PHP Arrays as Stacks, Queues and Sets

Arrays as Stacks, Queues and Sets

Arrays are often used as stacks (Last In, First Out, or LIFO) and queue (First In, First Out, or FIFO) structures. PHP simplifies this approach by providing a set of functions can be used to push and pull (for stacks) and shift and unshift (for queues) elements from an array.

We’ll take a look at stacks first:

$stack = array();

array_push($stack, ’bar’, ’baz’);

var_dump($stack);

$last_in = array_pop($stack);

var_dump($last_in, $stack);

In this example, we first, create an array, and we then add two elements to it using array_push(). Next, using array_pop(), we extract the last element added to the array, resulting in this output:

array(2) {

[0]=>

string(3) "bar"

[1]=>

string(3) "baz"

}

string(3) "baz"

array(1) {

[0]=>

string(3) "bar"

}

If you intend to use an array as a queue, you can add elements to the beginning and

extract them from the end by using the array_unshift() and array_shift() func-tions:

$stack = array(’qux’, ’bar’, ’baz’);

$first_element = array_shift($stack);

var_dump($stack);

array_unshift($stack, ’foo’);

var_dump($stack);

In this example, we use array_shift() to push the first element out of the array. Next,

using array_unshift(), we do the reverse and add a value to the beginning of the

array. This example results in:

array(2) {

[0]=>

string(3) "bar"

[1]=>

string(3) "baz"

}

array(3) {

[0]=>

string(3) "foo"

[1]=>

string(3) "bar"

[2]=>

string(3) "baz"

}

Set Functionality

Some PHP functions are designed to perform set operations on arrays. For example, array_diff() is used to compute the difference between two arrays:

$a = array (1, 2, 3);

$b = array (1, 3, 4);

var_dump (array_diff ($a, $b));

The call to array_diff() in the code above will cause all the values of $a that do not also appear in $b to be retained, while everything else is discarded:

array(1) {

[1]=>

int(2)

}

Note that the keys are ignored—if you want the difference to be computed based on key-value pairs, you will have to use array_diff_assoc() instead, whereas if you want it to be computed on keys alone, array_diff_key() will do the trick. Both of these functions have user-defined callback versions called array_diff_uassoc() and array_diff_ukey() respectively.

Conversely to array_diff(), array_intersect() will compute the intersection between two arrays:

$a = array (1, 2, 3);

$b = array (1, 3, 4);

var_dump (array_intersect ($a, $b));

In this case, only the values that are included in both arrays are retained in the result:

array(2) {

[0]=>

int(1)

[2]=>

int(3)

}

Like with array_diff(), array_intersect only keeps in consideration the value of each element; PHP provides array_intersect_key() and array_intersect_assoc() versions for key- and key/value-based intersection, together with their callback variants array_intersect_ukey() and array_intersect_uassoc().

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