Generators overview

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 offers a convenient way to provide data to foreach loops without having to build an array in memory ahead of time, which may cause the program to exceed a memory limit or require a considerable amount of processing time to generate. Instead, a generator function can be used, 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. Like with iterators, random data access is not possible.

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.

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

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

        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(1, 9, 2) as $number) {
    echo "$number ";
}
echo "\n";

echo 'Single digit odd numbers from xrange(): ';
foreach (xrange(1, 9, 2) as $number) {
    echo "$number ";
}
?>

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