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{{ProgLangCompare}}
{{TOC limit|3}}
This '''comparison of programming languages (
==Language support==
{{Dynamic list}}
The following is a comparison of [[associative array]]s (also "mapping", "hash", and "dictionary") in various programming languages.
===
[[
For example:
<
phonebook["Sally Smart"] = "555-9999"
phonebook["John Doe"] = "555-1212"
phonebook["J. Random Hacker"] = "555-1337"
</syntaxhighlight>
<
for (name in phonebook) {
print name, " ", phonebook[name]
}
</syntaxhighlight>
<
{ # for every input line
multi[$1 SUBSEP $2]++;
Line 42:
}
}
</syntaxhighlight>
===
There is no standard implementation of
Another 3rd
Finally, the [[
Similar to [[
===C#===
<!-- Note: Be aware when editing that this section and its subsections are linked to by the F# and Visual Basic .NET sections -->
[[C Sharp (programming language)|C#]] uses the collection classes provided by the [[.NET Framework]]. The most commonly used associative array type is <code>System.Collections.Generic.Dictionary<TKey, TValue></code>, which is implemented as a mutable hash table. The relatively new <code>System.Collections.Immutable</code> package, available in .NET Framework versions 4.5 and above, and in all versions of [[.NET Core]], also includes the <code>System.Collections.Immutable.Dictionary<TKey, TValue></code> type, which is implemented using an [[AVL tree]]. The methods that would normally mutate the object in-place instead return a new object that represents the state of the original object after mutation.
====Creation====
The following demonstrates three means of populating a mutable dictionary:
* the <code>Add</code> method, which adds a key and value and throws an [[exception handling|exception]] if the key already exists in the dictionary;
* assigning to the indexer, which overwrites any existing value, if present; and
* assigning to the backing property of the indexer, for which the indexer is [[syntactic sugar]] (not applicable to C#, see [[#F#|F#]] or [[#Visual Basic .NET|VB.NET]] examples).
<syntaxhighlight lang="csharp">
var dictionary = new Dictionary<string, string>();
dictionary.Add("Sally Smart", "555-9999");
dictionary["John Doe"] = "555-1212";
// Not allowed in C#.
// dictionary.Item("J. Random Hacker") = "553-1337";
</syntaxhighlight>
The dictionary can also be initialized during construction using a "collection initializer", which compiles to repeated calls to <code>Add</code>.
<syntaxhighlight lang="csharp">
{ "Sally Smart", "555-9999" },
{ "John Doe", "555-1212" },
{ "J. Random Hacker", "553-1337" }
};
</syntaxhighlight>
===={{anchor|C# access}}Access by key====
Values are primarily retrieved using the indexer (which throws an exception if the key does not exist) and the <code>TryGetValue</code> method, which has an output parameter for the sought value and a Boolean return-value indicating whether the key was found.
<syntaxhighlight lang="csharp">
var sallyNumber = dictionary["Sally Smart"];
</syntaxhighlight>
<syntaxhighlight lang="csharp">
var sallyNumber = (dictionary.TryGetValue("Sally Smart", out var result) ? result : "n/a";
</syntaxhighlight>
In this example, the <code>sallyNumber</code> value will now contain the string <code>"555-9999"</code>.
===={{anchor|C# enumeration}}Enumeration====
A dictionary can be viewed as a sequence of keys, sequence of values, or sequence of pairs of keys and values represented by instances of the <code>KeyValuePair<TKey, TValue></code> type, although there is no guarantee of order. For a sorted dictionary, the programmer could choose to use a <code>SortedDictionary<TKey, TValue></code> or use the <code>.Sort</code> [[Language Integrated Query|LINQ]] extension method when enumerating.
The following demonstrates enumeration using a [[foreach loop]]:
<syntaxhighlight lang="csharp">
// loop through the collection and display each entry.
foreach (KeyValuePair<string,string> kvp in dictionary)
{
}
</syntaxhighlight>
===C++===
[[C++]]
<
#include <map>
#include <string>
#include <utility>
int main() {
std::map<std::string, std::string> phone_book;
phone_book.insert(std::make_pair("Sally Smart", "555-9999"));
phone_book.insert(std::make_pair("John Doe", "555-1212"));
phone_book.insert(std::make_pair("J. Random Hacker", "553-1337"));
}
</syntaxhighlight>
Or less efficiently, as this creates temporary <code>std::string</code> values:
<syntaxhighlight lang="cpp">
#include <map>
#include <string>
Line 95 ⟶ 131:
phone_book["John Doe"] = "555-1212";
phone_book["J. Random Hacker"] = "553-1337";
}
</syntaxhighlight>
With the extension of [[C++11#Initializer lists|initialization lists]] in C++11, entries can be added during a map's construction as shown below:
<syntaxhighlight lang="cpp">
#include <map>
#include <string>
int main() {
std::map<std::string, std::string> phone_book {
{"Sally Smart", "555-9999"},
{"John Doe", "555-1212"},
{"J. Random Hacker", "553-1337"}
};
}
</syntaxhighlight>
You can iterate through the list with the following code (C++03):
<
std::map<std::string, std::string>::iterator curr, end;
for(curr = phone_book.begin(), end = phone_book.end(); curr != end;
std::cout << curr->first << " = " << curr->second << std::endl;
</syntaxhighlight>
The same task in
<
for(const auto& curr : phone_book)
std::cout << curr.first << " = " << curr.second << std::endl;
</syntaxhighlight>
Using the structured binding available in [[C++17]]:
<syntaxhighlight lang="cpp">
for (const auto& [name, number] : phone_book) {
std::cout << name << " = " << number << std::endl;
}
</syntaxhighlight>
In C++, the <code>std::map</code> class is [[Generic programming#Templates in C.2B.2B|templated]] which allows the [[data type]]s of keys and values to be different for different <code>map</code> instances. For a given instance of the <code>map</code> class the keys must be of the same base type. The same must be true for all of the values. Although <code>std::map</code> is typically implemented using a [[self-balancing binary search tree]], C++11 defines a second map called <code>[[std::unordered_map]]</code>, which has the algorithmic characteristics of a hash table. This is a common vendor extension to the [[Standard Template Library]] (STL) as well, usually called <code>hash_map</code>, available from such implementations as SGI and STLPort.
===Cobra===
Initializing an empty dictionary and adding items in [[Cobra (programming language)|Cobra]]:
{{sxhl|2=python|1=<nowiki/>
dic as Dictionary<of String, String> = Dictionary<of String, String>()
dic.add('Sally Smart', '555-9999')
dic.add('John Doe', '555-1212')
dic.add('J. Random Hacker', '553-1337')
assert dic['Sally Smart'] == '555-9999'
}}
Alternatively, a dictionary can be initialized with all items during construction:
{{sxhl|2=python|1=<nowiki/>
dic = {
'Sally Smart':'555-9999',
'John Doe':'555-1212',
'J. Random Hacker':'553-1337'
}
}}
The dictionary can be enumerated by a for-loop, but there is no guaranteed order:
{{sxhl|2=python|1=<nowiki/>
for key, val in dic
print "[key]'s phone number is [val]"
}}
===ColdFusion Markup Language===
A structure in [[ColdFusion Markup Language]] (CFML) is equivalent to an associative array:
<syntaxhighlight lang=CFS>
dynamicKeyName = "John Doe";
phoneBook = {
"Sally Smart" = "555-9999",
"#dynamicKeyName#" = "555-4321",
"J. Random Hacker" = "555-1337",
UnknownComic = "???"
