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In
computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as ana ...
, an iterator is an
object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ai ...
that enables a programmer to traverse a
container A container is any receptacle or enclosure for holding a product used in storage, packaging, and transportation, including shipping. Things kept inside of a container are protected on several sides by being inside of its structure. The term ...
, particularly
lists A ''list'' is any set of items in a row. List or lists may also refer to: People * List (surname) Organizations * List College, an undergraduate division of the Jewish Theological Seminary of America * SC Germania List, German rugby unio ...
. Various types of iterators are often provided via a
container A container is any receptacle or enclosure for holding a product used in storage, packaging, and transportation, including shipping. Things kept inside of a container are protected on several sides by being inside of its structure. The term ...
's
interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * ''Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Inte ...
. Though the interface and semantics of a given iterator are fixed, iterators are often implemented in terms of the structures underlying a container implementation and are often tightly
coupled ''Coupled'' is an American dating game show that aired on Fox Broadcasting Company, Fox from May 17 to August 2, 2016. It was hosted by television personality, Terrence J and created by Mark Burnett, of ''Survivor (U.S. TV series), Survivor'', ''T ...
to the container to enable the operational semantics of the iterator. An iterator performs traversal and also gives access to data elements in a container, but does not itself perform
iteration Iteration is the repetition of a process in order to generate a (possibly unbounded) sequence of outcomes. Each repetition of the process is a single iteration, and the outcome of each iteration is then the starting point of the next iteration. ...
(i.e., not without some significant liberty taken with that concept or with trivial use of the terminology). An iterator is behaviorally similar to a database cursor. Iterators date to the CLU programming language in 1974.


Description


Internal Iterators

Internal iterators are
higher order functions In mathematics and computer science, a higher-order function (HOF) is a function that does at least one of the following: * takes one or more functions as arguments (i.e. a procedural parameter, which is a parameter of a procedure that is itse ...
(often taking
anonymous functions In computer programming, an anonymous function (function literal, lambda abstraction, lambda function, lambda expression or block) is a function definition that is not bound to an identifier. Anonymous functions are often arguments being passed to ...
, but not necessarily) such as map(), reduce() etc., implementing the traversal across a container, applying the given function to every element in turn. An example might be Python's map function: digits = , 1, 2, 3, 4, 5, 6, 7, 8, 9 squared_digits = map(lambda x: x**2, digits) # Iterating over this iterator would result in 0, 1, 4, 9, 16, ..., 81.


External iterators and the iterator pattern

An external iterator may be thought of as a type of pointer that has two primary operations: referencing one particular element in the object collection (called ''element access''), and modifying itself so it points to the next element (called ''element traversal''). There must also be a way to create an iterator so it points to some first element as well as some way to determine when the iterator has exhausted all of the elements in the container. Depending on the language and intended use, iterators may also provide additional operations or exhibit different behaviors. The primary purpose of an iterator is to allow a user to process every element of a container while isolating the user from the internal structure of the container. This allows the container to store elements in any manner it wishes while allowing the user to treat it as if it were a simple sequence or list. An iterator class is usually designed in tight coordination with the corresponding container class. Usually, the container provides the methods for creating iterators. A
loop counter In computer science a for-loop or for loop is a control flow statement for specifying iteration. Specifically, a for loop functions by running a section of code repeatedly until a certain condition has been satisfied. For-loops have two part ...
is sometimes also referred to as a loop iterator. A
loop counter In computer science a for-loop or for loop is a control flow statement for specifying iteration. Specifically, a for loop functions by running a section of code repeatedly until a certain condition has been satisfied. For-loops have two part ...
, however, only provides the traversal functionality and not the element access functionality.


