banner
Ibuildings Blogs > Implementing Iterators
Search Nederlands | English
Jobs at Ibuildings
Are you passionate about PHP and do you thrive in a culture that values teamwork? Then check out our vacancies.
PHP Events

Tag cloud

Subscribe to this blog

Home  ->  Ibuildings Blogs

Ruuds Blog

ing. Ruud Alberts ing. Ruud Alberts
Software Engineer

Ruud is a software engineer at our Sittard office. He has an interest in design patterns and their implementation in PHP. He is a strong believer in the MVC pattern and his favourite MVC framework is Kohana, a fork of the CodeIgniter framework that makes use of the power of PHP5. 		Zend Certified Engineer

Tuesday, August 19. 2008

Implementing Iterators

Let's kickstart this blogpost by defining what an iterator actually is. According to wikipedia, an iterator is:
"an object which allows a programmer to traverse through all the elements of a collection, regardless of its specific implementation."
A collection can pretty much be anything. The most obvious sources would be arrays, but other than that, iterations can be done over database resultsets, strings, datetime intervals, directories, file content and XML listings, to name a few. The real benefit of using a standard iterator implementation, is that every implemented iterator does its job obeying a standard interface. Whether said collection is a database resultset or a directory structure; it can be iterated using the same method names. And that's neat, because it will save us developers a trip to the manual on many occasions (... yeah it does... just admit it... ).

SPL
SPL, or the Standard PHP Library:
"is a collection of interfaces and classes that are meant to solve standard problems and implements some efficient data access interfaces and classes."
The SPL extension is available and compiled by default since PHP 5.0.0 and since PHP 5.3 it can no longer be disabled, making it a standard asset to your (or more importantly: your host's) PHP installation. One of the great benefits of SPL, is the way it handles Iterators.

The Iterator interface is defined as listed below. Obviously meaning that every time you implement an Iterator, you have to at least implement the methods mentioned.

PHP:

<?php

interface Iterator
{
    // Return the key of the current element. 
    public function key();

    // Return the current element. 
    public function current();

    // Move forward to next element.
    public function next();

    // Check if there is a current element after calls to rewind() or next(). 
    public function valid();

    // Rewind the Iterator to the first element. 
    public function rewind();
}

?>



When you instantiate your Iterator object, the following methods will be called on the object you iterate:
  • __construct() (if you implement a constructor, for example to preset some class variables)
  • rewind()
  • valid()
  • current() // if valid() evaluated to true


Iterating over your collection (using foreach) will then call

  • next()
  • valid()
  • current() // if valid() evaluated to true


Implementing your own iterator
SPL provides many iterators that are waiting for you to be used. An extensive list can be found here. On top of handling iteration for you, many of these provide additional functionality. The ArrayIterator, for example, gives you an extensive range of methods to run on your collection; like sorting, setting and seeking values. Also available are more complex iterators,like those that handle recursion for you. Instead of getting in depth with all that's possible using SPL's iterators (for that I would recommend visiting SPL's homepage) I would like to point out how you can implement your very own iterator.

Despite all that's readily available for you to be used, it's still thinkable that you'll encounter situations that just beg for a different kind of iterator to be implemented. I mean: wouldn't you just love to loop over a string and give each character a different color? That's something I run into each and every single day. Here's how it can be done.

First we implement our very own StringIterator, which is basically a class that provides means to loop over each character of a string separately.
PHP:

<?php

class StringIterator implements Iterator
{
    protected $collection;
    protected $pointer_position;
    
    public function __construct( $collection )
    {
      $this->collection = $collection;
    }
    
    public function key()
    {
      return $this->pointer_position;
    }

    public function current()
    { 
      return $this->collection[$this->pointer_position];
    }

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

    public function rewind()
    {
      $this->pointer_position = 0;
    }
    
    public function valid()
    {
      return strlen( $this->collection ) > $this->pointer_position;
    }

}

?>



Time for some cheerful colors. By extending our very own StringIterator we will be able to add alternative behaviour. Editing the current character can be achieved by wrapping the initial current() returnvalue in a custom function.