};
writeOutput(phoneBook.UnknownComic); // ???
writeDump(phoneBook); // entire struct
</syntaxhighlight>
===D===
[[D programming language|D]] offers direct support for associative arrays in the core language; such arrays are implemented as a chaining hash table with binary trees.<ref>{{Cite web|title=Associative Arrays - D Programming Language|url=https://dlang.org/spec/hash-map.html|access-date=2021-05-07|website=dlang.org}}</ref> The equivalent example would be:
<syntaxhighlight lang="d">
int main() {
string[ string ] phone_book;
Line 154 ⟶ 224:
return 0;
}
</syntaxhighlight>
Keys and values can be any types, but all the keys in an associative array must be of the same type, and the same goes for dependent values.
<
foreach (key, value; phone_book) {
writeln("Number for " ~ key ~ ": " ~ value );
}
</syntaxhighlight>
A property can be removed as follows:
<
phone_book.remove("Sally Smart");
</syntaxhighlight>
===Delphi===
[[Delphi (software)|Delphi]] supports several standard containers, including TDictionary<T>:
<syntaxhighlight lang="delphi">
uses
SysUtils,
Generics.Collections;
var
PhoneBook: TDictionary<string, string>;
Entry: TPair<string, string>;
begin
PhoneBook := TDictionary<string, string>.Create;
PhoneBook.Add('Sally Smart', '555-9999');
PhoneBook.Add('John Doe', '555-1212');
PhoneBook.Add('J. Random Hacker', '553-1337');
for Entry in PhoneBook do
Writeln(Format('Number for %s: %s',[Entry.Key, Entry.Value]));
end.
</syntaxhighlight>
Pre-2009 Delphi versions do not support associative arrays directly. Such arrays can be simulated using the TStrings class:
<
procedure TForm1.Button1Click(Sender: TObject);
var
Line 199 ⟶ 290:
DataField.Free;
end;
</syntaxhighlight>
===Erlang===
[[Erlang (programming language)|Erlang]] offers many
====Keylists====
Keylists are lists of [[tuple]]s, where the first element of each tuple is a key, and the second is a value. Functions for operating on keylists are provided in the <code>lists</code> module.
<
PhoneBook = [{"Sally
{"John Doe", "555-1212"},
{"J. Random Hacker", "553-1337"}].
</syntaxhighlight>
Accessing an element of the keylist can be done with the <code>lists:keyfind/3</code> function:
<
{_, Phone} = lists:keyfind("Sally
io:format("Phone number: ~s~n", [Phone]).
</syntaxhighlight>
====Dictionaries====
Dictionaries are implemented in the <code>dict</code> module of the standard library. A new dictionary is created using the <code>dict:new/0</code> function and new key/value pairs are stored using the <code>dict:store/3</code> function:
<
PhoneBook1 = dict:new(),
PhoneBook2 = dict:store("Sally Smith", "555-9999", Dict1),
PhoneBook3 = dict:store("John Doe", "555-1212", Dict2),
PhoneBook = dict:store("J. Random Hacker", "553-1337", Dict3).
</syntaxhighlight>
Such a serial initialization would be more idiomatically represented in Erlang with the appropriate function:
<
PhoneBook = dict:from_list([{"Sally Smith", "555-9999"},
{"John Doe", "555-1212"}, {"J. Random Hacker", "553-1337"}]).
</syntaxhighlight>
The dictionary can be accessed using the <code>dict:find/2</code> function:
<
{ok, Phone} = dict:find("Sally Smith", PhoneBook),
io:format("Phone: ~s~n", [Phone]).
</syntaxhighlight>
In both cases, any Erlang term can be used as the key. Variations include the <code>orddict</code> module, implementing ordered dictionaries, and <code>gb_trees</code>, implementing general balanced trees.
====Maps====
Maps were introduced in OTP 17.0,<ref>{{Cite web|title=Erlang -- maps|url=https://erlang.org/doc/man/maps.html|access-date=2021-03-07|website=erlang.org}}</ref> and combine the strengths of keylists and dictionaries. A map is defined using the syntax <code>#{ K1 => V1, ... Kn => Vn }</code>:
<syntaxhighlight lang="erlang">
PhoneBook = #{"Sally Smith" => "555-9999",
"John Doe" => "555-1212",
"J. Random Hacker" => "553-1337"}.
</syntaxhighlight>
Basic functions to interact with maps are available from the <code>maps</code> module. For example, the <code>maps:find/2</code> function returns the value associated with a key:
<syntaxhighlight lang="erlang">
{ok, Phone} = maps:find("Sally Smith", PhoneBook),
io:format("Phone: ~s~n", [Phone]).
</syntaxhighlight>
Unlike dictionaries, maps can be pattern matched upon:
<syntaxhighlight lang="erlang">
#{"Sally Smith", Phone} = PhoneBook,
io:format("Phone: ~s~n", [Phone]).
</syntaxhighlight>
Erlang also provides syntax sugar for functional updates—creating a new map based on an existing one, but with modified values or additional keys:
<syntaxhighlight lang="erlang">
PhoneBook2 = PhoneBook#{
% the `:=` operator updates the value associated with an existing key
"J. Random Hacker" := "355-7331",
% the `=>` operator adds a new key-value pair, potentially replacing an existing one
"Alice Wonderland" => "555-1865"
}
</syntaxhighlight>
===F#===
===={{code|Map<'Key,'Value>}}====
At runtime, [[F Sharp (programming language)|F#]] provides the <code>Collections.Map<'Key,'Value></code> type, which is an immutable [[AVL tree]].
=====Creation=====
The following example calls the <code>Map</code> constructor, which operates on a list (a semicolon delimited sequence of elements enclosed in square brackets) of tuples (which in F# are comma-delimited sequences of elements).
<
let numbers =
[
"John Doe", "555-1212";
"J. Random Hacker", "555-1337"
] |> Map
</syntaxhighlight>
=====Access by key=====
Values can be looked up via one of the <code>Map</code> members, such as its indexer or <code>Item</code> property (which throw an [[Exception handling|exception]] if the key does not exist) or the <code>TryFind</code> function, which returns an [[option type]] with a value of {{code|Some <result>|f#}}, for a successful lookup, or <code>None</code>, for an unsuccessful one. [[Pattern matching]] can then be used to extract the raw value from the result, or a default value can be set.
<syntaxhighlight lang="fsharp">
let sallyNumber = numbers.["Sally Smart"]
// or
let sallyNumber = numbers.Item("Sally Smart")
</syntaxhighlight>
<syntaxhighlight lang=fsharp>
let sallyNumber =
match numbers.TryFind("Sally Smart") with
| Some(number) -> number
| None -> "n/a"
</syntaxhighlight>
In both examples above, the <code>sallyNumber</code> value would contain the string <code>"555-9999"</code>.
===={{code|Dictionary<'TKey,'TValue>}}====
Because F# is a .NET language, it also has access to features of the [[.NET Framework]], including the {{code|System.Collections.Generic.Dictionary<'TKey,'TValue>|f#}} type (which is implemented as a [[hash table]]), which is the primary associative array type used in C# and Visual Basic. This type may be preferred when writing code that is intended to operate with other languages on the .NET Framework, or when the performance characteristics of a hash table are preferred over those of an AVL tree.
=====Creation=====
The <code>dict</code> function provides a means of conveniently creating a .NET dictionary that is not intended to be mutated; it accepts a sequence of tuples and returns an immutable object that implements {{code|IDictionary<'TKey,'TValue>|f#}}.
