Use of the Y Combinator in Ruby] - Implements autovivification in Ruby with the Y Combinator.
• [http://rdoc.info/github/rubyworks/facets/master/Hash#autonew-class_method Hash#autonew in the Ruby gem "facets" adds autovivification on hash reads]
• [http://rubygems.org/gems/xkeys The Ruby gem "xkeys" facilitates nested structure traversal and autovivifies on array or hash writes]

Category:Perl

Category:Evaluation strategy

Category:Management cybernetics

Category:Articles with example Perl code

Category:Articles with example Python (programming language) code

Category:Articles with example PHP code

Category:Articles with example Java code

Category:Articles with example C++ code

Category:Articles with example Ruby code