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<title>Indexer (programming)</title>

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Indexer (programming)

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In <a href="Object-oriented_programming" title="Object-oriented programming">object-oriented programming</a>, an indexer allows instances of a particular class or struct to be indexed just like arrays.<a href="#cite_note-1">[1]</a> It is a form of <a href="Operator_overloading" title="Operator overloading">operator overloading</a>.

<div class="mw-heading mw-heading2"><h2 id="Implementations">Implementations</h2></div>

In C++ one can emulate indexing by overloading the <code class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr">[]</code> operator. The expression <code class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr">a[b...]</code> translates to a call to the user-defined function <code class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr">operator[]</code> as <code class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr">(a).operator[](b...)</code>.<a href="#cite_note-2">[2]</a> Here is an example,

<div class="mw-highlight mw-highlight-lang-c++ mw-content-ltr" dir="ltr"><pre>struct vector {

int size; double* data;

vector(int n) { size = n; data = new double[n](); }

~vector(){ size = 0; delete[] data; }

double& operator[](int i) { return data[i]; }

};

#include <iostream>

int main() {

vector v(3);

for (int i = 0; i < v.size; i++) v[i] = i + 1;

for (int i = 0; i < v.size; i++) std::cout << v[i] << "\n";

return 0;

}

</pre></div>

Indexers are implemented through the get and set <a href="Mutator_method" title="Mutator method">accessors</a> for the <code class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr">operator[]</code>. They are similar to <a href="Property_(programming)" title="Property (programming)">properties</a>, but differ by not being <a href="Static_method" class="mw-redirect" title="Static method">static</a>, and the fact that indexers' accessors take parameters. The get and set accessors are called as methods using the parameter list of the indexer declaration, but the set accessor still has the implicit <code class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr">value</code> parameter.

<div class="mw-heading mw-heading4"><h4 id="Example_1">Example 1</h4></div>

<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>public class Vector

{

private double[] _data;

public Vector(int n)

{

_data = new double[n];

}

public int Size => _data.Length;

public double this[int i]

{

get => _data[i];

set => _data[i] = value;

}

public static void Main()

{

var vector = new Vector(3);

for (var i = 0; i < vector.Size; i++)

vector[i] = i + 1;

System.Console.WriteLine(vector[i]);

}

}

</pre></div>

<div class="mw-heading mw-heading4"><h4 id="Example_2">Example 2</h4></div>

Here is a C# example of the usage of an indexer in a class:

<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>class Family

{

private List<string> _familyMembers = new List<string>();

public Family(params string[] members)

{

_familyMembers.AddRange(members);

}

public string this[int index]

{

// The get accessor

// The set accessor with

set => _familyMembers[index] = value;

}

public int this[string val]

{

// Getting index by value (first element found)

get => _familyMembers.FindIndex(m => m == val);

}

public int Length => _familyMembers.Count;

}

</pre></div>

Usage example:

<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>void Main()

{

var doeFamily = new Family("John", "Jane");

{

var member = doeFamily[i];

var index = doeFamily[member]; // same as i in this case, but it demonstrates indexer overloading allowing to search doeFamily by value.

Console.WriteLine($"{member} is the member number {index} of the {nameof(doeFamily)}");

}

}

</pre></div>

In this example, the indexer is used to get the value at the nth position, and then to get the position in the list referenced by its value.

The output of the code is:

<pre>John is the member number 0 of the doeFamily

Jane is the member number 1 of the doeFamily

</pre>

In <a href="PHP" title="PHP">PHP</a> indexing can be implemented via the predefined <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">ArrayAccess</code> interface,<a href="#cite_note-4">[4]</a>

<div class="mw-highlight mw-highlight-lang-php mw-content-ltr" dir="ltr"><pre>class Vector implements ArrayAccess

{

function __construct(int $n) {

$this->size = $n;

$this->data = array_fill(0, $n, 0);

}

public function offsetGet($offset): mixed {

return $this->data[$offset];

}

public function offsetSet($offset, $value): void {

$this->data[$offset] = $value;

}

public function offsetExists($offset): bool {}

public function offsetUnset($offset): void {}

}

$vector = new Vector(3);

for ($i = 0; $i < $vector->size; $i++) $vector[$i] = $i + 1;

for ($i = 0; $i < $vector->size; $i++) print "{$vector[$i]}\n";

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Python">Python</h3></div>

In <a href="Python_(programming_language)" title="Python (programming language)">Python</a> one implements indexing by overloading the <code class="mw-highlight mw-highlight-lang-python mw-content-ltr" dir="ltr">__getitem__</code> and <code class="mw-highlight mw-highlight-lang-python mw-content-ltr" dir="ltr">__setitem__</code> methods,