<syntaxhighlight lang="fsharp">
let numbers =
[
"Sally Smart", "555-9999";
"John Doe", "555-1212";
"J. Random Hacker", "555-1337"
] |> dict
</syntaxhighlight>
When a mutable dictionary is needed, the constructor of {{code|System.Collections.Generic.Dictionary<'TKey,'TValue>|f#}} can be called directly. See [[#C#|the C# example on this page]] for additional information.
<syntaxhighlight lang="fsharp">
let numbers = System.Collections.Generic.Dictionary<string, string>()
numbers.Add("Sally Smart", "555-9999")
numbers.["John Doe"] <- "555-1212"
numbers.Item("J. Random Hacker") <- "555-1337"
</syntaxhighlight>
=====Access by key=====
<code>IDictionary</code> instances have an indexer that is used in the same way as <code>Map</code>, although the equivalent to <code>TryFind</code> is <code>TryGetValue</code>, which has an output parameter for the sought value and a Boolean return value indicating whether the key was found.
<syntaxhighlight lang="fsharp">
let sallyNumber =
let mutable result = ""
if numbers.TryGetValue("Sally Smart", &result) then result else "n/a"
</syntaxhighlight>
F# also allows the function to be called as if it had no output parameter and instead returned a tuple containing its regular return value and the value assigned to the output parameter:
<syntaxhighlight lang="fsharp">
let sallyNumber =
match numbers.TryGetValue("Sally Smart") with
| true, number -> number
| _ -> "n/a"
</syntaxhighlight>
====Enumeration====
A dictionary or map can be enumerated using <code>Seq.map</code>.
<syntaxhighlight lang=fsharp>
// loop through the collection and display each entry.
numbers |> Seq.map (fun kvp -> printfn "Phone number for %O is %O" kvp.Key kvp.Value)
</syntaxhighlight>
===FoxPro===
[[Visual FoxPro]] implements mapping with the Collection Class.
<syntaxhighlight lang="foxpro">
mapping = NEWOBJECT("Collection")
mapping.Add("Daffodils", "flower2") && Add(object, key) – key must be character
Line 294 ⟶ 465:
object = mapping("flower2") && returns "Daffodils" (retrieve by key)
object = mapping(1) && returns "Daffodils" (retrieve by index)
</syntaxhighlight>
GetKey returns 0 if the key is not found.
===Go===
[[Go (programming language)|Go]] has built-in, language-level support for associative arrays, called
A map type is written Adding elements one at a time:
<syntaxhighlight lang="go">
phone_book := make(map[string] string) // make an empty map
phone_book["Sally Smart"] = "555-9999"
phone_book["John Doe"] = "555-1212"
phone_book["J. Random Hacker"] = "553-1337"
</syntaxhighlight>
A map literal:
<syntaxhighlight lang="go">
phone_book := map[string] string {
"Sally Smart": "555-9999",
Line 318 ⟶ 491:
"J. Random Hacker": "553-1337",
}
</syntaxhighlight>
Iterating through a map:
<syntaxhighlight lang="go">
// over both keys and values
for key, value := range phone_book {
Line 331 ⟶ 505:
fmt.Printf("Name: %s\n", key)
}
</syntaxhighlight>
===Haskell===
The [[
<
m = [("Sally Smart", "555-9999"), ("John Doe", "555-1212"), ("J. Random Hacker", "553-1337")]
main = print (lookup "John Doe" m)
</syntaxhighlight>
output:
Just "555-1212"
Note that the lookup function returns a "Maybe" value, which is "Nothing" if not found, or "Just
The [[Glasgow Haskell Compiler
One is polymorphic functional maps (represented as immutable balanced binary trees):
<
import qualified Data.Map as M
Line 358 ⟶ 532:
main = print (M.lookup "John Doe" m'' :: Maybe String)
</syntaxhighlight>
output:
Just "555-1212"
Line 366 ⟶ 540:
Finally, a polymorphic hash table:
<
import qualified Data.HashTable as H
Line 375 ⟶ 549:
foo <- H.lookup m "John Doe"
print foo
</syntaxhighlight>
output:
Just "555-1212"
Lists of pairs and functional maps both provide a purely functional interface, which is more idiomatic in Haskell. In contrast, hash tables provide an imperative interface in the [[Monad (functional programming)#IO monad|IO monad]].
===Java===
In [[Java (programming language)|Java]] associative arrays are implemented as "maps"
<
Map<String, String> phoneBook = new HashMap<String, String>();
phoneBook.put("Sally Smart", "555-9999");
phoneBook.put("John Doe", "555-1212");
phoneBook.put("J. Random Hacker", "555-1337");
</syntaxhighlight>
The {{Javadoc:SE|java/util|Map|get(java.lang.Object)|name=get}} method is used to access a key; for example, the value of the expression <code>phoneBook.get("Sally Smart")</code> is <code>"555-9999"</code>. This code uses a hash map to store the associative array, by calling the constructor of the {{Javadoc:SE|java/util|HashMap}} class. However, since the code only uses methods common to the interface {{Javadoc:SE|java/util|Map}}, a self-balancing binary tree could be used by calling the constructor of the {{Javadoc:SE|java/util|TreeMap}} class (which implements the subinterface {{Javadoc:SE|java/util|SortedMap}}), without changing the definition of the <code>phoneBook</code> variable, or the rest of the code, or using other underlying data structures that implement the <code>Map</code> interface.
The hash function in Java, used by HashMap and HashSet, is provided by the {{Javadoc:SE|java/lang|Object|hashCode()}} method. Since every class in Java [[Inheritance (object-oriented programming)|inherits]] from {{Javadoc:SE|java/lang|Object}}, every object has a hash function. A class can [[Method overriding (programming)|override]] the default implementation of <code>hashCode()</code> to provide a custom hash function more in accordance with the properties of the object.
The <code>Object</code> class also contains the {{Javadoc:SE|name=equals(Object)|java/lang|Object|equals(java.lang.Object)}} method, which tests an object for equality with another object. Hashed data structures in Java rely on objects maintaining the following contract between their <code>hashCode()</code> and <code>equals()</code> methods:
For two objects ''a'' and ''b'',
<syntaxhighlight lang="java">
a.equals(b) == b.equals(a)
if a.equals(b), then a.hashCode() == b.hashCode()
</syntaxhighlight>
In order to maintain this contract, a class that overrides <code>equals()</code> must also override <code>hashCode()</code>, and
A further contract that a hashed data
Analogously, TreeMap, and other sorted data structures,
===JavaScript===
[[JavaScript]] (and its standardized version, [[ECMAScript]]) is a [[Prototype-based programming|prototype-based]] [[Object-oriented programming|object-oriented]] language.
====Map and WeakMap====
Modern JavaScript handles associative arrays, using the <code>Map</code> and <code>WeakMap</code> classes. A map does not contain any keys by default; it only contains what is explicitly put into it. The keys and values can be any type (including functions, objects, or any primitive).
=====Creation=====
A map can be initialized with all items during construction:
<syntaxhighlight lang="javascript">
const phoneBook = new Map([
["Sally Smart", "555-9999"],
["John Doe", "555-1212"],
["J. Random Hacker", "553-1337"],
]);
</syntaxhighlight>
Alternatively, you can initialize an empty map and then add items:
<syntaxhighlight lang="javascript">
const phoneBook = new Map();
phoneBook.set("Sally Smart", "555-9999");
phoneBook.set("John Doe", "555-1212");
phoneBook.set("J. Random Hacker", "553-1337");
</syntaxhighlight>
=====Access by key=====
Accessing an element of the map can be done with the <code>get</code> method:
<syntaxhighlight lang="javascript">
const sallyNumber = phoneBook.get("Sally Smart");
</syntaxhighlight>
In this example, the value <code>sallyNumber</code> will now contain the string "555-9999".