Generators

One way of implementing iterators is to use a restricted form of
coroutine Coroutines are computer program components that generalize subroutines for non-preemptive multitasking, by allowing execution to be suspended and resumed. Coroutines are well-suited for implementing familiar program components such as cooperative ...
, known as a
generator Generator may refer to: * Signal generator, electronic devices that generate repeating or non-repeating electronic signals * Electric generator, a device that converts mechanical energy to electrical energy. * Generator (circuit theory), an eleme ...
. By contrast with a
subroutine In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed. Functions may ...
, a generator coroutine can ''yield'' values to its caller multiple times, instead of returning just once. Most iterators are naturally expressible as generators, but because generators preserve their local state between invocations, they're particularly well-suited for complicated, stateful iterators, such as tree traversers. There are subtle differences and distinctions in the use of the terms "generator" and "iterator", which vary between authors and languages. In
Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...
, a generator is an iterator constructor: a function that returns an iterator. An example of a Python generator returning an iterator for the
Fibonacci number In mathematics, the Fibonacci numbers, commonly denoted , form a sequence, the Fibonacci sequence, in which each number is the sum of the two preceding ones. The sequence commonly starts from 0 and 1, although some authors start the sequence from ...
s using Python's yield statement follows: def fibonacci(limit): a, b = 0, 1 for _ in range(limit): yield a a, b = b, a + b for number in fibonacci(100): # The generator constructs an iterator print(number)


Implicit iterators

Some object-oriented languages such as C#,
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
(later versions),
Delphi Delphi (; ), in legend previously called Pytho (Πυθώ), in ancient times was a sacred precinct that served as the seat of Pythia, the major oracle who was consulted about important decisions throughout the ancient classical world. The oracle ...
(later versions), Go,
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
(later versions),
Lua Lua or LUA may refer to: Science and technology * Lua (programming language) * Latvia University of Agriculture * Last universal ancestor, in evolution Ethnicity and language * Lua people, of Laos * Lawa people, of Thailand sometimes referred t ...
,
Perl Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offici ...
,
Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...
,
Ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sa ...
provide an
intrinsic In science and engineering, an intrinsic property is a property of a specified subject that exists itself or within the subject. An extrinsic property is not essential or inherent to the subject that is being characterized. For example, mass ...
way of iterating through the elements of a container object without the introduction of an explicit iterator object. An actual iterator object may exist in reality, but if it does it is not exposed within the source code of the language. Implicit iterators are often manifested by a "
foreach In computer programming, foreach loop (or for each loop) is a control flow statement for traversing items in a collection. is usually used in place of a standard loop statement. Unlike other loop constructs, however, loops usually maintain ...
" statement (or equivalent), such as in the following Python example: for value in iterable: print(value) In Python, an iterable is an object which can be converted to an iterator, which is then iterated through during the for loop; this is done implicitly. Or other times they may be created by the collection object itself, as in this Ruby example: iterable.each do , value, puts value end This iteration style is sometimes called "internal iteration" because its code fully executes within the context of the iterable object (that controls all aspects of iteration), and the programmer only provides the operation to execute at each step (using an
anonymous function In computer programming, an anonymous function (function literal, lambda abstraction, lambda function, lambda expression or block) is a function definition that is not bound to an identifier. Anonymous functions are often arguments being passed to ...
). Languages that support
list comprehension A list comprehension is a Syntax of programming languages, syntactic construct available in some programming languages for creating a list based on existing list (computing), lists. It follows the form of the mathematical ''set-builder notation'' ( ...
s or similar constructs may also make use of implicit iterators during the construction of the result list, as in Python: names = erson.name for person in roster if person.male Sometimes the implicit hidden nature is only partial. The
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
language has a few function templates for implicit iteration, such as for_each(). These functions still require explicit iterator objects as their initial input, but the subsequent iteration does not expose an iterator object to the user.


Streams

Iterators are a useful abstraction of
input stream In computer science, a stream is a sequence of data elements made available over time. A stream can be thought of as items on a conveyor belt being processed one at a time rather than in large batches. Streams are processed differently from ...
s – they provide a potentially infinite iterable (but not necessarily indexable) object. Several languages, such as Perl and Python, implement streams as iterators. In Python, iterators are objects representing streams of data. Alternative implementations of stream include data-driven languages, such as
AWK AWK (''awk'') is a domain-specific language designed for text processing and typically used as a data extraction and reporting tool. Like sed and grep, it is a filter, and is a standard feature of most Unix-like operating systems. The AWK langu ...
and
sed sed ("stream editor") is a Unix utility that parses and transforms text, using a simple, compact programming language. It was developed from 1973 to 1974 by Lee E. McMahon of Bell Labs, and is available today for most operating systems. sed wa ...
.


