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Generator syntax

A generator function looks just like a normal function, except that instead of returning a value, a generator yields as many values as it needs to. Any function containing yield is a generator function.

When a generator function is called, it returns an object that can be iterated over. When you iterate over that object (for instance, via a foreach loop), PHP will call the object's iteration methods each time it needs a value, then saves the state of the generator when the generator yields a value so that it can be resumed when the next value is required.

Once there are no more values to be yielded, then the generator can simply return, and the calling code continues just as if an array has run out of values.

Note:

A generator can return values, which can be retrieved using Generator::getReturn().

yield keyword

The heart of a generator function is the yield keyword. In its simplest form, a yield statement looks much like a return statement, except that instead of stopping execution of the function and returning, yield instead provides a value to the code looping over the generator and pauses execution of the generator function.

Example #1 A simple example of yielding values

<?php
function gen_one_to_three() {
for (
$i = 1; $i <= 3; $i++) {
// Note that $i is preserved between yields.
yield $i;
}
}

$generator = gen_one_to_three();
foreach (
$generator as $value) {
echo
"$value\n";
}
?>

The above example will output:

1
2
3

Note:

Internally, sequential integer keys will be paired with the yielded values, just as with a non-associative array.

Yielding values with keys

PHP also supports associative arrays, and generators are no different. In addition to yielding simple values, as shown above, you can also yield a key at the same time.

The syntax for yielding a key/value pair is very similar to that used to define an associative array, as shown below.

Example #2 Yielding a key/value pair

<?php
/*
* The input is semi-colon separated fields, with the first
* field being an ID to use as a key.
*/

$input = <<<'EOF'
1;PHP;Likes dollar signs
2;Python;Likes whitespace
3;Ruby;Likes blocks
EOF;

function
input_parser($input) {
foreach (
explode("\n", $input) as $line) {
$fields = explode(';', $line);
$id = array_shift($fields);

yield
$id => $fields;
}
}

foreach (
input_parser($input) as $id => $fields) {
echo
"$id:\n";
echo
" $fields[0]\n";
echo
" $fields[1]\n";
}
?>

The above example will output:

1:
    PHP
    Likes dollar signs
2:
    Python
    Likes whitespace
3:
    Ruby
    Likes blocks

Yielding null values

Yield can be called without an argument to yield a null value with an automatic key.

Example #3 Yielding nulls

<?php
function gen_three_nulls() {
foreach (
range(1, 3) as $i) {
yield;
}
}

var_dump(iterator_to_array(gen_three_nulls()));
?>

The above example will output:

array(3) {
  [0]=>
  NULL
  [1]=>
  NULL
  [2]=>
  NULL
}

Yielding by reference

Generator functions are able to yield values by reference as well as by value. This is done in the same way as returning references from functions: by prepending an ampersand to the function name.

Example #4 Yielding values by reference

<?php
function &gen_reference() {
$value = 3;

while (
$value > 0) {
yield
$value;
}
}

/*
* Note that we can change $number within the loop, and
* because the generator is yielding references, $value
* within gen_reference() changes.
*/
foreach (gen_reference() as &$number) {
echo (--
$number).'... ';
}
?>

The above example will output:

2... 1... 0...

Generator delegation via yield from

Generator delegation allows you to yield values from another generator, Traversable object, or array by using the yield from keyword. The outer generator will then yield all values from the inner generator, object, or array until that is no longer valid, after which execution will continue in the outer generator.

If a generator is used with yield from, the yield from expression will also return any value returned by the inner generator.

Caution

Storing into an array (e.g. with iterator_to_array())

yield from does not reset the keys. It preserves the keys returned by the Traversable object, or array. Thus some values may share a common key with another yield or yield from, which, upon insertion into an array, will overwrite former values with that key.

A common case where this matters is iterator_to_array() returning a keyed array by default, leading to possibly unexpected results. iterator_to_array() has a second parameter preserve_keys which can be set to false to collect all the values while ignoring the keys returned by the Generator.