=====Enumeration=====
The keys in a map are ordered. Thus, when iterating through it, a map object returns keys in order of insertion. The following demonstrates enumeration using a for-loop:
<syntaxhighlight lang="javascript">
// loop through the collection and display each entry.
for (const [name, number] of phoneBook) {
console.log(`Phone number for ${name} is ${number}`);
}
</syntaxhighlight>
A key can be removed as follows:
<syntaxhighlight lang="javascript">
phoneBook.delete("Sally Smart");
</syntaxhighlight>
====Object====
An object is similar to a map—both let you set keys to values, retrieve those values, delete keys, and detect whether a value is stored at a key. For this reason (and because there were no built-in alternatives), objects historically have been used as maps.
However, there are important differences that make a map preferable in certain cases. In JavaScript an object is a mapping from property names to values—that is, an associative array with one caveat: the keys of an object must be either a string or a symbol (native objects and primitives implicitly converted to a string keys are allowed). Objects also include one feature unrelated to associative arrays: an object has a prototype, so it contains default keys that could conflict with user-defined keys. So, doing a lookup for a property will point the lookup to the prototype's definition if the object does not define the property.
An object literal is written as <code>{ property1: value1, property2: value2, ... }</code>. For example:
<syntaxhighlight lang="javascript">
const myObject = {
"Sally Smart": "555-9999",
"John Doe": "555-1212",
"J. Random Hacker": "553-1337",
};
</syntaxhighlight>
To prevent the lookup from using the prototype's properties, you can use the <code>Object.setPrototypeOf</code> function:
<syntaxhighlight lang=JavaScript>
Object.setPrototypeOf(myObject, null);
</syntaxhighlight>
As of ECMAScript 5 (ES5), the prototype can also be bypassed by using <code>Object.create(null)</code>:
<syntaxhighlight lang=JavaScript>
const myObject = Object.create(null);
Object.assign(myObject, {
"Sally Smart": "555-9999",
"John Doe": "555-1212",
"J. Random Hacker": "553-1337",
});
</syntaxhighlight>
If the property name is a valid identifier, the quotes can be omitted, e.g.:
<
</syntaxhighlight>
Lookup is written using property
<
myObject["John Doe"]
myOtherObject.foo
</syntaxhighlight>
You can also loop through all enumerable properties and associated values as follows (a for-in loop):
<
for (
}
</syntaxhighlight>
Or (a for-of loop):
<syntaxhighlight lang=JavaScript>
for (const [property, value] of Object.entries(myObject)) {
console.log(`${property} = ${value}`);
}
</syntaxhighlight>
A property can be removed as follows:
<
delete myObject["Sally Smart"];
</syntaxhighlight>
As mentioned before, properties are strings
<
myObject[1]
myObject[[
myObject[{ toString
</syntaxhighlight>
In modern JavaScript it's considered bad form to use the Array type as an associative array. Consensus is that the Object type and <code>Map</code>/<code>WeakMap</code> classes are best for this purpose. The reasoning behind this is that if Array is extended via prototype and Object is kept pristine, for and for-in loops will work as expected on associative 'arrays'. This issue has been brought to the fore by the popularity of JavaScript frameworks that make heavy and sometimes indiscriminate use of prototypes to extend JavaScript's inbuilt types.
See [http://blog.metawrap.com/2006/05/30/june-6th-is-javascript-array-and-object-prototype-awareness-day/ JavaScript Array And Object Prototype Awareness Day] for more information on the issue.
===Julia===
In [[Julia (programming language)|Julia]], the following operations manage associative arrays.
Declare dictionary:
<syntaxhighlight lang="julia">
phonebook = Dict( "Sally Smart" => "555-9999", "John Doe" => "555-1212", "J. Random Hacker" => "555-1337" )
</syntaxhighlight>
Access element:
<syntaxhighlight lang="julia">
</syntaxhighlight>
Add element:
<syntaxhighlight lang="julia">
phonebook["New Contact"] = "555-2222"
</syntaxhighlight>
Delete element:
<syntaxhighlight lang="julia">
delete!(phonebook, "Sally Smart")
</syntaxhighlight>
Get keys and values as [[Iterator#Implicit iterators|iterables]]:
<syntaxhighlight lang="julia">
keys(phonebook)
values(phonebook)
</syntaxhighlight>
===KornShell 93, and compliant shells===
In [[Korn Shell|KornShell]] 93, and compliant shells (ksh93, bash4...), the following operations can be used with associative arrays.
<!-- Tested in ksh93 r -->
Definition:
<syntaxhighlight lang="ksh">
typeset -A phonebook; # ksh93; in bash4+, "typeset" is a synonym of the more preferred "declare", which works identically in this case
phonebook=(["Sally Smart"]="555-9999" ["John Doe"]="555-1212" ["[[J. Random Hacker]]"]="555-1337");
</syntaxhighlight>
Dereference:
<syntaxhighlight lang="ksh">
${phonebook["John Doe"]};
</syntaxhighlight>
===Lisp===
[[Lisp programming language|Lisp]] was originally conceived as a "LISt Processing" language, and one of its most important data types is the linked list, which can be treated as an [[association list]] ("alist").
<
'(("Sally Smart" . "555-9999")
("John Doe" . "555-1212")
("J. Random Hacker" . "553-1337"))
</syntaxhighlight>
The syntax <code>(x . y)</code> is used to indicate a [[cons|<code>cons</code>ed]] pair. Keys and values need not be the same type within an alist. Lisp and [[Scheme (programming language)|Scheme]] provide operators such as <code>assoc</code> to manipulate alists in ways similar to associative arrays.
A set of operations specific to the handling of association lists exists for [[Common Lisp]], each of these working non-destructively.
To add an entry the <code>acons</code> function is employed, creating and returning a new association list. An association list in Common Lisp mimicks a stack, that is, adheres to the last-in-first-out (LIFO) principle, and hence prepends to the list head.
<syntaxhighlight lang=Lisp>
(let ((phone-book NIL))
(setf phone-book (acons "Sally Smart" "555-9999" phone-book))
(setf phone-book (acons "John Doe" "555-1212" phone-book))
(setf phone-book (acons "J. Random Hacker" "555-1337" phone-book)))
</syntaxhighlight>
This function can be construed as an accommodation for <code>cons</code> operations.<ref>{{cite web |title=Common Lisp the Language, 2nd Edition: 15.6. Association Lists |url=https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node153.html |website=Carnegie Mellon University |access-date=3 August 2020}}</ref>
<syntaxhighlight lang=Lisp>
;; The effect of
;; (cons (cons KEY VALUE) ALIST)
;; is equivalent to
;; (acons KEY VALUE ALIST)
(let ((phone-book '(("Sally Smart" . "555-9999") ("John Doe" . "555-1212"))))
(cons (cons "J. Random Hacker" "555-1337") phone-book))
</syntaxhighlight>
Of course, the destructive <code>push</code> operation also allows inserting entries into an association list, an entry having to constitute a key-value cons in order to retain the mapping's validity.
<syntaxhighlight lang=Lisp>
(push (cons "Dummy" "123-4567") phone-book)
</syntaxhighlight>
Searching for an entry by its key is performed via <code>assoc</code>, which might be configured for the test predicate and direction, especially searching the association list from its end to its front. The result, if positive, returns the entire entry cons, not only its value. Failure to obtain a matching key leads to a return of the <code>NIL</code> value.
