PHP: Object Iteration

Object Iteration

PHP 5 provides a way for objects to be defined so it is possible to iterate through a list of items, with, for example a foreach statement. By default, all visible properties will be used for the iteration.

Example #1 Simple Object Iteration


<?php

class MyClass

{

    public $var1 = 'value 1';

    public $var2 = 'value 2';

    public $var3 = 'value 3';



    protected $protected = 'protected var';

    private   $private   = 'private var';



    function iterateVisible() {

       echo "MyClass::iterateVisible:\n";

       foreach($this as $key => $value) {

           print "$key => $value\n";

       }

    }

}



$class = new MyClass();



foreach($class as $key => $value) {

    print "$key => $value\n";

}

echo "\n";





$class->iterateVisible();



?>

The above example will output:

var1 => value 1
var2 => value 2
var3 => value 3

MyClass::iterateVisible:
var1 => value 1
var2 => value 2
var3 => value 3
protected => protected var
private => private var

As the output shows, the foreach iterated through all visible variables that can be accessed. To take it a step further you can implement one of PHP 5's internal interface named Iterator. This allows the object to decide what and how the object will be iterated.

Example #2 Object Iteration implementing Iterator


<?php

class MyIterator implements Iterator

{

    private $var = array();



    public function __construct($array)

    {

        if (is_array($array)) {

            $this->var = $array;

        }

    }



    public function rewind() {

        echo "rewinding\n";

        reset($this->var);

    }



    public function current() {

        $var = current($this->var);

        echo "current: $var\n";

        return $var;

    }



    public function key() {

        $var = key($this->var);

        echo "key: $var\n";

        return $var;

    }



    public function next() {

        $var = next($this->var);

        echo "next: $var\n";

        return $var;

    }



    public function valid() {

        $var = $this->current() !== false;

        echo "valid: {$var}\n";

        return $var;

    }

}



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

$it = new MyIterator($values);



foreach ($it as $a => $b) {

    print "$a: $b\n";

}

?>

The above example will output:

rewinding
current: 1
valid: 1
current: 1
key: 0
0: 1
next: 2
current: 2
valid: 1
current: 2
key: 1
1: 2
next: 3
current: 3
valid: 1
current: 3
key: 2
2: 3
next:
current:
valid:

You can also define your class so that it doesn't have to define all the Iterator functions by simply implementing the PHP 5 IteratorAggregate interface.

Example #3 Object Iteration implementing IteratorAggregate


<?php

class MyCollection implements IteratorAggregate

{

    private $items = array();

    private $count = 0;



    // Required definition of interface IteratorAggregate

    public function getIterator() {

        return new MyIterator($this->items);

    }



    public function add($value) {

        $this->items[$this->count++] = $value;

    }

}



$coll = new MyCollection();

$coll->add('value 1');

$coll->add('value 2');

$coll->add('value 3');



foreach ($coll as $key => $val) {

    echo "key/value: [$key -> $val]\n\n";

}

?>

The above example will output:

rewinding
current: value 1
valid: 1
current: value 1
key: 0
key/value: [0 -> value 1]

next: value 2
current: value 2
valid: 1
current: value 2
key: 1
key/value: [1 -> value 2]

next: value 3
current: value 3
valid: 1
current: value 3
key: 2
key/value: [2 -> value 3]

next:
current:
valid:

Note: For more examples of iterators, see the SPL Extension.