spO0q

Posted on Feb 7, 2020 • Edited on Nov 23

Things you probably don't know about PHP arrays

#php #beginners #programmation

Everybody uses arrays whether it's Php or not. Regarding Php arrays there are special functions you can use to make the most of them.

Arrays are easy to use and quite efficient.

What you probably already know

Arrays are data structures. This is what it looks like :

$alphabet  = ["a", "b", "c"];
$_alphabet = range("a", "c");

It creates lists. Far far better than :

$a = "a";
$b = "b";
$c = "c";
... etc

And when dealing with specific values we can easily access them e.g :

$a  = $alphabet[0];
$_a = reset($_alphabet);// it's just another way to get "a"

There are different types of arrays, some of them can have complex structure because specific keys can be defined as strings. You can even create multiple nested levels.

Php deeply

There are related terms for arrays :

lists
dictionaries
collections
trees

source

Php stores these structures in let's say "slots". Under the hood (internally) there is a hash function that computes data with integers in these slots.

Besides data are added in a way that Php remembers the exact ascending order they were added.

If you want to know how it's done you have to go deeper.

Ordered lists of elements

Simple arrays do not have named keys, if you want to access a particular value you can do the following :

$alphabet  = ["a", "b", "c"];
$a         = $alphabet[0];

In this case it works because obviously I have only 3 elements in that list so who cares... but in real life arrays can contain hundreds, thousands of entries. So that would be very lame this way.

Fortunately arrays can contain key/value pairs with named keys and when you specify names for your keys (as strings) the storing part is quite different.

Going a little bit deeper with hash tables

Ok we just saw data are stored in slots but that's still kind of abstract.

Arrays are actually implemented with hash tables. It's how Php indexes memory, it's how the internal hash function computes data with integers in slots.

When you use named keys (associative array), it's first converted to integers before indexing memory. The index stored as integer in memory contains the key/value pairs.

When you do this, you create maps.

source

Hash tables are used to store pretty much everything such as functions, methods, objects, arrays, variables.

C - origins

Php is implemented with C. The hash function computes your keys with integers. These integers are offsets in C arrays which contain linked list of possible values. Php runs all these possible values until it finds the matching element.

You don't have to specify key if it's an integer

Non-associative arrays do no need to be written like that :

$actresses = [ 0 => 'Scarlett Johansson', 1 => 'Ann Hathaway', 2 => 'Charlize Theron' ];

instead just write :

$actresses = [ 'Scarlett Johansson', 'Ann Hathaway', 'Charlize Theron' ];

Php automatically increments keys as integers starting at 0.

Array Destructuring

You can do the following :

$actresses = [
    [ 'Scarlett Johansson', 'Lost in Translation' ],
    [ 'Ann Hathaway', 'Interstellar' ],
    [ 'Charlize Theron', 'Aeon Flux' ],
];

foreach ($actresses as $actress) {
    [ $person, $movie ] = $actress;
    echo "$person in $movie" . PHP_EOL;
}

Here [ $person, $movie ] = $actress;is the shorthand for list($person, $movie) = $actress;

In this example if you just want movies without actresses indeed do not use person and add a comma before movie.

Array chunks

array_chunk($actresses, 2) will split the array and may simplify futher processing:

Array
(
    [0] => Array
        (
            [0] => Array
                (
                    [0] => Scarlett Johansson
                    [1] => Lost in Translation
                )

            [1] => Array
                (
                    [0] => Ann Hathaway
                    [1] => Interstellar
                )

        )

    [1] => Array
        (
            [0] => Array
                (
                    [0] => Charlize Theron
                    [1] => Aeon Flux
                )

        )

)

Wrap up

Php arrays are very powerful and fun. You have to go a little bit deeper to understand how it's stored in memory. I hope this introduction encourages you to read more about it.

Top comments (6)

Ben Sinclair • Jun 10

Your demonstration of array_chunk is a little confusing as it doesn't result in anything other than the original array wrapped in another. Could I suggest demoing with a size parameter of 2, which would split the array into two chunks of lengths 2 and 1, and make it a bit clearer what's going on?

spO0q • Nov 23

done :)

hbgl • Jun 5 '21 • Edited

Arrays are actually implemented with hash tables.

Internally, PHP has the notion of packed arrays. These are arrays that have consecutive integer indices, e.g. ['foo', 'bar', 'baz'], with some additional caveats. The interesting thing is, that they are not implemented with a hash table, but with a contiguous list. I thought this would be interesting to know because it can give you a performance boost in some applications.