Iterators in PHP and their benefits and disadvantages

Dmytro Tus

Full Stack Web developer

Definition of Iterators

In PHP, iterators are special objects that allow us to iterate through a data set sequentially. They implement the Iterator interface, which defines methods for iterating over elements. Iterators are especially useful when working with large data sets, as they allow you to process elements one by one instead of loading the entire data set into memory.

The Iterator interface includes the following methods:

• current(): Returns the current element.
• key(): Returns the key of the current element.
• next(): Moves the pointer to the next element.
• rewind(): Moves the pointer to the first element.
• valid(): Checks if the current element is valid.

Iterators usage example

Let's imagine that we have dataset. For example

$arr = [1,2,3,4,5,6,7];

And we need to make a list from this array.

If we wil use foreach we will load entire array into the ram. Let's see simple and bad approach.

<?php

$arr = [1, 2, 3, 4, 5, 6, 7];

// Load the entire array into the memory and process
echo '<ul>';
foreach ($arr as $value) {
    echo '<li>' . $value . '</li>';
}
echo '</ul>';

And now let's make the same but with Iterator.

<?php

$arr = [1, 2, 3, 4, 5, 6, 7];
$iterator = new ArrayIterator($arr);

// using iterator for processing array
echo '<ul>';
foreach ($iterator as $value) {
    echo '<li>' . $value . '</li>';
}
echo '</ul>';

That's all.

In this can I used built-in ArrayIterator class from php.

PHP has a lot of Iterators which are used for different purposes ( processing arrays, objects, etc. )

If we want, we can make own implementation of iterator in php. If we want to do it, we need to use IteratorInterface. Here is an example.

<?php

class MyNewArrayIterator implements Iterator {
    private $array = [];
    private $position = 0;

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

    public function current(): mixed {
        return $this->array[$this->position];
    }

    public function key(): mixed {
        return $this->position;
    }

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

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

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

In a nutshell iterators can help us to process the data on large datasets and make it efficiently.

Generators

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

This definition can be complicated. But the main what is needed to remember that when we need to create generator we need to use yield keyword instead of return.

Let's jump into the code and see the benefits.

<?php

$arr = range(1,20);

$onlyEven = [];

foreach($arr as $val)
{
    if($val % 2 === 0) {
        $onlyEven[] = $val;
    }
}

// this code will load all 20 digits into the memory
print_r($onlyEven); // [2,4,6,8,10,12,14,16,18,20]


$arr = range(1,20);

function onlyEvenGenerator($start, $end) {
    for ($i = $start; $i <= $end; $i++) {
        if ($i % 2 === 0) {
            yield $i; //we executing the function only when our condition is true
        }
    }
}

$generator = onlyEvenGenerator(1,20); // here we have Generator class instead of array

$onlyEvenViaGenerator = [];
foreach($generator as $val)
{
    $onlyEvenViaGenerator[] = $val;
}

// this code will load only 10 digits into the memory
print_r($onlyEvenViaGenerator); // [2,4,6,8,10,12,14,16,18,20]

Disadvantages of generators

Important is that we can not use loop on generator more than once. 

Let's provide the example.

<?php

function createGenerator($start, $end) {
    for ($i = $start; $i <= $end; $i++) {
        yield $i;
    }
}

$generator = createGenerator(1,20);

$array = [];
foreach($generator as $val)
{
    $array[] = $val;
}

$arrayPlus50 = [];
foreach($generator as $val)
{
    $arrayPlus1[] = $val + 50;
}

print_r($arrayPlus1);

///Fatal error: Uncaught Exception: Cannot traverse an already closed generator in php-wasm run script:18 Stack trace: #0 {main} thrown in php-wasm run script on line 18

But with normal array we can do it

<?php

$notGenerator = range(1,20);

$array = [];
foreach($notGenerator as $val)
{
    $array[] = $val;
}

print_r($array);

$arrayPlus50 = [];
foreach($notGenerator as $val)
{
    $arrayPlus1[] = $val + 50;
}

print_r($arrayPlus1);

///it is works and print for us two arrays

Conclution. Iterators are powerfull instrument which can improve perfomance of php application. But it has disadvantages, so we need to decide whenever to use that. 

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