PHP 5.6.0beta1 released

Generators overview

(PHP 5 >= 5.5.0)

Generators provide an easy way to implement simple iterators without the overhead or complexity of implementing a class that implements the Iterator interface.

A generator allows you to write code that uses foreach to iterate over a set of data without needing to build an array in memory, which may cause you to exceed a memory limit, or require a considerable amount of processing time to generate. Instead, you can write a generator function, which is the same as a normal function, except that instead of returning once, a generator can yield as many times as it needs to in order to provide the values to be iterated over.

A simple example of this is to reimplement the range() function as a generator. The standard range() function has to generate an array with every value in it and return it, which can result in large arrays: for example, calling range(0, 1000000) will result in well over 100 MB of memory being used.

As an alternative, we can implement an xrange() generator, which will only ever need enough memory to create an Iterator object and track the current state of the generator internally, which turns out to be less than 1 kilobyte.

Example #1 Implementing range() as a generator

<?php
function xrange($start$limit$step 1) {
    if (
$start $limit) {
        if (
$step <= 0) {
            throw new 
LogicException('Step must be +ve');
        }

        for (
$i $start$i <= $limit$i += $step) {
            yield 
$i;
        }
    } else {
        if (
$step >= 0) {
            throw new 
LogicException('Step must be -ve');
        }

        for (
$i $start$i >= $limit$i += $step) {
            yield 
$i;
        }
    }
}

/*
 * Note that both range() and xrange() result in the same
 * output below.
 */

echo 'Single digit odd numbers from range():  ';
foreach (
range(192) as $number) {
    echo 
"$number ";
}
echo 
"\n";

echo 
'Single digit odd numbers from xrange(): ';
foreach (
xrange(192) as $number) {
    echo 
"$number ";
}
?>

The above example will output:

Single digit odd numbers from range():  1 3 5 7 9 
Single digit odd numbers from xrange(): 1 3 5 7 9 
add a note add a note

User Contributed Notes 4 notes

up
9
bloodjazman at gmail dot com
8 months ago
for the protection from the leaking of resources
see RFC https://wiki.php.net/rfc/generators#closing_a_generator

and use finnaly

sample code

function getLines($file) {
    $f = fopen($file, 'r');
    try {
        while ($line = fgets($f)) {
            yield $line;
        }
    } finally {
        fclose($f);
    }
}

foreach (getLines("file.txt") as $n => $line) {
    if ($n > 5) break;
    echo $line;
}
up
1
dc at libertyskull dot com
2 months ago
Same example, different results:

----------------------------------
           |  time  | memory, mb |
----------------------------------
| not gen  | 0.7589 | 146.75     |
|---------------------------------
| with gen | 0.7469 | 8.75       |
|---------------------------------

Time in results varying from 6.5 to 7.8 on both examples.
So no real drawbacks concerning processing speed.
up
1
lubaev
3 months ago
Abstract test.
<?php

$start_time
=microtime(true);
$array = array();
$result = '';
for(
$count=1000000; $count--;)
{
 
$array[]=$count/2;
}
foreach(
$array as $val)
{
 
$val += 145.56;
 
$result .= $val;
}
$end_time=microtime(true);

echo
"time: ", bcsub($end_time, $start_time, 4), "\n";
echo
"memory (byte): ", memory_get_peak_usage(true), "\n";

?>

<?php

$start_time
=microtime(true);
$result = '';
function
it()
{
  for(
$count=1000000; $count--;)
  {
   
yield $count/2;
  }
}
foreach(
it() as $val)
{
 
$val += 145.56;
 
$result .= $val;
}
$end_time=microtime(true);

echo
"time: ", bcsub($end_time, $start_time, 4), "\n";
echo
"memory (byte): ", memory_get_peak_usage(true), "\n";

?>
Result:
----------------------------------
           |  time  | memory, mb |
----------------------------------
| not gen  | 2.1216 | 89.25      |
|---------------------------------
| with gen | 6.1963 | 8.75       |
|---------------------------------
| diff     | < 192% | > 90%      |
----------------------------------
up
0
swen dot zanon at geoglis dot de
27 days ago
I always get these results (approximately), no matter how many times I request:

without generator / with array:
time: 2.3667 memory (byte): 298057728

with generator:
time: 8.4556 memory (byte): 9175040
To Top