The Iterator interface

(PHP 5 >= 5.0.0, PHP 7)

Introduction

Interface for external iterators or objects that can be iterated themselves internally.

Interface synopsis

Iterator extends Traversable {

/* Methods */

abstract public mixed current ( void )

abstract public scalar key ( void )

abstract public void next ( void )

abstract public void rewind ( void )

abstract public boolean valid ( void )

}

Predefined iterators

PHP already provides a number of iterators for many day to day tasks. See SPL iterators for a list.

Examples

Example #1 Basic usage

This example demonstrates in which order methods are called when using foreach with an iterator.


<?php
class myIterator implements Iterator {
    private $position = 0;
    private $array = array(
        "firstelement",
        "secondelement",
        "lastelement",
    );  

    public function __construct() {
        $this->position = 0;
    }

    function rewind() {
        var_dump(__METHOD__);
        $this->position = 0;
    }

    function current() {
        var_dump(__METHOD__);
        return $this->array[$this->position];
    }

    function key() {
        var_dump(__METHOD__);
        return $this->position;
    }

    function next() {
        var_dump(__METHOD__);
        ++$this->position;
    }

    function valid() {
        var_dump(__METHOD__);
        return isset($this->array[$this->position]);
    }
}

$it = new myIterator;

foreach($it as $key => $value) {
    var_dump($key, $value);
    echo "\n";
}
?>

The above example will output something similar to:

string(18) "myIterator::rewind"
string(17) "myIterator::valid"
string(19) "myIterator::current"
string(15) "myIterator::key"
int(0)
string(12) "firstelement"

string(16) "myIterator::next"
string(17) "myIterator::valid"
string(19) "myIterator::current"
string(15) "myIterator::key"
int(1)
string(13) "secondelement"

string(16) "myIterator::next"
string(17) "myIterator::valid"
string(19) "myIterator::current"
string(15) "myIterator::key"
int(2)
string(11) "lastelement"

string(16) "myIterator::next"
string(17) "myIterator::valid"

Iterator::current — Return the current element
Iterator::key — Return the key of the current element
Iterator::next — Move forward to next element
Iterator::rewind — Rewind the Iterator to the first element
Iterator::valid — Checks if current position is valid

User Contributed Notes

robert_e_lee at dell dot com

6 years ago


Order of operations when using a foreach loop:

1. Before the first iteration of the loop, Iterator::rewind() is called.
2. Before each iteration of the loop, Iterator::valid() is called.
3a. It Iterator::valid() returns false, the loop is terminated.
3b. If Iterator::valid() returns true, Iterator::current() and
Iterator::key() are called.
4. The loop body is evaluated.
5. After each iteration of the loop, Iterator::next() is called and we repeat from step 2 above.

This is roughly equivalent to:

<?php
$it->rewind();

while ($it->valid())
{
    $key = $it->key();
    $value = $it->current();

    // ...

    $it->next();
}
?>

The loop isn't terminated until Iterator::valid() returns false or the body of the loop executes a break statement.

The only two methods that are always executed are Iterator::rewind() and Iterator::valid() (unless rewind throws an exception).

The Iterator::next() method need not return anything. It is defined as returning void. On the other hand, sometimes it is convenient for this method to return something, in which case you can do so if you want.

If your iterator is doing something expensive, like making a database query and iterating over the result set, the best place to make the query is probably in the Iterator::rewind() implementation.

In this case, the construction of the iterator itself can be cheap, and after construction you can continue to set the properties of the query all the way up to the beginning of the foreach loop since the
Iterator::rewind() method isn't called until then.

Things to keep in mind when making a database result set iterator:

* Make sure you close your cursor or otherwise clean up any previous query at the top of the rewind method. Otherwise your code will break if the same iterator is used in two consecutive foreach loops when the first loop terminates with a break statement before all the results are iterated over.

* Make sure your rewind() implementation tries to grab the first result so that the subsequent call to valid() will know whether or not the result set is empty. I do this by explicitly calling next() from the end of my rewind() implementation.

* For things like result set iterators, there really isn't always a "key" that you can return, unless you know you have a scalar primary key column in the query. Unfortunately, there will be cases where either the iterator doesn't know the primary key column because it isn't providing the query, the nature of the query is such that a primary key isn't applicable, the iterator is iterating over a table that doesn't have one, or the iterator is iterating over a table that has a compound primary key. In these cases, key() can return either:
the row index (based on a simple counter that you provide), or can simply return null.

Iterators can also be used to:

* iterate over the lines of a file or rows of a CSV file
* iterate over the characters of a string
* iterate over the tokens in an input stream
* iterate over the matches returned by an xpath expression
* iterate over the matches returned by a regexp
* iterate over the files in a folder
* etc...