Contrasting with indexing

In procedural languages it is common to use the subscript operator and a
loop counter In computer science a for-loop or for loop is a control flow statement for specifying iteration. Specifically, a for loop functions by running a section of code repeatedly until a certain condition has been satisfied. For-loops have two part ...
to loop through all the elements in a sequence such as an array. Although indexing may also be used with some object-oriented containers, the use of iterators may have some advantages: * Counting loops are not suitable to all data structures, in particular to data structures with no or slow
random access Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any othe ...
, like
lists A ''list'' is any set of items in a row. List or lists may also refer to: People * List (surname) Organizations * List College, an undergraduate division of the Jewish Theological Seminary of America * SC Germania List, German rugby unio ...
or
trees In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are u ...
. * Iterators can provide a consistent way to iterate on data structures of all kinds, and therefore make the code more readable, reusable, and less sensitive to a change in the data structure. * An iterator can enforce additional restrictions on access, such as ensuring that elements cannot be skipped or that a previously visited element cannot be accessed a second time. * An iterator may allow the container object to be modified without invalidating the iterator. For instance, once an iterator has advanced beyond the first element it may be possible to insert additional elements into the beginning of the container with predictable results. With indexing this is problematic since the index numbers must change. The ability of a container to be modified while iterating through its elements has become necessary in modern
object-oriented Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pro ...
programming, where the interrelationships between objects and the effects of operations may not be obvious. By using an iterator one is isolated from these sorts of consequences. This assertion must however be taken with a grain of salt, because more often than not, for efficiency reasons, the iterator implementation is so tightly bound to the container that it does preclude modification of the underlying container without invalidating itself. For containers that may move around their data in memory, the only way to not invalidate the iterator is, for the container, to somehow keep track of all the currently alive iterators and update them on the fly. Since the number of iterators at a given time may be arbitrarily large in comparison to the size of the tied container, updating them all will drastically impair the complexity guarantee on the container's operations. An alternative way to keep the number of updates bound relatively to the container size would be to use a kind of handle mechanism, that is a collection of indirect pointers to the container's elements that must be updated with the container, and let the iterators point to these handles instead of directly to the data elements. But this approach will negatively impact the iterator performance, since it must effectuate a double pointer following to access the actual data element. This is usually not desirable, because many algorithms using the iterators invoke the iterators data access operation more often than the advance method. It is therefore especially important to have iterators with very efficient data access. All in all, this is always a trade-off between security (iterators remain always valid) and efficiency. Most of the time, the added security is not worth the efficiency price to pay for it. Using an alternative container (for example a singly linked list instead of a vector) would be a better choice (globally more efficient) if the stability of the iterators is needed.


Classifying iterators


Iterator categories

Iterators can be categorised according to their functionality. Here is a (non-exhaustive) list of iterator categories:


Iterator types

Different languages or libraries used with these languages define iterator types. Some of them are