<syntaxhighlight lang=Lisp>
(assoc "John Doe" phone-book :test #'string=)
</syntaxhighlight>
Two generalizations of <code>assoc</code> exist: <code>assoc-if</code> expects a predicate function that tests each entry's key, returning the first entry for which the predicate produces a non-<code>NIL</code> value upon invocation. <code>assoc-if-not</code> inverts the logic, accepting the same arguments, but returning the first entry generating <code>NIL</code>.
<syntaxhighlight lang=Lisp>
;; Find the first entry whose key equals "John Doe".
(assoc-if
#'(lambda (key)
(string= key "John Doe"))
phone-book)
;; Finds the first entry whose key is neither "Sally Smart" nor "John Doe"
(assoc-if-not
#'(lambda (key)
(member key '("Sally Smart" "John Doe") :test #'string=))
phone-book)
</syntaxhighlight>
The inverse process, the detection of an entry by its value, utilizes <code>rassoc</code>.
<syntaxhighlight lang=Lisp>
;; Find the first entry with a value of "555-9999".
;; We test the entry string values with the "string=" predicate.
(rassoc "555-9999" phone-book :test #'string=)
</syntaxhighlight>
The corresponding generalizations <code>rassoc-if</code> and <code>rassoc-if-not</code> exist.
<syntaxhighlight lang=Lisp>
;; Finds the first entry whose value is "555-9999".
(rassoc-if
#'(lambda (value)
(string= value "555-9999"))
phone-book)
;; Finds the first entry whose value is not "555-9999".
(rassoc-if-not
#'(lambda (value)
(string= value "555-9999"))
phone-book)
</syntaxhighlight>
All of the previous entry search functions can be replaced by general list-centric variants, such as <code>find</code>, <code>find-if</code>, <code>find-if-not</code>, as well as pertinent functions like <code>position</code> and its derivates.
<syntaxhighlight lang=Lisp>
;; Find an entry with the key "John Doe" and the value "555-1212".
(find (cons "John Doe" "555-1212") phone-book :test #'equal)
</syntaxhighlight>
Deletion, lacking a specific counterpart, is based upon the list facilities, including destructive ones.
<syntaxhighlight lang=Lisp>
;; Create and return an alist without any entry whose key equals "John Doe".
(remove-if
#'(lambda (entry)
(string= (car entry) "John Doe"))
phone-book)
</syntaxhighlight>
Iteration is accomplished with the aid of any function that expects a list.
<syntaxhighlight lang=Lisp>
;; Iterate via "map".
(map NIL
#'(lambda (entry)
(destructuring-bind (key . value) entry
(format T "~&~s => ~s" key value)))
phone-book)
;; Iterate via "dolist".
(dolist (entry phone-book)
(destructuring-bind (key . value) entry
(format T "~&~s => ~s" key value)))
</syntaxhighlight>
These being structured lists, processing and transformation operations can be applied without constraints.
<syntaxhighlight lang=Lisp>
;; Return a vector of the "phone-book" values.
(map 'vector #'cdr phone-book)
;; Destructively modify the "phone-book" via "map-into".
(map-into phone-book
#'(lambda (entry)
(destructuring-bind (key . value) entry
(cons (reverse key) (reverse value))))
phone-book)
</syntaxhighlight>
Because of their linear nature, alists are used for relatively small sets of data. [[Common Lisp]] also supports a [[hash table]] data type, and for [[Scheme (programming language)|Scheme]] they are implemented in [[Scheme Requests for Implementation|SRFI]] 69. Hash tables have greater overhead than alists, but provide much faster access when there are many elements. A further characteristic is the fact that Common Lisp hash tables do not, as opposed to association lists, maintain the order of entry insertion.
Common Lisp hash tables are constructed via the <code>make-hash-table</code> function, whose arguments encompass, among other configurations, a predicate to test the entry key. While tolerating arbitrary objects, even heterogeneity within a single hash table instance, the specification of this key <code>:test</code> function is confined to distinguishable entities: the Common Lisp standard only mandates the support of <code>eq</code>, <code>eql</code>, <code>equal</code>, and <code>equalp</code>, yet designating additional or custom operations as permissive for concrete implementations.
<syntaxhighlight lang=Lisp>
(let ((phone-book (make-hash-table :test #'equal)))
(setf (gethash "Sally Smart" phone-book) "555-9999")
(setf (gethash "John Doe" phone-book) "555-1212")
(setf (gethash "J. Random Hacker" phone-book) "553-1337"))
</syntaxhighlight>
The <code>gethash</code> function permits obtaining the value associated with a key.
<syntaxhighlight lang=Lisp>
(gethash "John Doe" phone-book)
</syntaxhighlight>
Additionally, a default value for the case of an absent key may be specified.
<syntaxhighlight lang=Lisp>
(gethash "Incognito" phone-book 'no-such-key)
</syntaxhighlight>
An invocation of <code>gethash</code> actually returns two values: the value or substitute value for the key and a boolean indicator, returning <code>T</code> if the hash table contains the key and <code>NIL</code> to signal its absence.
<syntaxhighlight lang=Lisp>
(multiple-value-bind (value contains-key) (gethash "Sally Smart" phone-book)
(if contains-key
(format T "~&The associated value is: ~s" value)
(format T "~&The key could not be found.")))
</syntaxhighlight>
Use <code>remhash</code> for deleting the entry associated with a key.
<syntaxhighlight lang=Lisp>
(remhash "J. Random Hacker" phone-book)
</syntaxhighlight>
<code>clrhash</code> completely empties the hash table.
<syntaxhighlight lang=Lisp>
(clrhash phone-book)
</syntaxhighlight>
The dedicated <code>maphash</code> function specializes in iterating hash tables.
<syntaxhighlight lang=Lisp>
(maphash
#'(lambda (key value)
(format T "~&~s => ~s" key value))
phone-book)
</syntaxhighlight>
Alternatively, the <code>loop</code> construct makes provisions for iterations, through keys, values, or conjunctions of both.
<syntaxhighlight lang=Lisp>
;; Iterate the keys and values of the hash table.
(loop
for key being the hash-keys of phone-book
using (hash-value value)
do (format T "~&~s => ~s" key value))
;; Iterate the values of the hash table.
(loop
for value being the hash-values of phone-book
do (print value))
</syntaxhighlight>
A further option invokes <code>with-hash-table-iterator</code>, an iterator-creating macro, the processing of which is intended to be driven by the caller.
<syntaxhighlight lang=Lisp>
(with-hash-table-iterator (entry-generator phone-book)
(loop do
(multiple-value-bind (has-entry key value) (entry-generator)
(if has-entry
(format T "~&~s => ~s" key value)
(loop-finish)))))
</syntaxhighlight>
It is easy to construct composite abstract data types in Lisp, using structures or object-oriented programming features, in conjunction with lists, arrays, and hash tables.
