PHP 8.3.4 Released!

A interface Iterator

(PHP 5, PHP 7, PHP 8)

Introdução

Interface para iteradores externos ou objetos poderem ser iterados por eles mesmos internamente.

Resumo da Interface

interface Iterator extends Traversable {
/* Métodos */
public current(): mixed
public key(): mixed
public next(): void
public rewind(): void
public valid(): bool
}

Iterators predefinidos

O PHP já possui diversos iteradores para muitas das tarefas do dia a dia. Veja iteradores SPL para uma lista.

Exemplos

Exemplo #1 Utilização básica

Este exemplo demonstra em qual ordem os métodos são chamados quando estiver usando um foreach em um iterador.

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

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

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

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

#[
\ReturnTypeWillChange]
public function
key() {
var_dump(__METHOD__);
return
$this->position;
}

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

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

$it = new myIterator;

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

O exemplo acima produzirá algo semelhante a:

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"

Veja Também

Veja também a iteração em objetos.

Índice

add a note

User Contributed Notes 18 notes

up
241
robert_e_lee at dell dot com
14 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...
up
45
RocketInABog at techno-monks dot net
14 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;
}
}

?>
up
25
fetidfrog at gmail dot com
11 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;
}

# ...
}
?>
up
4
jeroen at asystance dot nl
4 years ago
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.
up
13
mike dot thornton at firstroi dot com
14 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));
}
?>
up
5
Anonymous
8 years 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";
}
?>
up
11
Geoffrey Sneddon
14 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;
}
?>
up
6
Gilles A
9 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());
}
?>
up
7
Anthony Sterling
14 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.
up
3
geompse at gmail dot com
12 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

?>
up
1
ct
12 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();

?>
up
-1
InvisibleSmiley
1 year ago
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.
up
0
Andrew Roffey
3 years ago
/**
* 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");
}
up
0
sofe2038 at gmail dot com
3 years ago
The "scalar" restriction on key() is no longer true. A simple example is that Generators can yield non-scalar keys.
up
0
kent at marketacumen dot com
13 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.
up
-1
dwlnetnl at users dot sourceforge dot net
12 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.
up
-2
Daniel Lo Nigro
10 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
up
-4
shaun at slickdesign dot com dot au
5 years ago
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->i ] );
}
function
current() {
return
current( $this->iterations[ $this->i ] );
}
function
key() {
return
key( $this->iterations[ $this->i ] );
}
function
next() {
return
next( $this->iterations[ $this->i ] );
}
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( [1, 2] );
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,

?>
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