Iterator pattern

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In object-oriented programming, the iterator pattern is a design pattern in which an iterator is used to traverse a container and access the container's elements. The iterator pattern decouples algorithms from containers; in some cases, algorithms are necessarily container-specific and thus cannot be decoupled.

For example, the hypothetical algorithm SearchForElement can be implemented generally using a specified type of iterator rather than implementing it as a container-specific algorithm. This allows SearchForElement to be used on any container that supports the required type of iterator.

Overview[edit]

The Iterator [1] design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

What problems can the Iterator design pattern solve? [2]

  • The elements of an aggregate object should be accessed and traversed without exposing its representation (data structures).
  • New traversal operations should be defined for an aggregate object without changing its interface.

Defining access and traversal operations in the aggregate interface is inflexible because it commits the aggregate to particular access and traversal operations and makes it impossible to add new operations later without having to change the aggregate interface.

What solution does the Iterator design pattern describe?

  • Define a separate (iterator) object that encapsulates accessing and traversing an aggregate object.
  • Clients use an iterator to access and traverse an aggregate without knowing its representation (data structures).

Different iterators can be used to access and traverse an aggregate in different ways.
New access and traversal operations can be defined independently by defining new iterators.

See also the UML class and sequence diagram below.

Definition[edit]

The essence of the Iterator Pattern is to "Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.".[3]

Structure[edit]

UML class and sequence diagram[edit]

A sample UML class and sequence diagram for the Iterator design pattern. [4]

In the above UML class diagram, the Client class refers (1) to the Aggregate interface for creating an Iterator object (createIterator()) and (2) to the Iterator interface for traversing an Aggregate object (next(),hasNext()). The Iterator1 class implements the Iterator interface by accessing the Aggregate1 class.

The UML sequence diagram shows the run-time interactions: The Client object calls createIterator() on an Aggregate1 object, which creates an Iterator1 object and returns it to the Client. The Client uses then Iterator1 to traverse the elements of the Aggregate1 object.

UML class diagram[edit]

The iterator pattern

Language-specific implementation[edit]

Some languages standardize syntax. C++ and Python are notable examples.

C#[edit]

.NET Framework has special interfaces that support a simple iteration: System.Collections.IEnumerator over a non-generic collection and System.Collections.Generic.IEnumerator<T> over a generic collection.

C# statement foreach is designed to easily iterate through the collection that implements System.Collections.IEnumerator and/or System.Collections.Generic.IEnumerator<T> interface. Since C# v2, foreach is also able to iterate through types that implement System.Collections.Generic.IEnumerable<T> and System.Collections.Generic.IEnumerator<T> [5]

Example of using foreach statement:

var primes = new List<int>{ 2, 3, 5, 7, 11, 13, 17, 19};
long m = 1;
foreach (var p in primes)
    m *= p;

C++[edit]

C++ implements iterators with the semantics of pointers in that language. In C++, a class can overload all of the pointer operations, so an iterator can be implemented that acts more or less like a pointer, complete with dereference, increment, and decrement. This has the advantage that C++ algorithms such as std::sort can immediately be applied to plain old memory buffers, and that there is no new syntax to learn. However, it requires an "end" iterator to test for equality, rather than allowing an iterator to know that it has reached the end. In C++ language, we say that an iterator models the iterator concept.

Java[edit]

Java has the Iterator interface.

As of Java 5, objects implementing the Iterable interface, which returns an Iterator from its only method, can be traversed using Java's foreach loop syntax. The Collection interface from the Java collections framework extends Iterable.

JavaScript[edit]

JavaScript, as part of ECMAScript 6, supports the iterator pattern with any object that provides a next() method, which returns an object with two specific properties: done and value. Here's an example that shows a reverse array iterator:

function reverseArrayIterator(array) {
    var index = array.length - 1;
    return {
       next: () =>
          index >= 0 ?
           {value: array[index--], done: false} :
           {done: true}
    }
}

const it = reverseArrayIterator(['three', 'two', 'one']);
console.log(it.next().value);  //-> 'one'
console.log(it.next().value);  //-> 'two'
console.log(it.next().value);  //-> 'three'
console.log(`Are you done? ${it.next().done}`);  //-> true

Most of the time, though, what you want is to provide Iterator[6] semantics on objects so that they can be iterated automatically via for...of loops. Some of JavaScript's built-in types such as Array, Map, or Set already define their own iteration behavior. You can achieve the same effect by defining an object's meta @@iterator method, also referred to by Symbol.iterator. This creates an Iterable object.