PHP:

<?php

class ColorfulStringIterator extends StringIterator 
{
  
    public function current( )
    {
      return $this->randomColor( parent::current( ) );
    }
    
    private function randomColor( $character )
    {
      $color = sprintf( "%02X%02X%02X", mt_rand( 0, 255 ), mt_rand( 0, 255 ), mt_rand( 0, 255 ) );
      return '<span style="color:#'.$color.'">'.$character.'</span>';
    }    
}

?>



Now we will be able to iterate over our collection:
PHP:

<?php

$collection = 'Lorem ipsum dolor sit amet, consectetuer adipiscing elit.';

// using foreach
foreach( new ColorfulStringIterator( $collection ) as $current_element )
{
  echo $current_element;
}

// using for
for( $iterator = new ColorfulStringIterator( $collection ); $iterator->valid(); $iterator->next() )
{
  echo $iterator->current();
}

?>



Because of iterator standardization, we can then still use additional wrappers. An example would be the LimitIterator, preventing you from iterating out of pre-defined bounds:
PHP:

<?php

$collection = 'Lorem ipsum dolor sit amet, consectetuer adipiscing elit.';
foreach( new LimitIterator( new ColorfulStringIterator( $collection ), 0, 10 ) as $current_element )
{
  echo $current_element;
}

?>



Conclusion
To sum this all up: SPL provides great means to standardize the way in which iteration is achieved. Using these standardizations allows you to fully profit from that what is readily available, and that what you can extend with great ease. I've given you a glance at how to implement your own iterator, and this hopefully inspired you to take a closer look at this useful extension. If you hadn't done so already :-)
Posted by Ruud Alberts --> at 14:45 | Comments (5) | Trackbacks (0)
Defined tags for this entry: , , ,

Trackbacks
Trackback specific URI for this entry

No Trackbacks

Comments
Display comments as (Linear | Threaded)

Hi, nice introduction to PHP Iterators.

Any reason for returning the "current" value in your ::next() implementation ? This could be causing some overhead and/or erratic results in other contexts (iterating a database query result for example).
#1 Gromitt (Homepage) on 2008-08-21 07:18 (Reply)
Hello Grommit,

Reading back my code I can't find what (or where) it is that you mean? Is it possible that you've misread the actual iteration (in the for loop) for a class implementation?
#1.1 Ruud on 2008-08-21 09:19 (Reply)
Hi, I was talking about this part :

CODE:
    public function next()
    {        
      $this->pointer_position++;
      return $this->collection[$this->pointer_position];
    }


Why the "return" ?
#1.1.1 Gromitt (Homepage) on 2008-08-21 14:13 (Reply)
Ah yes, you're absolutely right. Returning the value is an unneeded step since this will be handled internally. I'll edit my example, thanks for noticing!
#1.1.1.1 Ruud on 2008-08-21 14:20 (Reply)
Nice little intro to Iterators.

However I have some remarks / additions

Assumption is the mother of all f*ck ups IMO.
Therefor I'd personally force array input.
PHP:

<?php

class StringIterator implements Iterator{
 
    public function __construct(array $collection){}

?>



As for using for loops, please note that in order to iterate with for loops more then once the array pointer has te be reset!

PHP:

<?php

$iterator = new ColorfulStringIterator( $collection ); 

for(; $iterator-&gt;valid(); $iterator-&gt;next()){
  echo $iterator-&gt;current();
}
//second for loop will not work without the reset..
$iterator-&gt;rewind();
//reset($iterator); //should also work...

for(; $iterator-&gt;valid(); $iterator-&gt;next()){
  echo $iterator-&gt;current();
}

?>



Enjoy Iterators ^^ I use them all over the place!
#2 Lode on 2008-08-21 09:18 (Reply)

Add Comment

BBCode format allowed
Enclosing asterisks marks text as bold (*word*), underscore are made via _word_.
Standard emoticons like :-) and ;-) are converted to images.
E-Mail addresses will not be displayed and will only be used for E-Mail notifications

To prevent automated Bots from commentspamming, please enter the string you see in the image below in the appropriate input box. Your comment will only be submitted if the strings match. Please ensure that your browser supports and accepts cookies, or your comment cannot be verified correctly.
CAPTCHA
 
 
sitemap | contact | privacy | general conditions | © 1999 - 2008 Ibuildings BV