Prepend associative array elements to an associative array

Question

Is it possible to prepend an associative array with literal key=>value pairs? I know that array_unshift() works with numerical keys, but I'm hoping for something that will work with literal keys.

As an example I'd like to do the following:

$array1 = array('fruit3'=>'apple', 'fruit4'=>'orange');
$array2 = array('fruit1'=>'cherry', 'fruit2'=>'blueberry');

// prepend magic

$resulting_array = ('fruit1'=>'cherry', 
                    'fruit2'=>'blueberry', 
                    'fruit3'=>'apple', 
                    'fruit4'=>'orange');

Trinitrobenzene

See also: Differences between array_replace() and array_merge() and + (array union operator) – Rage

Answer 1

Can't you just do:

$resulting_array = $array2 + $array1;

?

Hobie

See also array_merge() and its difference from using the + operator: br.php.net/manual/en/function.array-merge.php#92602 – Sundae

@cletus: Sheesh. Yeah, I can. Not sure what made me think I couldn't or what wasn't working before. Thanks for the response. – Trinitrobenzene

It is worth noting the difference but that difference is relevant for preserving numeric keys and this array is a "pure" associative array with string keys. – Hobie

it should be noted that the + operator only seems a good idea if you are sure that the keys (including iterative keys) are always different. – Englis

Answer 2

You cannot directly prepend an associative array with a key-value pair.

However, you can create a new array that contains the new key-value pair at the beginning of the array with the union operator +. The outcome is an entirely new array though and creating the new array has O(n) complexity.

The syntax is below.

$new_array = array('new_key' => 'value') + $original_array;

Note: Do not use array_merge(). array_merge() overwrites keys and does not preserve numeric keys.

Trackman

"overwrites keys and does not preserve numeric keys"... a) how does the union operator ("+") handle "duplicate" keys? re: not preserve numeric keys : likely desired – Leeth

Answer 3

In your situation, you want to use array_merge():

array_merge(array('fruit1'=>'cherry', 'fruit2'=>'blueberry'), array('fruit3'=>'apple', 'fruit4'=>'orange'));

To prepend a single value, for an associative array, instead of array_unshift(), again use array_merge():

array_merge(array($key => $value), $myarray);

Legge

Answer 4

Using the same method as @mvpetrovich, you can use the shorthand version of an array to shorten the syntax.

$_array = array_merge(["key1" => "key_value"], $_old_array);

---

References:

PHP: array_merge()

PHP: Arrays - Manual

As of PHP 5.4 you can also use the short array syntax, which replaces array() with [].

Nadianadine

Answer 5

@Cletus is spot on. Just to add, if the ordering of the elements in the input arrays are ambiguous, and you need the final array to be sorted, you might want to ksort:

$resulting_array = $array1 + $array2;
ksort($resulting_array);

Epa

As a late note, ksort returns boolean, so the above needs to be done as two statements not one, e.g. $a = $array1 + $array2; ksort($a);, otherwise $resulting_array will be a boolean value not the array you were expecting. – Marcos

Answer 6

If using Laravel, you can use prepend on a collection instance

 collect(['b' => 'b', 'c' => 'c'])->prepend('a','a');

// ['a'=>'a', 'b' => 'b', 'c' => 'c']

https://laravel.com/docs/9.x/collections#method-prepend

Plasmosome

Nicely done. I wish more users would find canonicals to post modern techniques on. This question is asking how to prepend more than one associative element to the array. Does this approach have the capacity to prepend multiple associative elements to the array/collection or do you have to call the prepend() method n times? – Rage