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7.7.2 Sets

Arrays let you implement the basic operations of set theory: union, intersection, and difference.
Each set is represented by an array, and various PHP functions implement the set operations.
The values in the set are the values in the array the keys are not used, but they are generally
preserved by the operations.

The union of two sets is all the elements from both sets, with duplicates removed. The array_
merge( ) and array_unique( ) functions let you calculate the union. Here’s how to find the union
of two arrays:

function array_union($a, $b)

{

$union = array_merge($a, $b); // duplicates may still exist
$union = array_unique($union); return $union;

}

$first = array(1, ‘two’, 3);

$second = array(‘two’, ‘three’, ‘four’);
$union = array_union($first, $second);

print_r($union);

Array ( [0] => 1 [1] => two [2] => 3[4] => three [5] => four )

The intersection of two sets is the set of elements they have in common. PHP’s built-in array_
intersect( ) function takes any number of arrays as arguments and returns an array of those values
that exist in each. If multiple keys have the same value, the first key with that value is preserved.

Another common function to perform on a set of arrays is to get the difference; that is, the values
in one array that are not present in another array. The array_diff( ) function calculates this,
returning an array with values from the first array that are not present in the second.

The following code takes the difference of two arrays:
$first = array(1, ‘two’, 3);

$second = array(‘two’, ‘three’, ‘four’);

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