RocketInABog at techno-monks dot net

6 years ago


<?php
# - Here is an implementation of the Iterator interface for arrays
#     which works with maps (key/value pairs)
#     as well as traditional arrays
#     (contiguous monotonically increasing indexes).
#   Though it pretty much does what an array
#     would normally do within foreach() loops,
#     this class may be useful for using arrays
#     with code that generically/only supports the
#     Iterator interface.
#  Another use of this class is to simply provide
#     object methods with tightly controlling iteration of arrays.

class tIterator_array implements Iterator {
  private $myArray;

  public function __construct( $givenArray ) {
    $this->myArray = $givenArray;
  }
  function rewind() {
    return reset($this->myArray);
  }
  function current() {
    return current($this->myArray);
  }
  function key() {
    return key($this->myArray);
  }
  function next() {
    return next($this->myArray);
  }
  function valid() {
    return key($this->myArray) !== null;
  }
}

?>

fetidfrog at gmail dot com

3 years ago


If you have a custom iterator that may throw an exception in it's current() method, there is no way to catch the exception without breaking a foreach loop.

The following for loop allows you to skip elements for which $iterator->current() throws an exception, rather than breaking the loop.

<?php
for ($iterator->rewind(); $iterator->valid(); $iterator->next()) {
    try {
        $value = $iterator->current();
    } catch (Exception $exception) {
        continue;
    }

    # ...
}
?>

mike dot thornton at firstroi dot com

6 years ago


It's important to note that following won't work if you have null values.

<?php
    function valid() {
        var_dump(__METHOD__);
        return isset($this->array[$this->position]);
    }
?>

Other examples have shown the following which won't work if you have false values:

<?php
    function valid() {
        return $this->current() !== false;
    }
?>

Instead use:

<?php
    function valid() {
        return array_key_exists($this->array, $this->position);
    }
?>

Or the following if you do not store the position.

<?php
    public function valid() {
        return !is_null(key($this->array));
    }
?>

Geoffrey Sneddon

6 years ago


So, playing around with iterators in PHP (coming from languages where I'm spoiled with generators to do things like this), I wrote a quick piece of code to give the Fibonacci sequence (to infinity, though only the first terms up to F_{10} are output).



<?php



class Fibonacci implements Iterator {

    private $previous = 1;

    private $current = 0;

    private $key = 0;

    

    public function current() {

        return $this->current;

    }

    

    public function key() {

        return $this->key;

    }

    

    public function next() {

        $newprevious = $this->current;

        $this->current += $this->previous;

        $this->previous = $newprevious;

        $this->key++;

    }

    

    public function rewind() {

        $this->previous = 1;

        $this->current = 0;

        $this->key = 0;

    }

    

    public function valid() {

        return true;

    }

}



$seq = new Fibonacci;

$i = 0;

foreach ($seq as $f) {

    echo "$f\n";

    if ($i++ === 10) break;

}

?>

4 years ago


<?php

/*
 * An implementation of the Iterator
 * with simpleXML to remove a node and generate a new XML file.
 * 
 * project.xml file:
 * <?xml version="1.0" encoding="UTF-8"?>
 * ...
 * <data>
 *  <item>
 *      <value>one</value>
 *  </item>
 *  <item>
 *      <value>two</value>
 *  </item>
 *  ...
 * </data>
 * 
 */

class parseXML implements Iterator {
    
    private $position;
    private $xml;
    public $item;

    public function __construct() {
        
        $this->position = 0;
        $this->xml = simplexml_load_file('project.xml');
                
    }
    
    public function unsetItem() {
        
        foreach ($this as $key => $value) {
        
            if ($value->value == $this->item ) {
        
                unset($this->xml->data->item[$key]);
        
            }
        }
        
        $this->mkXML();
    }
    
    public function mkXML() {
        
        file_put_contents('project.xml', $this->xml->asXML() );
        
    }

    function rewind() {
        $this->position = 0;
    }

    function current() {
        return $this->xml->data->item[$this->position];
    }

    function key() {
        return $this->position;
    }

    function next() {
        ++$this->position;
    }

    function valid() {
        return isset($this->xml->data->item[$this->position]);
    }
    
}

$itemRemove = new parseXML();

$itemRemove->item = "one";

$itemRemove->unsetItem();

?>

geompse at gmail dot com

4 years ago


Be carefull with Iterator when using nested loops or deleting items inside the collection while looping over it.
It can be tricky to detect.
This unexpected behavior is pertinent if you think about it long enough.