Example #5 yield from with iterator_to_array()

<?php
function inner() {
yield
1; // key 0
yield 2; // key 1
yield 3; // key 2
}
function
gen() {
yield
0; // key 0
yield from inner(); // keys 0-2
yield 4; // key 1
}
// pass false as second parameter to get an array [0, 1, 2, 3, 4]
var_dump(iterator_to_array(gen()));
?>

The above example will output:

array(3) {
  [0]=>
  int(1)
  [1]=>
  int(4)
  [2]=>
  int(3)
}

Example #6 Basic use of yield from

<?php
function count_to_ten() {
yield
1;
yield
2;
yield from [
3, 4];
yield from new
ArrayIterator([5, 6]);
yield from
seven_eight();
yield
9;
yield
10;
}

function
seven_eight() {
yield
7;
yield from
eight();
}

function
eight() {
yield
8;
}

foreach (
count_to_ten() as $num) {
echo
"$num ";
}
?>

The above example will output:

1 2 3 4 5 6 7 8 9 10

Example #7 yield from and return values

<?php
function count_to_ten() {
yield
1;
yield
2;
yield from [
3, 4];
yield from new
ArrayIterator([5, 6]);
yield from
seven_eight();
return yield from
nine_ten();
}

function
seven_eight() {
yield
7;
yield from
eight();
}

function
eight() {
yield
8;
}

function
nine_ten() {
yield
9;
return
10;
}

$gen = count_to_ten();
foreach (
$gen as $num) {
echo
"$num ";
}
echo
$gen->getReturn();
?>

The above example will output:

1 2 3 4 5 6 7 8 9 10
add a note

User Contributed Notes 9 notes

up
124
Adil lhan (adilmedya at gmail dot com)
11 years ago
For example yield keyword with Fibonacci:

function getFibonacci()
{
$i = 0;
$k = 1; //first fibonacci value
yield $k;
while(true)
{
$k = $i + $k;
$i = $k - $i;
yield $k;
}
}

$y = 0;

foreach(getFibonacci() as $fibonacci)
{
echo $fibonacci . "\n";
$y++;
if($y > 30)
{
break; // infinite loop prevent
}
}
up
51
info at boukeversteegh dot nl
9 years ago
[This comment replaces my previous comment]

You can use generators to do lazy loading of lists. You only compute the items that are actually used. However, when you want to load more items, how to cache the ones already loaded?

Here is how to do cached lazy loading with a generator:

<?php
class CachedGenerator {
protected
$cache = [];
protected
$generator = null;

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

public function
generator() {
foreach(
$this->cache as $item) yield $item;

while(
$this->generator->valid() ) {
$this->cache[] = $current = $this->generator->current();
$this->generator->next();
yield
$current;
}
}
}
class
Foobar {
protected
$loader = null;

protected function
loadItems() {
foreach(
range(0,10) as $i) {
usleep(200000);
yield
$i;
}
}

public function
getItems() {
$this->loader = $this->loader ?: new CachedGenerator($this->loadItems());
return
$this->loader->generator();
}
}

$f = new Foobar;

# First
print "First\n";
foreach(
$f->getItems() as $i) {
print
$i . "\n";
if(
$i == 5 ) {
break;
}
}

# Second (items 1-5 are cached, 6-10 are loaded)
print "Second\n";
foreach(
$f->getItems() as $i) {
print
$i . "\n";
}

# Third (all items are cached and returned instantly)
print "Third\n";
foreach(
$f->getItems() as $i) {
print
$i . "\n";
}
?>
up
20
Hayley Watson
8 years ago
If for some strange reason you need a generator that doesn't yield anything, an empty function doesn't work; the function needs a yield statement to be recognised as a generator.

<?php

function gndn()
{
}

foreach(
gndn() as $it)
{
echo
'FNORD';
}

?>

But it's enough to have the yield syntactically present even if it's not reachable:

<?php

function gndn()
{
if(
false) { yield; }
}

foreach(
gndn() as $it)
{
echo
'FNORD';
}

?>
up
13
zilvinas at kuusas dot lt
8 years ago
Do not call generator functions directly, that won't work.