===LPC===
[[LPC (programming language)|LPC]] implements associative arrays as a fundamental type known as either "map" or "mapping", depending on the driver. The keys and values can be of any type. A mapping literal is written as <code>([ key_1 : value_1, key_2 : value_2 ])</code>. Procedural
<
mapping phone_book = ([]);
phone_book["Sally Smart"] = "555-9999";
phone_book["John Doe"] = "555-1212";
phone_book["J. Random Hacker"] = "555-1337";
</syntaxhighlight>
Mappings are accessed for reading using the indexing operator in the same way as they are for writing, as shown above. So phone_book["Sally Smart"] would return the string "555-9999", and phone_book["John Smith"] would return 0. Testing for presence is done using the function member(), e.g. <code>if(member(phone_book, "John Smith")) write("John Smith is listed.\n");</code>
Deletion is accomplished using a function called either m_delete() or map_delete(), depending on the driver
LPC drivers of the
<
mapping phone_book = ([:2]);
phone_book["Sally Smart", 0] = "555-9999";
Line 524 ⟶ 1,000:
phone_book["J. Random Hacker", 0] = "555-1337";
phone_book["J. Random Hacker", 1] = "77 Massachusetts Avenue";
</syntaxhighlight>
LPC drivers modern enough to support a foreach() construct
===Lua===
In [[Lua programming language|Lua]], "table" is a fundamental type that can be used either as an array (numerical index, fast) or as an associative array.
The keys and values can be of any type, except nil. The following focuses on non-numerical indexes.
A table literal is written as <code>{ value, key = value, [index] = value, ["non id string"] = value }</code>. For example:
<
phone_book = {
["Sally Smart"] = "555-9999",
Line 548 ⟶ 1,025:
-- Table and function (and other types) can also be used as keys
}
</syntaxhighlight>
If the key is a valid identifier (not a
Lookup is written using either square brackets, which always works, or dot notation, which only works for identifier keys:
<
print(aTable["John Doe"](45))
x = aTable.subTable.k
</syntaxhighlight>
You can also loop through all keys and associated values with iterators or for
<
simple = { [true] = 1, [false] = 0, [3.14] = math.pi, x = 'x', ["!"] = 42 }
function FormatElement(key, value)
Line 577 ⟶ 1,054:
until k == nil
print""
</syntaxhighlight>
An entry can be removed by setting it to nil:
<
simple.x = nil
</syntaxhighlight>
Likewise, you can overwrite values or add them:
<
simple['%'] = "percent"
simple['!'] = 111
</syntaxhighlight>
===Mathematica and Wolfram Language===
[[Mathematica]] and [[Wolfram Language]] use the Association expression to represent associative arrays.<ref>{{cite web|url=https://reference.wolfram.com/language/ref/Association.html|title=Association (<-...->)—Wolfram Language Documentation|website=reference.wolfram.com}}</ref>
<syntaxhighlight lang="mathematica">
phonebook = <| "Sally Smart" -> "555-9999",
"John Doe" -> "555-1212",
"J. Random Hacker" -> "553-1337" |>;
</syntaxhighlight>
To access:<ref>{{cite web|url=https://reference.wolfram.com/language/ref/Key.html|title=Key—Wolfram Language Documentation|website=reference.wolfram.com}}</ref>
<syntaxhighlight lang="mathematica">
phonebook[[Key["Sally Smart"]]]
</syntaxhighlight>
If the keys are strings, the Key keyword is not necessary, so:
<syntaxhighlight lang="mathematica">
phonebook[["Sally Smart"]]
</syntaxhighlight>
To list keys:<ref>{{cite web|url=https://reference.wolfram.com/language/ref/Keys.html|title=Keys—Wolfram Language Documentation|website=reference.wolfram.com}}</ref> and values<ref>{{cite web|url=https://reference.wolfram.com/language/ref/Values.html|title=Values—Wolfram Language Documentation|website=reference.wolfram.com}}</ref>
Keys[phonebook]
Values[phonebook]
===MUMPS===
In [[MUMPS]] every array is an associative array. The built-in, language-level, direct support for associative arrays
applies to private, process-specific arrays stored in memory called "locals" as well as to the permanent, shared, global arrays stored on disk which are available concurrently
SET ^phonebook("Sally Smart")="555-9999" ;; storing permanent data
SET phonebook("John Doe")="555-1212" ;; storing temporary data
SET phonebook("J. Random Hacker")="553-1337" ;; storing temporary data
MERGE ^phonebook=phonebook ;; copying temporary data into permanent data
WRITE "Phone Number :",^phonebook("Sally Smart"),!
Line 610 ⟶ 1,114:
FOR S NAME=$ORDER(^phonebook(NAME)) QUIT:NAME="" WRITE NAME," Phone Number :",^phonebook(NAME),!
===Objective-C (Cocoa/GNUstep) ===
[[Cocoa (API)|Cocoa]] and [[GNUstep]], written in [[Objective-C]], handle associative arrays using <code>NSMutableDictionary</code> (a mutable version of <code>NSDictionary</code>) class cluster. This class allows assignments between any two objects
<
NSMutableDictionary *aDictionary = [[NSMutableDictionary alloc] init];
[aDictionary setObject:@"555-9999" forKey:@"Sally Smart"];
[aDictionary setObject:@"555-1212" forKey:@"John Doe"];
[aDictionary setObject:@"553-1337" forKey:@"Random Hacker"];
</syntaxhighlight>
To access assigned objects, this command may be used:
<
id anObject = [aDictionary objectForKey:@"Sally Smart"];
</syntaxhighlight>
All keys or values can be
<
NSEnumerator *keyEnumerator = [aDictionary keyEnumerator];
id key;
Line 635 ⟶ 1,139:
// ... process it here ...
}
</syntaxhighlight>
<
for (id key in aDictionary) {
// ... process it here ...
}
</syntaxhighlight>
What is even more practical, structured data graphs may be easily created using [[Cocoa (API)|Cocoa]], especially <code>NSDictionary</code> (<code>NSMutableDictionary</code>). This can be illustrated with this compact example:
<
NSDictionary *aDictionary =
[NSDictionary dictionaryWithObjectsAndKeys:
Line 658 ⟶ 1,162:
nil], @"hackers",
nil];
</syntaxhighlight>
<
id anObject = [aDictionary valueForKeyPath:@"students.Sally Smart"];
</syntaxhighlight>
===OCaml===
The [[OCaml]] programming language provides three different associative containers. The simplest is a list of pairs:
<
# let m = [
"Sally Smart", "555-9999";
Line 681 ⟶ 1,185:
# List.assoc "John Doe" m;;
- : string = "555-1212"
</syntaxhighlight>
The second is a polymorphic hash table:
<
# let m = Hashtbl.create 3;;
val m : ('_a, '_b) Hashtbl.t = <abstr>
Line 694 ⟶ 1,198:
# Hashtbl.find m "John Doe";;
- : string = "555-1212"
</syntaxhighlight>
The code above uses OCaml's default hash function <code>Hashtbl.hash</code>, which is defined automatically for all types.
Finally, functional maps (represented as immutable balanced binary trees):
<
# module StringMap = Map.Make(String);;
...
Line 709 ⟶ 1,213:
# StringMap.find "John Doe" m;;
- : string = "555-1212"
</syntaxhighlight>
Note that in order to use <code>Map</code>, you have to provide the functor <code>Map.Make</code> with a module which defines the key type and the comparison function. The third-party library ExtLib provides a polymorphic version of functional maps, called
Lists of pairs and functional maps both provide a purely functional interface.