In different programming languages


C# and other .NET languages

Iterators in the
.NET Framework The .NET Framework (pronounced as "''dot net"'') is a proprietary software framework developed by Microsoft that runs primarily on Microsoft Windows. It was the predominant implementation of the Common Language Infrastructure (CLI) until bein ...
are called "enumerators" and represented by the IEnumerator interface. IEnumerator provides a MoveNext() method, which advances to the next element and indicates whether the end of the collection has been reached; a Current property, to obtain the value of the element currently being pointed at; and an optional Reset() method, to rewind the enumerator back to its initial position. The enumerator initially points to a special value before the first element, so a call to MoveNext() is required to begin iterating. Enumerators are typically obtained by calling the GetEnumerator() method of an object implementing the IEnumerable interface. Container classes typically implement this interface. However, the
foreach In computer programming, foreach loop (or for each loop) is a control flow statement for traversing items in a collection. is usually used in place of a standard loop statement. Unlike other loop constructs, however, loops usually maintain ...
statement in C# can operate on any object providing such a method, even if it doesn't implement IEnumerable (
duck typing Duck typing in computer programming is an application of the duck test—"If it walks like a duck and it quacks like a duck, then it must be a duck"—to determine whether an object can be used for a particular purpose. With nominative t ...
). Both interfaces were expanded into
generic Generic or generics may refer to: In business * Generic term, a common name used for a range or class of similar things not protected by trademark * Generic brand, a brand for a product that does not have an associated brand or trademark, other ...
versions in .NET 2.0. The following shows a simple use of iterators in C# 2.0: // explicit version IEnumerator iter = list.GetEnumerator(); while (iter.MoveNext()) Console.WriteLine(iter.Current); // implicit version foreach (MyType value in list) Console.WriteLine(value); C# 2.0 also supports generators: a method that is declared as returning IEnumerator (or IEnumerable), but uses the "yield return" statement to produce a sequence of elements instead of returning an object instance, will be transformed by the compiler into a new class implementing the appropriate interface.


C++

The
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
language makes wide use of iterators in its
Standard Library In computer programming, a standard library is the library made available across implementations of a programming language. These libraries are conventionally described in programming language specifications; however, contents of a language's as ...
and describes several categories of iterators differing in the repertoire of operations they allow. These include ''forward iterators'', ''bidirectional iterators'', and ''random access iterators'', in order of increasing possibilities. All of the standard container template types provide iterators of one of these categories. Iterators generalize pointers to elements of an array (which indeed can be used as iterators), and their syntax is designed to resemble that of C
pointer arithmetic In computer science, a pointer is an object in many programming languages that stores a memory address. This can be that of another value located in computer memory, or in some cases, that of memory-mapped computer hardware. A pointer ''refe ...
, where the * and -> operators are used to reference the element to which the iterator points and pointer arithmetic operators like ++ are used to modify iterators in the traversal of a container. Traversal using iterators usually involves a single varying iterator, and two fixed iterators that serve to delimit a range to be traversed. The distance between the limiting iterators, in terms of the number of applications of the operator ++ needed to transform the lower limit into the upper one, equals the number of items in the designated range; the number of distinct iterator values involved is one more than that. By convention, the lower limiting iterator "points to" the first element in the range, while the upper limiting iterator does not point to any element in the range, but rather just beyond the end of the range. For traversal of an entire container, the begin() method provides the lower limit, and end() the upper limit. The latter does not reference any element of the container at all but is a valid iterator value that can be compared against. The following example shows a typical use of an iterator. std::vector items; items.push_back(5); // Append integer value '5' to vector 'items'. items.push_back(2); // Append integer value '2' to vector 'items'. items.push_back(9); // Append integer value '9' to vector 'items'. for (auto it = items.begin(); it != items.end(); ++it) // In C++11, the same can be done without using any iterators: for (auto x : items) // Both of the for loops print "529". Iterator types are separate from the container types they are used with, though the two are often used in concert. The category of the iterator (and thus the operations defined for it) usually depends on the type of container, with for instance arrays or vectors providing random access iterators, but sets (which use a linked structure as implementation) only providing bidirectional iterators. One same container type can have more than one associated iterator type; for instance the std::vector<T> container type allows traversal either using (raw) pointers to its elements (of type *<T>), or values of a special type std::vector<T>::iterator, and yet another type is provided for "reverse iterators", whose operations are defined in such a way that an algorithm performing a usual (forward) traversal will actually do traversal in reverse order when called with reverse iterators. Most containers also provide a separate const_iterator type, for which operations that would allow changing the values pointed to are intentionally not defined. Simple traversal of a container object or a range of its elements (including modification of those elements unless a const_iterator is used) can be done using iterators alone. But container types may also provide methods like insert or erase that modify the structure of the container itself; these are methods of the container class, but in addition require one or more iterator values to specify the desired operation. While it is possible to have multiple iterators pointing into the same container simultaneously, structure-modifying operations may invalidate certain iterator values (the standard specifies for each case whether this may be so); using an invalidated iterator is an error that will lead to undefined behavior, and such errors need not be signaled by the run time system. Implicit iteration is also partially supported by C++ through the use of standard function templates, such as std::for_each()
/code>, std::copy()
/code> and std::accumulate()
/code>. When used they must be initialized with existing iterators, usually begin and end, that define the range over which iteration occurs. But no explicit iterator object is subsequently exposed as the iteration proceeds. This example shows the use of for_each. ContainerType c; // Any standard container type of ItemType elements. void ProcessItem(const ItemType& i) std::for_each(c.begin(), c.end(), ProcessItem); // A for-each iteration loop. The same can be achieved using std::copy, passing a std::ostream_iterator
/code> value as third iterator: std::copy(c.begin(), c.end(), std::ostream_iterator(std::cout, "\n")); Since
C++11 C++11 is a version of the ISO/IEC 14882 standard for the C++ programming language. C++11 replaced the prior version of the C++ standard, called C++03, and was later replaced by C++14. The name follows the tradition of naming language versions by ...
, lambda function syntax can be used to specify to operation to be iterated inline, avoiding the need to define a named function. Here is an example of for-each iteration using a lambda function: ContainerType c; // Any standard container type of ItemType elements. // A for-each iteration loop with a lambda function. std::for_each(c.begin(), c.end(), [](const ItemType& i) );