Here's an example of a range function that generates a list of values starting from start to end, exclusive, using a regular for loop to generate the numbers:

function range(start, end) {
  return {
    [Symbol.iterator]() { //#A
      return this;
    },
    next() {
      if(start < end) {
        return { value: start++, done:false }; //#B
      }
      return { done: true, value:end }; //#B
    }
  }
}

for(number of range(1, 5)) {
   console.log(number);   //-> 1, 2, 3, 4
}

The iteration mechanism of built-in types, like strings, can also be manipulated:

let iter = ['I', 't', 'e', 'r', 'a', 't', 'o', 'r'][Symbol.iterator]();
iter.next().value; //-> I
iter.next().value; //-> t

PHP[edit]

PHP supports the iterator pattern via the Iterator interface, as part of the standard distribution.[7] Objects that implement the interface can be iterated over with the foreach language construct.

Example of patterns using PHP:

<?php

// BookIterator.php

namespace DesignPatterns;

class BookIterator implements \Iterator
{
    private $i_position = 0;
    private $booksCollection;

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

    public function current()
    {
        return $this->booksCollection->getTitle($this->i_position);
    }

    public function key()
    {
        return $this->i_position;
    }

    public function next()
    {
        $this->i_position++;
    }

    public function rewind()
    {
        $this->i_position = 0;
    }

    public function valid()
    {
        return !is_null($this->booksCollection->getTitle($this->i_position));
    }
}
<?php

// BookCollection.php

namespace DesignPatterns;

class BookCollection implements \IteratorAggregate
{
    private $a_titles = array();

    public function getIterator()
    {
        return new BookIterator($this);
    }

    public function addTitle($string)
    {
        $this->a_titles[] = $string;
    }

    public function getTitle($key)
    {
        if (isset($this->a_titles[$key])) {
            return $this->a_titles[$key];
        }
        return null;
    }

    public function is_empty()
    {
        return empty($a_titles);
    }
}
<?php

// index.php

require 'vendor/autoload.php';
use DesignPatterns\BookCollection;

$booksCollection = new BookCollection();
$booksCollection->addTitle('Design Patterns');
$booksCollection->addTitle('PHP7 is the best');
$booksCollection->addTitle('Laravel Rules');
$booksCollection->addTitle('DHH Rules');

foreach($booksCollection as $book){
    var_dump($book);
}

OUTPUT[edit]

string(15) "Design Patterns"
string(16) "PHP7 is the best"
string(13) "Laravel Rules"
string(9) "DHH Rules"

Python[edit]

Python prescribes a syntax for iterators as part of the language itself, so that language keywords such as for work with what Python calls sequences. A sequence has an __iter__() method that returns an iterator object. The "iterator protocol" requires next() return the next element or raise a StopIteration exception upon reaching the end of the sequence. Iterators also provide an __iter__() method returning themselves so that they can also be iterated over e.g., using a for loop. Generators are available since 2.2.

In Python 3, next() was renamed __next__().[8]

See also[edit]

References[edit]

  1. ^ Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison Wesley. pp. 257ff. ISBN 0-201-63361-2. 
  2. ^ "The Iterator design pattern - Problem, Solution, and Applicability". w3sDesign.com. Retrieved 2017-08-12. 
  3. ^ Gang Of Four
  4. ^ "The Iterator design pattern - Structure and Collaboration". w3sDesign.com. Retrieved 2017-08-12. 
  5. ^ https://docs.microsoft.com/en-us/dotnet/articles/csharp/language-reference/keywords/foreach-in
  6. ^ "Iterators and generators". Retrieved 18 March 2016. 
  7. ^ "PHP: Iterator". Retrieved 23 June 2013. 
  8. ^ "Python v2.7.1 documentation: The Python Standard Library: 5. Built-in Types". Retrieved 2 May 2011. 

External links[edit]