The Iterator interface

(PHP 5 >= 5.0.0)

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"

Table of Contents

Коментарии

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));
    }
?>
2009-05-05 16:40:32
http://php5.kiev.ua/manual/ru/class.iterator.html
<?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;
  }
}

?>
2009-05-11 19:35:33
http://php5.kiev.ua/manual/ru/class.iterator.html
Автор:
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;
}
?>
2009-05-23 13:42:06
http://php5.kiev.ua/manual/ru/class.iterator.html
Автор:
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(((sqrt(5)) / 2), $this->key) - pow((-/ (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.
2009-09-17 05:20:03
http://php5.kiev.ua/manual/ru/class.iterator.html
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...
2010-03-11 12:08:44
http://php5.kiev.ua/manual/ru/class.iterator.html
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.
2010-12-13 15:15:50
http://php5.kiev.ua/manual/ru/class.iterator.html
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

?>
2011-07-03 09:43:24
http://php5.kiev.ua/manual/ru/class.iterator.html
Автор:
<?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();

?>
2011-07-28 15:00:28
http://php5.kiev.ua/manual/ru/class.iterator.html
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.
2011-10-02 17:06:45
http://php5.kiev.ua/manual/ru/class.iterator.html
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;
    }

   
# ...
}
?>
2012-05-10 18:11:04
http://php5.kiev.ua/manual/ru/class.iterator.html
Автор:
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
2013-06-23 09:33:25
http://php5.kiev.ua/manual/ru/class.iterator.html
Автор:
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());
    }
?>
2014-04-04 18:41:51
http://php5.kiev.ua/manual/ru/class.iterator.html
Автор:
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";
}
?>
2016-02-14 14:51:59
http://php5.kiev.ua/manual/ru/class.iterator.html
With a large number of `current`, `next`, `key`, and `reset` array function implementations, care needs to be taken to ensure that deletions and nested loops are accounted for appropriately for each situation.

The following class has been copied from a previous comment and modified to allow it to be used within nested loops.

<?php
# Comment removed for brevity.

class tIterator_array implements Iterator {
  private 
$myArray;
 
// Store each iteration in a separate array.
 
private $iterations = [];
  private 
$i = -1;

  public function 
__construct$givenArray ) {
   
$this->myArray $givenArray;
  }
  function 
rewind() {
   
// Rewind is called at the start of the loop. This is where we can append the current array to start our new iteration.
   
$this->iterations[] = $this->myArray;
   
$this->i++;
    return 
reset$this->iterations$this->] );
  }
  function 
current() {
    return 
current$this->iterations$this->] );
  }
  function 
key() {
    return 
key$this->iterations$this->] );
  }
  function 
next() {
    return 
next$this->iterations$this->] );
  }
  function 
valid() {
    if ( 
null === $this->key() ) {
     
// Standard valid check. When null is returned the loop has finished, so we decrement the index and remove the latest iteration.
     
array_pop$this->iterations );
     
$this->i--;
      return 
false;
    }
    return 
true;
  }
}

// Example:
$a = new tIterator_array( [12] );
foreach ( 
$a as $k => $v ) {
  echo 
$k => $v:\n";
  foreach ( 
$a as $k => $v ) {
    echo 
$k => $v,\n";
  }
}
// Output:
# 0 => 1:
#  0 => 1,
#  1 => 2,
# 1 => 2:
#  0 => 1,
#  1 => 2,

?>
2018-12-12 05:09:32
http://php5.kiev.ua/manual/ru/class.iterator.html
RocketInABog's seemingly trivial tIterator_array class has one huge problem (which just cost me a couple of hours).

Consider this example, using their class:
<?php
$values 
= ['one''two''three'];
foreach (
$values as $v) {}
$current current($values);
// $current === 'one', as you would expect

$iterator = new tIterator_array($values);
foreach (
$iterator as $v) {}
$current $iterator->current();    // do NOT use current($iterator) or key($iterator)!!!
// $current === false, but why?
?>
The problem is that foreach resets arrays, but doesn't call Iterator::rewind on objects!