===
{{
The [[OptimJ]] programming language is an extension of Java 5. As
<
String[String] phoneBook = {
"Sally Smart" -> "555-9999",
"John Doe" -> "555-1212",
"J. Random Hacker" -> "553-1337"
};
Line 730 ⟶ 1,234:
// iterate over the values
for (String number : phoneBook) {
System.out.println(number);
}
Line 737 ⟶ 1,241:
// iterate over the keys
for (String name : phoneBook.keys) {
System.out.println(name + " -> " + phoneBook[name]);
}
// phoneBook[name] access a value by a key (it looks like java array access)
// i.e. phoneBook["John Doe"] returns "555-1212"
</syntaxhighlight>
Of course, it is possible to define multi-dimensional arrays, to mix
<
</syntaxhighlight>
===Perl 5===
[[Perl 5]] has built-in, language-level support for associative arrays. Modern Perl
A hash variable is marked by a <code>%</code> [[sigil (computer programming)|sigil]], to distinguish it from scalar, array, and other data types. A hash literal is a key-value list, with the preferred form using Perl's <code>=></code> token, which is
<
my %phone_book = (
'Sally Smart' => '555-9999',
'John Doe' => '555-1212',
'J. Random Hacker' => '553-1337',
);
</syntaxhighlight>
Accessing a hash element uses the syntax <code>$hash_name{$key}</code> – the key is surrounded by
The list of keys and values can be extracted using the built-in functions <code>keys</code> and <code>values</code>, respectively. So, for example, to print all the keys of a hash:
<
foreach $name (keys %phone_book) {
print $name, "\n";
}
</syntaxhighlight>
One can iterate through (key, value) pairs using the <code>each</code> function:
<
while (($name, $number) = each %phone_book) {
print 'Number for ', $name, ': ', $number, "\n";
}
</syntaxhighlight>
A hash
<
my $phone_book = {
'Sally Smart' => '555-9999',
'John Doe' => '555-1212',
'J. Random Hacker' => '553-1337',
};
</syntaxhighlight>
Values in a hash reference are accessed using the dereferencing operator:
<
print $phone_book->{'Sally Smart'};
</syntaxhighlight>
When the hash contained in the hash reference needs to be referred to as a whole, as with the <code>keys</code> function, the syntax is as follows:
<
foreach $name (keys %{$phone_book}) {
print 'Number for ', $name, ': ', $phone_book->{$name}, "\n";
}
</syntaxhighlight>
===Perl
[[Raku (programming language)|Perl 6]], renamed as "Raku", also has built-in, language-level support for associative arrays, which are referred to as ''hashes'' or as objects performing the "associative" role. As in Perl 5, Perl 6 default hashes are flat: keys are strings and values are scalars. One can define a hash to not coerce all keys to strings automatically: these are referred to as "object hashes", because the keys of such hashes remain the original object rather than a stringification thereof.
A hash variable is typically marked by a <code>%</code> [[sigil (computer programming)|sigil]], to visually distinguish it from scalar, array, and other data types, and to define its behaviour towards iteration. A hash literal is a key-value list, with the preferred form using Perl's <code>=></code> token, which makes the key-value association clearer:
<syntaxhighlight lang=Perl6>
my %phone-book =
'Sally Smart' => '555-9999',
'John Doe' => '555-1212',
'J. Random Hacker' => '553-1337',
;
</syntaxhighlight>
Accessing a hash element uses the syntax <code>%hash_name{$key}</code> – the key is surrounded by curly braces and the hash name (note that the sigil does '''not''' change, contrary to Perl 5). The value of <code>%phone-book{'John Doe'}</code> is <code>'555-1212'</code>.
The list of keys and values can be extracted using the built-in functions <code>keys</code> and <code>values</code>, respectively. So, for example, to print all the keys of a hash:
<syntaxhighlight lang=Perl6>
for %phone-book.keys -> $name {
say $name;
}
</syntaxhighlight>
By default, when iterating through a hash, one gets key–value pairs.
<syntaxhighlight lang=Perl6>
for %phone-book -> $entry {
say "Number for $entry.key(): $entry.value()"; # using extended interpolation features
}
</syntaxhighlight>
It is also possible to get alternating key values and value values by using the <code>kv</code> method:
<syntaxhighlight lang=Perl6>
for %phone-book.kv -> $name, $number {
say "Number for $name: $number";
}
</syntaxhighlight>
Raku doesn't have any references. Hashes can be passed as single parameters that are not flattened. If you want to make sure that a subroutine only accepts hashes, use the ''%'' sigil in the Signature.
<syntaxhighlight lang=Perl6>
sub list-phone-book(%pb) {
for %pb.kv -> $name, $number {
say "Number for $name: $number";
}
}
list-phone-book(%phone-book);
</syntaxhighlight>
In compliance with [[gradual typing]], hashes may be subjected to type constraints, confining a set of valid keys to a certain type.
<syntaxhighlight lang=Perl6>
# Define a hash whose keys may only be integer numbers ("Int" type).
my %numbersWithNames{Int};
# Keys must be integer numbers, as in this case.
%numbersWithNames.push(1 => "one");
# This will cause an error, as strings as keys are invalid.
%numbersWithNames.push("key" => "two");
</syntaxhighlight>
===PHP===
[[PHP]]'s built-in array type is, in reality, an associative array. Even when using numerical indexes, PHP internally stores arrays as associative arrays.<ref>About the implementation of [http://se.php.net/manual/en/language.types.array.php Arrays] in PHP</ref> So, PHP can have non-consecutively numerically indexed arrays. The keys have to be of integer (floating point numbers are truncated to integer) or string type, while values can be of arbitrary types, including other arrays and objects. The arrays are heterogeneous: a single array can have keys of different types. PHP's associative arrays can be used to represent trees, lists, stacks, queues, and other common data structures not built into PHP.
An associative array can be declared using the following syntax:
<
$phonebook = array();
$phonebook['Sally Smart'] = '555-9999';
Line 825 ⟶ 1,392:
'J. Random Hacker' => '555-1337',
);
// or, as of PHP 5.4
$phonebook = [
'Sally Smart' => '555-9999',
'John Doe' => '555-1212',
'J. Random Hacker' => '555-1337',
];
// or
Line 831 ⟶ 1,406:
$phonebook['contacts']['John Doe']['number'] = '555-1212';
$phonebook['contacts']['J. Random Hacker']['number'] = '555-1337';
</syntaxhighlight>
PHP can loop through an associative array as follows:
<
foreach ($phonebook as $name => $number) {
echo 'Number for ', $name, ': ', $number, "\n";
}
// For the last array example it is used like this
foreach ($phonebook['contacts'] as $name => $num) {
echo 'Name: ', $name, ', number: ', $num['number'], "\n";
}
</syntaxhighlight>
PHP has an
===Pike===
[[Pike (programming language)|Pike]] has built-in support for
<
mapping(string:string) phonebook = ([
"Sally Smart":"555-9999",
Line 859 ⟶ 1,434:
"J. Random Hacker":"555-1337"
]);
</syntaxhighlight>
Accessing and testing for presence in mappings is done using the indexing operator. So <code>phonebook["Sally Smart"]</code> would return the string <code>"555-9999"</code>, and <code>phonebook["John Smith"]</code> would return 0.