<div class="mw-highlight mw-highlight-lang-python mw-content-ltr" dir="ltr"><pre>import array

class Vector(object):

def __init__(self, n: int):

self.size = n

self.data = array.array("d", [0.0] * n)

def __getitem__(self, i: int):

return self.data[i]

def __setitem__(self, i: int, value):

self.data[i] = value

vector = Vector(3)

for i in range(vector.size):

vector[i] = i + 1

print(vector[i])

</pre></div>

<a href="Rust_(programming_language)" title="Rust (programming language)">Rust</a> provides the <style data-mw-deduplicate="TemplateStyles:r886049734">

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</style>std::ops::Index trait.<a href="#cite_note-5">[5]</a>

<div class="mw-highlight mw-highlight-lang-rust mw-content-ltr" dir="ltr"><pre>use std::ops::Index;

enum Nucleotide {

A,

C,

G,

T,

}

struct NucleotideCount {

a: usize,

c: usize,

g: usize,

t: usize,

}

impl Index<Nucleotide> for NucleotideCount {

type Output = usize;

fn index(&self, nucleotide: Nucleotide) -> &Self::Output {

match nucleotide {

Nucleotide::A => &self.a,

Nucleotide::C => &self.c,

Nucleotide::G => &self.g,

Nucleotide::T => &self.t,

}

}

let nucleotide_count = NucleotideCount {a: 14, c: 9, g: 10, t: 12};

assert_eq!(nucleotide_count[Nucleotide::A], 14);

assert_eq!(nucleotide_count[Nucleotide::C], 9);

assert_eq!(nucleotide_count[Nucleotide::G], 10);

assert_eq!(nucleotide_count[Nucleotide::T], 12);

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Smalltalk">Smalltalk</h3></div>

In <a href="Smalltalk" title="Smalltalk">Smalltalk</a> one can emulate indexing by (e.g.) defining the <code class="mw-highlight mw-highlight-lang-smalltalk mw-content-ltr" dir="ltr">get:</code> and <code class="mw-highlight mw-highlight-lang-smalltalk mw-content-ltr" dir="ltr">set:value:</code> instance methods. For example, in <a href="GNU_Smalltalk" title="GNU Smalltalk">GNU Smalltalk</a>,

<div class="mw-highlight mw-highlight-lang-smalltalk mw-content-ltr" dir="ltr"><pre>Object subclass: vector [ |data| ]

vector class extend [ new: n [ |v| v:=super new. v init: n. ^v] ]

vector extend [ init: n [ data:= Array new: n ] ]

vector extend [ size [ ^(data size) ] ]

vector extend [ get: i [ ^(data at: i) ] ]

vector extend [ set: i value: x [ data at: i put: x ] ]

v:=vector new: 3

1 to: (v size) do: [:i| v set: i value: (i+1) ]

1 to: (v size) do: [:i| (v get: i) printNl ]

</pre></div>

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<ul><li><a href="Mutator_method" title="Mutator method">Mutator method</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFjagadish9802008" class="citation web cs1">jagadish980 (2008-01-29). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20090922193214/http://forums.sureshkumar.net/vb-asp-net-interview-technical-questions/16320-c-what-indexer-c.html">"C# - What is an indexer in C#"</a>. SURESHKUMAR.NET FORUMS. Archived from <a rel="nofollow" class="external text" href="http://forums.sureshkumar.net/vb-asp-net-interview-technical-questions/16320-c-what-indexer-c.html">the original</a> on September 22, 2009. Retrieved 2011-08-01.</cite><code class="cs1-code">{{cite web}}</code>: CS1 maint: numeric names: authors list (link)

</li>

<li id="cite_note-2"><a href="#cite_ref-2">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://en.cppreference.com/w/cpp/language/operators">"C++ operator overloading"</a>.</cite>

</li>

<li id="cite_note-3"><a href="#cite_ref-3">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.dotnetfunda.com/interview/exam4161-what-is-an-indexer-in-csharp-.aspx">"C# Interview Questions"</a>. .net Funda. Retrieved 2011-08-01.</cite>

</li>

<li id="cite_note-4"><a href="#cite_ref-4">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.php.net/manual/en/class.arrayaccess.php">"PHP ArrayAccess interface"</a>.</cite>

</li>

<li id="cite_note-5"><a href="#cite_ref-5">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://doc.rust-lang.org/std/ops/trait.Index.html">"Index in std::ops - Rust"</a>. doc.rust-lang.org. Retrieved 11 January 2025.</cite>

</li>

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