How Can I Reliably Determine if a PHP Array is a Sequential List?

Differentiating Associative and Sequential PHP Arrays

PHP inherently recognizes all arrays as associative, making it challenging to distinguish between arrays that behave like lists. To resolve this, we explore a reliable method to determine if an array qualifies as a list.

List vs. Associative Arrays

• List (Sequential Array): Contains only numeric keys starting from 0, forming a sequence.

$sequentialArray = [
    'apple', 'orange', 'tomato', 'carrot'
];

• Associative Array: Consists of both string and numeric keys, allowing for more versatile data organization.

$assocArray = [
    'fruit1' => 'apple',
    'fruit2' => 'orange',
    'veg1' => 'tomato',
    'veg2' => 'carrot'
];

Identifying Lists in PHP 8.1 and Beyond

PHP 8.1 introduced array_is_list() for conveniently checking if an array is a list:

array_is_list($sequentialArray); // true

Legacy Solution for Earlier PHP Versions

To cater to pre-8.1 PHP versions, we can define a custom function:

function array_is_list(array $arr)
{
    if ($arr === []) {
        return true;
    }
    return array_keys($arr) === range(0, count($arr) - 1);
}

Testing the Function

var_dump(array_is_list([])); // true
var_dump(array_is_list(['a', 'b', 'c'])); // true
var_dump(array_is_list(["0" => 'a', "1" => 'b', "2" => 'c'])); // true
var_dump(array_is_list(["1" => 'a', "0" => 'b', "2" => 'c'])); // false
var_dump(array_is_list(["a" => 'a', "b" => 'b', "c" => 'c'])); // false

By leveraging these methods, developers can effectively distinguish between associative and sequential arrays, enabling more precise and efficient array handling in their PHP applications.

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