Java

Introduced in the
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
JDK 1.2 release, the interface allows the iteration of container classes. Each Iterator provides a and method, and may optionally support a method. Iterators are created by the corresponding container class, typically by a method named iterator(). The next() method advances the iterator and returns the value pointed to by the iterator. The first element is obtained upon the first call to next(). To determine when all the elements in the container have been visited the hasNext() test method is used. The following example shows a simple use of iterators: Iterator iter = list.iterator(); // Iterator iter = list.iterator(); // in J2SE 5.0 while (iter.hasNext()) To show that hasNext() can be called repeatedly, we use it to insert commas between the elements but not after the last element. This approach does not properly separate the advance operation from the actual data access. If the data element must be used more than once for each advance, it needs to be stored in a temporary variable. When an advance is needed without data access (i.e. to skip a given data element), the access is nonetheless performed, though the returned value is ignored in this case. For collection types that support it, the remove() method of the iterator removes the most recently visited element from the container while keeping the iterator usable. Adding or removing elements by calling the methods of the container (also from the same thread) makes the iterator unusable. An attempt to get the next element throws the exception. An exception is also thrown if there are no more elements remaining (hasNext() has previously returned false). Additionally, for there is a with a similar API but that allows forward and backward iteration, provides its current index in the list and allows setting of the list element at its position. The J2SE 5.0 release of Java introduced the interface to support an enhanced for (
foreach In computer programming, foreach loop (or for each loop) is a control flow statement for traversing items in a collection. is usually used in place of a standard loop statement. Unlike other loop constructs, however, loops usually maintain ...
) loop for iterating over collections and arrays. Iterable defines the method that returns an Iterator. Using the enhanced for loop, the preceding example can be rewritten as for (MyType obj : list) Some containers also use the older (since 1.0) Enumeration class. It provides hasMoreElements() and nextElement() methods but has no methods to modify the container.


Scala

In Scala, iterators have a rich set of methods similar to collections, and can be used directly in for loops. Indeed, both iterators and collections inherit from a common base trait - scala.collection.TraversableOnce. However, because of the rich set of methods available in the Scala collections library, such as map, collect, filter etc., it is often not necessary to deal with iterators directly when programming in Scala. Java iterators and collections can be automatically converted into Scala iterators and collections, respectively, simply by adding the single line import scala.collection.JavaConversions._ to the file. The JavaConversions object provides implicit conversions to do this. Implicit conversions are a feature of Scala: methods that, when visible in the current scope, automatically insert calls to themselves into relevant expressions at the appropriate place to make them typecheck when they otherwise wouldn't.