<?php

foreach($it as $key => $value)
    echo $value;
#output: value1, value2, value3

foreach($it as $key => $value)
    foreach($it as $key => $value)
        echo $value;
#output: value1, value2, value3

foreach($it as $key => $value)
    foreach(clone $it as $key => $value)
        echo $value;
#output: value1, value2, value3, value1, value2, value3, value1, value2, value3

foreach($it as $key => $value)
{
    echo $value;
    array_shift($it->values);
}
#ouput: value1, value3

?>

Anthony Sterling

6 years ago


Here's a Fibonacci example using the formula, rather than addition.

<?php
/**
 * @author    Anthony Sterling
 */
class FibonacciSequence implements Iterator
{
    protected
        $limit        = 0;
        
    protected
        $key        = 0;    
        
    public function __construct($limit = 0)
    {
        $this->limit = (integer)$limit;
    }
    
    public function current()
    {
        return round(
            (pow(((1 + sqrt(5)) / 2), $this->key) - pow((-1 / (1 + sqrt(5)) / 2), $this->key)) / sqrt(5),
            null
        );
    }

    public function key()
    {
        return $this->key;
    }

    public function next()
    {
        $this->key++;
    }

    public function rewind()
    {
        $this->key = 0;
    }

    public function valid()
    {
        return $this->key < $this->limit;
    } 
}

foreach(new FibonacciSequence() as $number)
{
    printf(
        '%d<br />',
        $number
    );
}
/*
    0
    1
    1
    2
    3
    5
    8
    13
    21
    34
    55
    89
    144
    233
    377
    610
    987
    1597
    2584
    4181
    6765
    10946
    17711
    28657
    46368
    75025
    121393
    196418
    317811
    514229
*/
?>

Anthony.

Gilles A

2 years ago


Examples of use
<?php
class myIterator implements Iterator 
    {
    private 
        $_array = array();
    public function __construct(array $array) 
        {
        $this->_array = $array;
        }
    public function rewind() 
        {
        reset($this->_array);
        }
    public function current() 
        {
        return current($this->_array);
        }
    public function key() 
        {
        return key($this->_array);
        }
    public function next() 
        {
        next($this->_array);
        }
    public function valid() 
        {
        return $this->key() !== null;
        }
    }

$it = new myIterator(array('foo_1' => 'bar_1','foo_2' => 'bar_2'));

//example 1 : foreach

foreach($it as $key => $value) 
    {
    var_dump($key, $value);
    }
    
//example 2 : while

$it -> rewind();
while($it->valid())
    {
    var_dump($it->key(), $it->current());
    
    $it->next();
    }
 
//example 3 : for

for($it->rewind();$it->valid();$it->next())
    {
   var_dump($it->key(), $it->current());
    }
?>

Anonymous

2 months ago


An interesting fact that I didn't read in the doc:

the key() method is called only if your foreach loop needs it.

For instance, the following loop calls the key() method:

<?php
foreach($it as $key => $value) {
    var_dump($key, $value);
    echo "\n";
}
?>

But the following loop doesn't:

<?php
foreach($it as $value) {
    var_dump($value);
    echo "\n";
}
?>

Daniel Lo Nigro

2 years ago


If you're using PHP 5.5 or above and are creating a simple iterator, consider using a generator function instead. There is significantly less boilerplate code and the code is easier to read. http://au1.php.net/generators

kent at marketacumen dot com

5 years ago


For Iterators implement database queries, what I've found is that if you want to chain multiple iterators together using a "MultipleIterator" then you *do not* want ::rewind() to actually execute your query, especially if it's expensive.

Instead, what I've done is implement that portion in "valid." 

e.g.

<?php
class Database_Result_Iterator {
    ...
    private $_db_resource = null;
    private $_loaded = false;
    private $_valid = false;

    function rewind() {
        if ($this->_db_resource) {
            mysql_free($this->_db_resource);
            $this->_db_resource = null;
        }
        $this->_loaded = false;
        $this->_valid = false;
    }

    function valid() {
        if ($this->_loaded) {
            $this->load();
        }
        return $this->_valid;
    }

    private function load() {
        $this->_db_resource = mysql_query(...);
        $this->_loaded = true;
        $this->next(); // Sets _valid
    }
}
?>

That way if you chain multiple queries in a "MultipleIterator" together, the "rewind" call (which rewinds all iterators at once) does not execute every query at once. 

In addition, I found that the MultipleIterator may now work best for other reasons, but still, the above is a good idea to postpone queries until the last possible moment they are needed.

dwlnetnl at users dot sourceforge dot net

4 years ago


Be aware that when you call a method like current($this) within the Iterator class, the properties of the class are returned and the Iterator's current() isn't called. This is because current() applies to arrays and the Iterator class is then interpretated as an array.

The Iterator interface

Introduction

Interface synopsis

Predefined iterators

Examples

Table of Contents

User Contributed Notes