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Set Theory Union of arrays in PHP

There are 3 operations with sets in mathematics: intersection, difference and union (unification). In PHP we can do this operations with arrays:

  • intersection: array_intersect
  • difference: array_diff

What function is for union?

No duplicates can be in the result array (like array_intersect and array_diff).

If indexes are numeric then array_merge will not overwrite the original value, but will be appended (PHP docs).

Try array_merge:

array_unique(array_merge($array1, $array2));

PHP Manual

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      • 1
    • I don't understand why this is accepted and upvoted more than the answers pointing out the union operator (+). Am I missing something about the union operator?
      • 1
    • PHP's "union operator" does not perform a mathematical union. var_dump([1,2]+[3,4]) has the mathematical union of [1,2,3,4] (due to 4 values), but the two arrays only have two unique keys, therefore the script will output [1,2]. PHP's "union" operator is totally unrelated to a mathematical union
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    • Note that array_unique won't reindex the keys for numeric arrays - you may need to use array_values as well if the array indexes need to be sequential.
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    • @JoshRibakoff PHP's union operator is a mathematical union of the array's keys, not its values. It is certainly not "totally unrelated" to a mathematical union. You can use array_flip to put the values in the keys so the array union operator applies - see my answer.
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    • Beware that this will not work if the input arrays contain string keys. For example: php -r 'var_dump(array_unique(array_merge(["a"=>"cat","b"=>"dog"], ["b"=>"cow","c"=>"hen"])));' produces ['cat','cow','hen'] because values with the same string key are overwritten. To avoid this problem, call array_values on each input array before merging, as follows: php -r 'var_dump(array_unique(array_merge(array_values(["a"=>"cat","b"=>"dog"]), array_values(["b"=>"cow","c"=>"hen"]))));'

Adrien's answer won't necessary produce a sequentially numbered array from two sequentially numbered arrays - here are some options that will:

array_values(array_unique(array_merge($array1, $array2)));

(Adrien's answer with renumbering the keys afterward)


(Put the values in the keys, and use the array union operator)

array_merge($array1, array_diff($array2, $array1))

(Remove the shared values from the second array before merging)

Benchmark results (for merging two arrays of length 1000 a thousand times on my system):

  • Unique (Adrien's version): 2.862163066864 seconds
  • Values_Unique: 3.12 seconds
  • Keys_Flip: 2.34 seconds
  • Merge_Diff: 2.64 seconds

Same test, but with the two arrays being very similar (at least 80% duplicate):

  • Unique (Adrien's version): 2.92 seconds
  • Values_Unique: 3.15 seconds
  • Keys_Flip: 1.84 seconds
  • Merge_Diff: 2.36 seconds

It seems using the array union operator to do the actual union is the fastest method. Note however that array_flip is only safe if the array's values are all strings or all integers; if you have to produce the union of an array of objects, I recommend the version with array_merge and array_diff.

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use "+" operator to do so. See the link Array Operators

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From the code in the PHP: Array Operators documentation:

$a = array("a" => "apple", "b" => "banana");
$b = array("a" => "pear", "b" => "strawberry", "c" => "cherry");

$c = $a + $b; // Union of $a and $b
echo "Union of \$a and \$b: \n";

$c = $b + $a; // Union of $b and $a
echo "Union of \$b and \$a: \n";

When executed, this script will print the following:

Union of $a and $b:
array(3) {
  string(5) "apple"
  string(6) "banana"
  string(6) "cherry"
Union of $b and $a:
array(3) {
  string(4) "pear"
  string(10) "strawberry"
  string(6) "cherry"
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      • 1
    • From the top comment on the PHP: Array Operators documentation. "The union operator did not behave as I thought it would on first glance. It implements a union (of sorts) based on the keys of the array, not on the values." He then goes on to suggest array_unique(array_merge($a,$b))
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    • I've added the link, but if you are going to copy and paste directly from the PHP source code you should add in a reference.

The OP did not specify whether the union is by value or by key and PHP has array_merge for merging values and + for merging the keys. The results depends on whether the array is indexed or keyed and which array comes first.

$a = ['a', 'b'];
$b = ['b', 'c'];

$c = ['a' => 'A',  'b' => 'B'];
??$d = ['a' => 'AA', 'c' => 'C'];

Indexed array

See array_merge

By value using array_merge

??array_merge($a, $b); // [?0 => 'a', 1 => 'b', 2 => 'b', 3 => 'c']
??array_merge($b, $a); // ?[0 => 'b', 1 => 'c', 2 => 'a', 3 => 'b']

merge by key using + operator

See + operator

??$a + $b; ?// [0 => 'a', 1 => 'b']
$b + $a; // [0 => 'b', 1 => 'c']

Keyed array

By value using array_merge

?array_merge($c, $d); // ?['a' => 'AA', 'b' => 'B', 'c' => 'C']
array_merge($d, $c); // ['a' => 'A',  'c' => 'C', 'b' => 'B']

merge by key using + operator

$c + $d; // [?'a' => 'A',  'b' => 'B', 'c' => 'C']
??$d + $c; // ?['a' => 'AA', 'c' => 'C', 'b' => 'B']
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Warm tip !!!

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