MATLAB

MATLAB MATLAB (an abbreviation of "MATrix LABoratory") is a proprietary multi-paradigm programming language and numeric computing environment developed by MathWorks. MATLAB allows matrix manipulations, plotting of functions and data, implementation ...
supports both external and internal implicit iteration using either "native" arrays or cell arrays. In the case of external iteration where the onus is on the user to advance the traversal and request next elements, one can define a set of elements within an array storage structure and traverse the elements using the for-loop construct. For example, % Define an array of integers myArray = ,3,5,7,11,13 for n = myArray % ... do something with n disp(n) % Echo integer to Command Window end traverses an array of integers using the for keyword. In the case of internal iteration where the user can supply an operation to the iterator to perform over every element of a collection, many built-in operators and MATLAB functions are overloaded to execute over every element of an array and return a corresponding output array implicitly. Furthermore, the arrayfun and cellfun functions can be leveraged for performing custom or user defined operations over "native" arrays and cell arrays respectively. For example, function simpleFun % Define an array of integers myArray = ,3,5,7,11,13 % Perform a custom operation over each element myNewArray = arrayfun(@(a)myCustomFun(a),myArray); % Echo resulting array to Command Window myNewArray function outScalar = myCustomFun(inScalar) % Simply multiply by 2 outScalar = 2*inScalar; defines a primary function simpleFun that implicitly applies custom subfunction myCustomFun to each element of an array using built-in function arrayfun. Alternatively, it may be desirable to abstract the mechanisms of the array storage container from the user by defining a custom object-oriented MATLAB implementation of the Iterator Pattern. Such an implementation supporting external iteration is demonstrated in MATLAB Central File Exchange ite
Design Pattern: Iterator (Behavioral)
This is written in the new class-definition syntax introduced with MATLAB software version 7.6 (R2008a) and features a one-dimensional cell array realization of the List Abstract Data Type (ADT) as the mechanism for storing a heterogeneous (in data type) set of elements. It provides the functionality for explicit forward
List A ''list'' is any set of items in a row. List or lists may also refer to: People * List (surname) Organizations * List College, an undergraduate division of the Jewish Theological Seminary of America * SC Germania List, German rugby union ...
traversal with the hasNext(), next() and reset() methods for use in a while-loop.


PHP

PHP PHP is a general-purpose scripting language geared toward web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementation is now produced by The PHP Group ...
's foreach loop was introduced in version 4.0 and made compatible with objects as values in 4.0 Beta 4. However, support for iterators was added in PHP 5 through the introduction of the internal Traversable interface. The two main interfaces for implementation in PHP scripts that enable objects to be iterated via the foreach loop are Iterator and IteratorAggregate. The latter does not require the implementing class to declare all required methods, instead it implements an accessor method (getIterator) that returns an instance of Traversable. The
Standard PHP Library PHP is a General-purpose programming language, general-purpose scripting language geared toward web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementati ...
provides several classes to work with special iterators. PHP also supports Generators since 5.5. The simplest implementation is by wrapping an array, this can be useful for type hinting and
information hiding In computer science, information hiding is the principle of segregation of the ''design decisions'' in a computer program that are most likely to change, thus protecting other parts of the program from extensive modification if the design decisio ...
. namespace Wikipedia\Iterator; final class ArrayIterator extends \Iterator All methods of the example class are used during the execution of a complete foreach loop (foreach ($iterator as $key => $current) ). The iterator's methods are executed in the following order: # $iterator->rewind() ensures that the internal structure starts from the beginning. # $iterator->valid() returns ''true'' in this example. # $iterator->current() returned value is stored in $value. # $iterator->key() returned value is stored in $key. # $iterator->next() advances to the next element in the internal structure. # $iterator->valid() returns ''false'' and the loop is aborted. The next example illustrates a PHP class that implements the Traversable interface, which could be wrapped in an IteratorIterator class to act upon the data before it is returned to the foreach loop. The usage together with the MYSQLI_USE_RESULT constant allows PHP scripts to iterate result sets with billions of rows with very little memory usage. These features are not exclusive to PHP nor to its MySQL class implementations (e.g. the PDOStatement class implements the Traversable interface as well). mysqli_report(MYSQLI_REPORT_ERROR , MYSQLI_REPORT_STRICT); $mysqli = new \mysqli('host.example.com', 'username', 'password', 'database_name'); // The \mysqli_result class that is returned by the method call implements the internal Traversable interface. foreach ($mysqli->query('SELECT `a`, `b`, `c` FROM `table`', MYSQLI_USE_RESULT) as $row)


