PHP : Arrays - [4.24] Finding the Union, Intersection, or Difference of Two Arrays

4.24.1 Problem

You have a pair of arrays, and you want to find their union (all the elements), intersection (elements in both, not just one), or difference (in one but not both).

4.24.2 Solution

To compute the union:

$union = array_unique(array_merge($a, $b));

To compute the intersection:

$intersection = array_intersection($a, $b);

To find the simple difference:

$difference = array_diff($a, $b);

And for the symmetric difference:

$difference = array_merge(array_diff($a, $b), array_diff($b, $a));

4.24.3 Discussion

Many necessary components for these calculations are built into PHP, it's just a matter of combining them in the proper sequence.

To find the union, you merge the two arrays to create one giant array with all values. But, array_merge( ) allows duplicate values when merging two numeric arrays, so you call array_unique( ) to filter them out. This can leave gaps between entries because array_unique( ) doesn't compact the array. It isn't a problem, however, as foreach and each( ) handle sparsely filled arrays without a hitch.

The function to calculate the intersection is simply named array_intersection( ) and requires no additional work on your part.

The array_diff( ) function returns an array containing all the unique elements in $old that aren't in $new. This is known as the simple difference:

$old = array('To', 'be', 'or', 'not', 'to', 'be');
$new = array('To', 'be', 'or', 'whatever');
$difference = array_diff($old, $new);
Array
(
    [3] => not
    [4] => to
)

The resulting array, $difference contains 'not' and 'to', because array_diff( ) is case-sensitive. It doesn't contain 'whatever' because it doesn't appear in $old.

To get a reverse difference, or in other words, to find the unique elements in $new that are lacking in $old, flip the arguments:

$old = array('To', 'be', 'or', 'not', 'to', 'be');
$new = array('To', 'be', 'or', 'whatever');
$reverse_diff = array_diff($new, $old);
Array
(
    [3] => whatever
)

The $reverse_diff array contains only 'whatever'.

If you want to apply a function or other filter to array_diff( ), roll your own diffing algorithm:

// implement case-insensitive diffing; diff -i

$seen = array( );
foreach ($new as $n) {
    $seen[strtolower($n)]++;
}

foreach ($old as $o) {
    $o = strtolower($o);
    if (!$seen[$o]) { $diff[$o] = $o; }
}

The first foreach builds an associative array lookup table. You then loop through $old and, if you can't find an entry in our lookup, add the element to $diff.

It can be a little faster to combine array_diff( ) with array_map( ):

$diff = array_diff(array_map('strtolower', $old), array_map('strtolower', $new));

The symmetric difference is what's in $a, but not $b, and what's in $b, but not $a:

$difference = array_merge(array_diff($a, $b), array_diff($b, $a));

Once stated, the algorithm is straightforward. You call array_diff( ) twice and find the two differences. Then you merge them together into one array. There's no need to call array_unique( ), since you've intentionally constructed these arrays to have nothing in common.