I also think it's a design mistake that foreach works with Iterator, but current(), key() and end() don't - these iterate over the objects fields.

I just refactored some code to use an Iterator instead of an array, and it broke in several very unexpected ways because of these differences.
2019-12-16 17:57:34
http://php5.kiev.ua/manual/ru/class.iterator.html
The "scalar" restriction on key() is no longer true. A simple example is that Generators can yield non-scalar keys.
2020-07-22 06:26:22
http://php5.kiev.ua/manual/ru/class.iterator.html
Автор:
/**
 * Iterate  a directory tree by walking the tree. For each directory in
 * the tree rooted at directory $parent_dir (including $parent_dir
 * itself), it returns $dirpath => array($dirnames, $filenames).
 *
 * $dirpath is a string, the path to the directory. $dirnames is a list
 * of the names of the subdirectories in dirpath (excluding '.' and
 * '..'). $filenames is a list of the names of the non-directory files in
 * $dirpath.
 *
 * Inspired by Python os.walk; see
 * https://docs.python.org/3/library/os.html#os.walk
 * Implemented as an iterator rather than a generator.
 *
 * @return array
 */

class walker implements Iterator {
  private $parent_dir = '';
  private $d = 0; // iterator
  private $dirs = array(); // indexed by iterator
  private $dirstack;
  private $discovered = array(); // indexed by directory
  private $v; // current directory
  private $cur_dirnames = array(); // current subdirs
  private $cur_filenames = array(); // current files in directory

  public function __construct($parent_dir) {
    $this->parent_dir = $parent_dir;
    $this->d = 0;
    $this->dirs[$this->d] = $parent_dir;
    $this->dirstack = new SplStack();
    $this->dirstack->push($parent_dir);
    $this->discovered = array($parent_dir => true);
    $this->v = $parent_dir;
    $this->cur_dirnames = array();
    $this->cur_filenames = array();
    $this->next();
  }

  public function rewind() {
    $this->__construct($this->parent_dir);
  }

  public function current() {
    return array($this->cur_dirnames, $this->cur_filenames);
  }

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

  public function next() {
    ++$this->d;
    $this->v = $this->dirstack->pop();
    $this->dirs[$this->d] = $this->v;
    $this->cur_dirnames = array();
    $this->cur_filenames = array();
    if (!$dh = opendir($this->v)) {
      // opendir emits E_WARNING if unable to open directory, likely due
      // to a permissions issue or directory removed before we could get
      // there
      return;
    }
    // discover the directories, return directories and files
    while (false !== ($fn = readdir($dh))) {
      if ($fn != '.' && $fn !== '..') {
        $fullfn = $this->v . '/' . $fn;
        if (is_dir($fullfn)) {
      $this->cur_dirnames[] = $fn;
          if (!array_key_exists($fullfn, $this->discovered)) {
            $this->discovered[$fullfn] = true;
        $this->dirstack->push($fullfn);
      }
        } else {
          $this->cur_filenames[] = $fn;
        }
      }
    }
    closedir($dh);
  }

  public function valid() {
    return $this->dirstack->count();
  }
}

$tree = new walker("/tmp");
foreach ($tree as $parent_dir => $nodes) {
  $subdirs = $nodes[0];
  $files = $nodes[1];
  printf("%s\n", $parent_dir);
  if ($subdirs) printf("  %s\n", implode("/\n  ", $subdirs));
  if ($files) printf("  %s\n", implode("\n  ", $files));
  print("\n");
}
2021-02-10 00:41:52
http://php5.kiev.ua/manual/ru/class.iterator.html
Автор:
If you implemented Iterator methods next() and rewind() by calling array functions next() and reset() and returning their results, be advised that this violates the tentative return types (void in both cases) introduced with PHP 8.1.

You can add the #[\ReturnTypeWillChange] attribute to both method implementations but that will only delay the issue until PHP 9.0 comes around.

Better adapt your implementations now (stop returning anything from these methods) and, if need be, only add the return type declarations later.
2022-07-20 10:32:29
http://php5.kiev.ua/manual/ru/class.iterator.html

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