Python

Iterators in
Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...
are a fundamental part of the language and in many cases go unseen as they are implicitly used in the for (
foreach In computer programming, foreach loop (or for each loop) is a control flow statement for traversing items in a collection. is usually used in place of a standard loop statement. Unlike other loop constructs, however, loops usually maintain ...
) statement, in
list comprehension A list comprehension is a Syntax of programming languages, syntactic construct available in some programming languages for creating a list based on existing list (computing), lists. It follows the form of the mathematical ''set-builder notation'' ( ...
s, and in generator expressions. All of Python's standard built-in
collection Collection or Collections may refer to: * Cash collection, the function of an accounts receivable department * Collection (church), money donated by the congregation during a church service * Collection agency, agency to collect cash * Collectio ...
types support iteration, as well as many classes that are part of the standard library. The following example shows typical implicit iteration over a sequence: for value in sequence: print(value) Python dictionaries (a form of
associative array In computer science, an associative array, map, symbol table, or dictionary is an abstract data type that stores a collection of (key, value) pairs, such that each possible key appears at most once in the collection. In mathematical terms an as ...
) can also be directly iterated over, when the dictionary keys are returned; or the items() method of a dictionary can be iterated over where it yields corresponding key,value pairs as a tuple: for key in dictionary: value = dictionary ey print(key, value) for key, value in dictionary.items(): print(key, value) Iterators however can be used and defined explicitly. For any iterable sequence type or class, the built-in function iter() is used to create an iterator object. The iterator object can then be iterated with the next() function, which uses the __next__() method internally, which returns the next element in the container. (The previous statement applies to Python 3.x. In Python 2.x, the next() method is equivalent.) A StopIteration exception will be raised when no more elements are left. The following example shows an equivalent iteration over a sequence using explicit iterators: it = iter(sequence) while True: try: value = it.next() # in Python 2.x value = next(it) # in Python 3.x except StopIteration: break print(value) Any user-defined class can support standard iteration (either implicit or explicit) by defining an __iter__() method that returns an iterator object. The iterator object then needs to define a __next__() method that returns the next element. Python's generators implement this iteration
protocol Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technology ...
.


