PHP autovivification

Question

Update: My original intention for this question was to determine if PHP actually has this feature. This has been lost in the answers' focus on the scalar issue. Please see this new question instead: "Does PHP have autovivification?" This question is left here for reference.

According to Wikipedia, PHP doesn't have autovivification, yet this code works:

$test['a']['b'] = 1;
$test['a']['c'] = 1;
$test['b']['b'] = 1;
$test['b']['c'] = 1;

var_dump($test);

Output:

array
  'a' => 
    array
      'b' => int 1
      'c' => int 1
  'b' => 
    array
      'b' => int 1
      'c' => int 1

I found that this code works too:

$test['a'][4] = 1;
$test['b'][4]['f'] = 3;

But adding this line causes an warning ("Warning: Cannot use a scalar value as an array")

$test['a'][4]['f'] = 3;

What's going on here? Why does it fail when I add the associative element after the index? Is this 'true' Perl-like autovivification, or some variation of it, or something else?

Edit: oh, I see the error with the scalar now, oops! These work as expected:

$test['a'][4]['a'] = 1;
$test['a'][4]['b'] = 2;
$test['a'][5]['c'] = 3;
$test['a'][8]['d'] = 4;

So, php does have autovivification? Searching Google for "php autovivification" doesn't bring up a canonical answer or example of it.

PHP does have its version of what Perl developers know as `autovifify`, but PHP has a design history that is ... different from Perl ... and there are some design artifacts and anomalies that are present because of it. This means some design aspects are not clearly identified according to a systematic nomenclature, as is the case with Perl, Java, Python and other mainstream programming languages. See also: https://eev.ee/blog/2012/04/09/php-a-fractal-of-bad-design/ — dreftymac, Sep 06 '16 at 18:51

score 7 · Accepted Answer · edited Jun 20 '20 at 09:12

7

From the PHP manual on the square bracket syntax:

$arr[] = value;

If $arr doesn't exist yet, it will be created, so this is also an alternative way to create an array

With your example:

$test['a'][4] = 1;

Since $test and $test['a'] don't currently exist; they are both created as arrays.

$test['b'][4]['f'] = 3;

$test['b'] and $test['b'][4] don't currently exist; they are both created as arrays.

$test['a'][4]['f'] = 3;

$test['a'][4] does exist, but it is an integer (1). This is the "scalar value" that cannot be used as an array. You can't use the square bracket [] syntax on number values; it doesn't convert an existing value to an array.

edited Jun 20 '20 at 09:12

Community

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answered May 11 '11 at 09:59

lonesomeday

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1

For completeness: `[]` is also a string operator so this code will not trigger a warning: `$foo = 'Hello, world!'; $foo[5] = 2;` – Álvaro González May 11 '11 at 10:06
Oops, that was a basic error with the scalar. Thanks for pointing it out. – Ollie Glass May 11 '11 at 10:09
Good answer but should link "the PHP manual" if you're going to reference it. – Shovas May 07 '18 at 14:11
1

@Shovas A good point. But do note that little "edit" button at the bottom of the answer: nothing stops you from doing it yourself! – lonesomeday May 07 '18 at 14:23

score 3 · Answer 2 · answered May 11 '11 at 10:08

3

According to the results, PHP has autovivification. The error comes from the way it works.

When you say: $a[1][2] = 55, PHP wants to insert the 55 into $a[1] as [2]=>55. Since the $a[1] is non-existent, PHP is creating automatically an empty node, cca. $a[1] = Array(). But when the node already exists, PHP does not create $a[1], just performs the [2]=>55, which is an error, if $a[1] is not array-like (array, object).

The last language I've seen, where nodes may have value and children too, is MUMPS. There were also a function, called $DATA(), which told wheter a node has any child (10), value (1) or both (11), or it's non-existent (0). I think, that's the correct handling of associative arrays.

(Anyway, I like this behavior of PHP.)

answered May 11 '11 at 10:08

ern0

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I do too, but I'm surprised that a Google search for "php autovivification" doesn't bring up a canonical statement that it exists or details of how it works. – Ollie Glass May 11 '11 at 10:10
1

Yep, and I think, it's a very important feature. That makes script languages easier-to-use than compiled ones, even they have nice container classes, like map, hash and so on. My pain is that JavaScript has no autovivification. Creating a "deep" node is one line in PHP, ** line in JavaScript, and * x 2 + 2* in C++ or Java (cca.). – ern0 May 11 '11 at 10:30

score 1 · Answer 3 · answered May 11 '11 at 09:58

1

With:

$test['b'][4]['f'] = 3;

You're not adding an element to

$test['a'][4]

bacause it's not initialized as an array.

If you'd write:

$test['a'][4] = array(1);

Then it would work.

With:

$test['a']['b'] = 1;
$test['a']['c'] = 1;
$test['b']['b'] = 1;
$test['b']['c'] = 1;

You're implicitly initializing $test['a'] and $test['b'] as an array. But $test['a']['b'] (and so on) as an int

answered May 11 '11 at 09:58

Yoshi

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So no autovivifying on indexed arrays, but it's ok with associative arrays? – Ollie Glass May 11 '11 at 09:59
Not exaclty. It depends on the value you set. You just can't access int as arrays. – Yoshi May 11 '11 at 10:01

PHP autovivification

3 Answers3

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