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The Iterator interface

Introduction

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

Interface synopsis

Iterator
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 )
}

Example #2 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


User Contributed Notes
Iterator
robert_e_lee at dell dot com
11-Mar-2010 06:08
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...
Anthony Sterling
17-Sep-2009 09:20
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.
Geoffrey Sneddon
23-May-2009 05:42
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;
}
?>
RocketInABog at techno-monks dot net
11-May-2009 11:35
<?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;
  }
}

?>
mike dot thornton at firstroi dot com
05-May-2009 08:40
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));
    }
?>
nullhility at gmail dot com
19-Feb-2009 03:13
Because the purpose of the above example is to illustrate the foreach statement's call sequence I made my own example of the Iterator interface just to give other user's some idea of possible solutions that might benefit from the interface.

<?php
class MysqlResult implements Iterator
{
    private
$result;
    private
$position;
    private
$row_data;
   
    public function
__construct ($result)
    {
       
$this->result = $result;
       
$this->position = 0;
    }
   
    public function
current ()
    {
        return
$this->row_data;
    }
   
    public function
key ()
    {
        return
$this->position;
    }
   
    public function
next ()
    {
       
$this->position++;
       
$this->row_data = mysql_fetch_assoc($this->result);
    }

    public function
rewind ()
    {
       
$this->position = 0;
       
mysql_data_seek($this->result, 0);
       
       
/* The initial call to valid requires that data
            pre-exists in $this->row_data
        */
       
$this->row_data = mysql_fetch_assoc($this->result);
    }

    public function
valid ()
    {
        return (boolean)
$this->row_data;
    }
}
$link = mysql_connect("localhost", "user");
mysql_select_db('example_db', $link);
$result = new MysqlResult(mysql_query("SELECT * FROM `example_tbl`"));
foreach (
$result as $pos => $row) {
   
var_dump($pos, $row);
}
?>
http://chanibal.net
29-Jan-2009 04:11
For anyone interested in how exactly does a iterator work in a foreach:

<?php
class SomeIterator implements Iterator {/*...implement...*/}

$it=new SomeIterator();
foreach(
$it as $key => $val) {
    print
"{$key}=>{$val}\n";
}

// works exactly the same as

$it=new SomeIterator();
for(
   
$it->rewind();
   
$it->valid();
   
$val=$it->current(), $key=$it->key(), $it->next()
) {
    print
"{$key}=>{$val}\n";
}

// and if someone forgot how for(;;) works...

$it=new SomeIterator();
$it->rewind;
while(
$it->valid()) {
   
$val=$it->current();
   
$key=$it->key();
   
$it->next();

    print
"{$key}=>{$val}\n";
}
?>
 

 
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