Raku

Iterators in Raku are a fundamental part of the language, although usually users don't have to care about iterators. Their usage is hidden behind iteration APIs such as the for statement, map, grep, list indexing with .
idx The acronym IDX may refer to: * .idx file extension, used by various programs to represent an index of some kind, commonly used by the DirectVobSub add-on for subtitle files * IDX Systems, a software company, portions now owned by GE and athenaheal ...
/code>, etc. The following example shows typical implicit iteration over a collection of values: my @values = 1, 2, 3; for @values -> $value # OUTPUT: # 1 # 2 # 3 Raku hashes can also be directly iterated over; this yields key-value Pair objects. The kv method can be invoked on the hash to iterate over the key and values; the keys method to iterate over the hash's keys; and the values method to iterate over the hash's values. my %word-to-number = 'one' => 1, 'two' => 2, 'three' => 3; for %word-to-number -> $pair # OUTPUT: # three => 3 # one => 1 # two => 2 for %word-to-number.kv -> $key, $value # OUTPUT: # three: 3 # one: 1 # two: 2 for %word-to-number.keys -> $key # OUTPUT: # three => 3 # one => 1 # two => 2 Iterators however can be used and defined explicitly. For any iterable type, there are several methods that control different aspects of the iteration process. For example, the iterator method is supposed to return an Iterator object, and the pull-one method is supposed to produce and return the next value if possible, or return the sentinel value IterationEnd if no more values could be produced. The following example shows an equivalent iteration over a collection using explicit iterators: my @values = 1, 2, 3; my $it := @values.iterator; # grab iterator for @values loop # OUTPUT: # 1 # 2 # 3 All iterable types in Raku compose the Iterable role, Iterator role, or both. The Iterable is quite simple and only requires the iterator to be implemented by the composing class. The Iterator is more complex and provides a series of methods such as pull-one, which allows for a finer operation of iteration in several contexts such as adding or eliminating items, or skipping over them to access other items. Thus, any user-defined class can support standard iteration by composing these roles and implementing the iterator and/or pull-one methods. The DNA class represents a DNA strand and implements the iterator by composing the Iterable role. The DNA strand is split into a group of trinucleotides when iterated over: subset Strand of Str where ; class DNA does Iterable ; for DNA.new('GATTACATA') # OUTPUT: # (G A T) # (T A C) # (A T A) say DNA.new('GATTACATA').map(*.join).join('-'); # OUTPUT: # GAT-TAC-ATA The Repeater class composes both the Iterable and Iterator roles: class Repeater does Iterable does Iterator for Repeater.new("Hello", 3) # OUTPUT: # Hello # Hello # Hello


Ruby

Ruby implements iterators quite differently; all iterations are done by means of passing callback closures to container methods - this way Ruby not only implements basic iteration but also several patterns of iteration like function mapping, filters and reducing. Ruby also supports an alternative syntax for the basic iterating method each, the following three examples are equivalent: (0...42).each do , n, puts n end ...and... for n in 0...42 puts n end or even shorter 42.times do , n, puts n end Ruby can also iterate over fixed lists by using Enumerators and either calling their #next method or doing a for each on them, as above.


Rust

With Rust one can iterate on element of vectors, or create own iterators. Each iterator has adapters (map, filter, skip, take, ...). for n in 0..42 Below the fibonacci() function returns a custom iterator. for i in fibonacci().skip(4).take(4)


See also

*
Iteratee In functional programming, an iteratee is a composable abstraction for incrementally processing sequentially presented chunks of input data in a purely functional fashion. With iteratees, it is possible to lazily transform how a resource will emi ...
, in which, instead of the developer calling the iterator repeatedly to get new values, the iteratee is called repeatedly to process new chunks of data - an example of
inversion of control In software engineering, inversion of control (IoC) is a design pattern in which custom-written portions of a computer program receive the flow of control from a generic framework. A software architecture with this design inverts control as compa ...
. *
Design pattern A design pattern is the re-usable form of a solution to a design problem. The idea was introduced by the architect Christopher Alexander and has been adapted for various other disciplines, particularly software engineering. The " Gang of Four" b ...
*
Iteration Iteration is the repetition of a process in order to generate a (possibly unbounded) sequence of outcomes. Each repetition of the process is a single iteration, and the outcome of each iteration is then the starting point of the next iteration. ...
*
Iterator pattern 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 a ...
*
Range Range may refer to: Geography * Range (geographic), a chain of hills or mountains; a somewhat linear, complex mountainous or hilly area (cordillera, sierra) ** Mountain range, a group of mountains bordered by lowlands * Range, a term used to i ...
*
Visitor pattern In object-oriented programming and software engineering, the visitor design pattern is a way of separating an algorithm from an object structure on which it operates. A practical result of this separation is the ability to add new operations to ...
*
Pointer (computer programming) In computer science, a pointer is an object in many programming languages that stores a memory address. This can be that of another value located in computer memory, or in some cases, that of memory-mapped computer hardware. A pointer ''ref ...


References


External links

{{Wiktionary
Java's Iterator, Iterable and ListIterator Explained

.NET interface
* Article

by Joshua Gatcomb * Article
A Technique for Generic Iteration and Its Optimization
(217 KB) by Stephen M. Watt
Iterators





PHP: Object Iteration






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