<?php

function my_transform($value) {
var_dump($value);
return
$value * 2;
}

function
my_function(array $values) {
foreach (
$values as $value) {
yield
my_transform($value);
}
}

$data = [1, 5, 10];
// my_transform() won't be called inside my_function()
my_function($data);

# my_transform() will be called.
foreach (my_function($data) as $val) {
// ...
}
?>
up
13
Harun Yasar harunyasar at mail dot com
9 years ago
That is a simple fibonacci generator.

<?php
function fibonacci($item) {
$a = 0;
$b = 1;
for (
$i = 0; $i < $item; $i++) {
yield
$a;
$a = $b - $a;
$b = $a + $b;
}
}

# give me the first ten fibonacci numbers
$fibo = fibonacci(10);
foreach (
$fibo as $value) {
echo
"$value\n";
}
?>
up
11
christophe dot maymard at gmail dot com
10 years ago
<?php
//Example of class implementing IteratorAggregate using generator

class ValueCollection implements IteratorAggregate
{
private
$items = array();

public function
addValue($item)
{
$this->items[] = $item;
return
$this;
}

public function
getIterator()
{
foreach (
$this->items as $item) {
yield
$item;
}
}
}

//Initializes a collection
$collection = new ValueCollection();
$collection
->addValue('A string')
->
addValue(new stdClass())
->
addValue(NULL);

foreach (
$collection as $item) {
var_dump($item);
}
up
7
Shumeyko Dmitriy
10 years ago
This is little example of using generators with recursion. Used version of php is 5.5.5
[php]
<?php
define
("DS", DIRECTORY_SEPARATOR);
define ("ZERO_DEPTH", 0);
define ("DEPTHLESS", -1);
define ("OPEN_SUCCESS", True);
define ("END_OF_LIST", False);
define ("CURRENT_DIR", ".");
define ("PARENT_DIR", "..");

function
DirTreeTraversal($DirName, $MaxDepth = DEPTHLESS, $CurrDepth = ZERO_DEPTH)
{
if ((
$MaxDepth === DEPTHLESS) || ($CurrDepth < $MaxDepth)) {
$DirHandle = opendir($DirName);
if (
$DirHandle !== OPEN_SUCCESS) {
try{
while ((
$FileName = readdir($DirHandle)) !== END_OF_LIST) { //read all file in directory
if (($FileName != CURRENT_DIR) && ($FileName != PARENT_DIR)) {
$FullName = $DirName.$FileName;
yield
$FullName;
if(
is_dir($FullName)) { //include sub files and directories
$SubTrav = DirTreeTraversal($FullName.DS, $MaxDepth, ($CurrDepth + 1));
foreach(
$SubTrav as $SubItem) yield $SubItem;
}
}
}
} finally {
closedir($DirHandle);
}
}
}
}

$PathTrav = DirTreeTraversal("C:".DS, 2);
print
"<pre>";
foreach(
$PathTrav as $FileName) printf("%s\n", $FileName);
print
"</pre>";
[/
php]
up
1
harl at gmail dot com
2 months ago
If you mix yielding values with keys and yielding values without keys, the result is the same as adding values to an array with or without keys.

<?php
function gen() {
yield
'a';
yield
4 => 'b';
yield
'c';
}

$t = iterator_to_array(gen());
var_dump($t);
?>

The result is an array [0 => 'a', 4 => 'b', 5 => 'c'], just as if you had written

<?php
$t
= [];
$t[] = 'a';
$t[4] = 'b';
$t[] = 'c';
var_dump($t);
?>

With the key given to 'c' being incremented from the previous numeric index.
up
-1
christianggimenez at gmail dot com
5 years ago
Module list of a number from 1 to 100.

<?php

function list_of_modulo(){

for(
$i = 1; $i <= 100; $i++){

if((
$i % 2) == 0){
yield
$i;
}
}
}

$modulos = list_of_modulo();

foreach(
$modulos as $value){

echo
"$value\n";
}

?>
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