Iterating through a mapping can be done using
<
foreach(phonebook; string key; string value) {
write("%s:%s\n", key, value);
}
</syntaxhighlight>
Or using an iterator object:
<
Mapping.Iterator i = get_iterator(phonebook);
while (i->index()) {
Line 879 ⟶ 1,454:
i->next();
}
</syntaxhighlight>
Elements of a mapping can be removed using <code>m_delete</code>, which returns the value of the removed index:
<
string sallys_number = m_delete(phonebook, "Sally Smart");
</syntaxhighlight>
===PostScript===
In [[PostScript]], associative arrays are called dictionaries. In Level 1 PostScript they must be created explicitly, but Level 2 introduced direct declaration using
<syntaxhighlight lang="postscript">
% Level 1 declaration
3 dict dup begin
Line 905 ⟶ 1,480:
>>
% Both methods leave the dictionary on the operand stack
</syntaxhighlight>
Dictionaries can be accessed directly, using
<syntaxhighlight lang="postscript">
% With the previous two dictionaries still on the operand stack
/red get print % outputs 'rot'
Line 915 ⟶ 1,491:
begin
green print % outputs 'vert'
end
</syntaxhighlight>
Dictionary contents can be iterated through using
<syntaxhighlight lang="postscript">
% Level 2 example
<<
Line 925 ⟶ 1,502:
/That 2
/Other 3
>> {exch =print ( is ) print ==} forall
</syntaxhighlight>
Which may output:
<syntaxhighlight lang="postscript">
That is 2
This is 1
Other is 3
</syntaxhighlight>
Dictionaries can be augmented (up to their defined size only in Level 1) or altered using
<syntaxhighlight lang="postscript">
% define a dictionary for easy reuse:
/MyDict <<
Line 947 ⟶ 1,528:
% remove something
MyDict /rouge undef
</syntaxhighlight>
=== Prolog ===
Some versions of [[Prolog]] include dictionary ("dict") utilities.<ref>[http://www.swi-prolog.org/pldoc/man?section=dicts "Dicts: structures with named arguments"]</ref>
===Python===
In [[Python (programming language)|Python]], associative arrays are called
<
phonebook = {
}
</syntaxhighlight>
Dictionary items can be accessed using the array indexing operator:
<syntaxhighlight lang="python">
>>> phonebook["Sally Smart"]
'555-9999'
</syntaxhighlight>
<
>>> for key in phonebook:
Sally Smart 555-9999
J. Random Hacker 553-1337
John Doe 555-1212
</syntaxhighlight>
Iterating through (key, value) tuples:
<
>>> for key, value in phonebook.items():
Sally Smart 555-9999
J. Random Hacker 553-1337
John Doe 555-1212
</syntaxhighlight>
Dictionary keys can be individually deleted using the <code>del</code> statement. The corresponding value can be returned before the key-value pair is deleted using the "pop" method of "dict" type:
<
>>> del phonebook["John Doe"]
>>> val = phonebook.pop("Sally Smart")
>>> phonebook.keys() # Only one key left
['J. Random Hacker']
</syntaxhighlight>
Python 2.7 and 3.x also support [https://peps.python.org/pep-0274/ dict comprehensions] (similar to [[list comprehension]]s), a compact syntax for generating a dictionary from any iterator:
<syntaxhighlight lang="python">
>>> square_dict = {i: i*i for i in range(5)}
>>> square_dict
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
>>> {key: value for key, value in phonebook.items() if "J" in key}
{'J. Random Hacker': '553-1337', 'John Doe': '555-1212'}
</syntaxhighlight>
Strictly speaking, a dictionary is a super-set of an associative array, since neither the keys or values are limited to a single datatype. One could think of a dictionary as an "associative list" using the nomenclature of Python. For example, the following is also legitimate:
<syntaxhighlight lang="python">
phonebook = {
"Sally Smart": "555-9999",
"John Doe": None,
"J. Random Hacker": -3.32,
14: "555-3322",
}
</syntaxhighlight>
The dictionary keys must be of an [[Immutable object|immutable]] data type. In Python, strings are immutable due to their method of implementation.
===Red===
In [[Red (programming language)|Red]] the built-in <code>map!</code><ref>{{cite web|url=https://doc.red-lang.org/en/datatypes/map.html|title=Map! datatype|website=doc.red-lang.org}}</ref> datatype provides an associative array that maps values of word, string, and scalar key types to values of any type. A hash table is used internally for lookup.
A map can be written as a literal, such as <code>#(key1 value1 key2 value2 ...)</code>, or can be created using <code>make map! [key1 value1 key2 value2 ...]</code>:
<syntaxhighlight lang="red">
Red [Title:"My map"]
my-map: make map! [
"Sally Smart" "555-9999"
"John Doe" "555-1212"
"J. Random Hacker" "553-1337"
]
; Red preserves case for both keys and values, however lookups are case insensitive by default; it is possible to force case sensitivity using the <code>/case</code> refinement for <code>select</code> and <code>put</code>.
; It is of course possible to use <code>word!</code> values as keys, in which case it is generally preferred to use <code>set-word!</code> values when creating the map, but any word type can be used for lookup or creation.
my-other-map: make map! [foo: 42 bar: false]
; Notice that the block is not reduced or evaluated in any way, therefore in the above example the key <code>bar</code> is associated with the <code>word!</code> <code>false</code> rather than the <code>logic!</code> value false; literal syntax can be used if the latter is desired:
my-other-map: make map! [foo: 42 bar: #[false]]
; or keys can be added after creation:
my-other-map: make map! [foo: 42]
my-other-map/bar: false
; Lookup can be written using <code>path!</code> notation or using the <code>select</code> action:
select my-map "Sally Smart"
my-other-map/foo
; You can also loop through all keys and values with <code>foreach</code>:
foreach [key value] my-map [
print [key "is associated to" value]
]
; A key can be removed using <code>remove/key</code>:
remove/key my-map "Sally Smart"
</syntaxhighlight>
===REXX===
In [[REXX]], associative arrays are called
<
KEY = 'Sally Smart'
PHONEBOOK.KEY = '555-9999'
KEY = 'John Doe'
PHONEBOOK.KEY = '555-1212'
KEY = 'J.
PHONEBOOK.KEY = '553-1337'
</syntaxhighlight>
Stem variables with numeric keys typically start at 1 and go up from there. The 0
<
NAME.1 = 'Sally Smart'
NAME.2 = 'John Doe'
NAME.3 = 'J. Random Hacker'
NAME.0 = 3
</syntaxhighlight>
keys are stored in a separate associative array, with numeric keys.
===
In [[Ruby programming language|Ruby]] a hash table is used as follows:
<syntaxhighlight lang="rb">
phonebook = {
}
phonebook['John Doe']
</syntaxhighlight>
Ruby supports hash looping and iteration with the following syntax:
<
irb(main):007:0> ### iterate over keys and values
irb(main):008:0* phonebook.each {|key, value| puts key + " => " + value}
Sally Smart => 555-9999
John Doe => 555-1212
J. Random Hacker => 553-1337
=> {"Sally Smart"=>"555-9999", "John Doe"=>"555-1212", "J. Random Hacker"=>"553-1337"}
irb(main):009:0> ### iterate keys only
irb(main):010:0* phonebook.each_key {|key| puts key}
Sally Smart
John Doe
J. Random Hacker
=> {"Sally Smart"=>"555-9999", "John Doe"=>"555-1212", "J. Random Hacker"=>"553-1337"}
irb(main):011:0> ### iterate values only
irb(main):012:0* phonebook.each_value {|value| puts value}
555-9999
555-1212
553-1337
=> {"Sally Smart"=>"555-9999", "John Doe"=>"555-1212", "J. Random Hacker"=>"553-1337"}
</syntaxhighlight>
Ruby also supports many other useful operations on hashes, such as merging hashes, selecting or rejecting elements that meet some criteria, inverting (swapping the keys and values), and flattening a hash into an array.
===Rust===
The [[Rust (programming language)|Rust]] standard library provides a hash map (<code>std::collections::HashMap</code>) and a [[B-tree]] map (<code>std::collections::BTreeMap</code>). They share several methods with the same names, but have different requirements for the types of keys that can be inserted. The <code>HashMap</code> requires keys to implement the <code>Eq</code> ([[equivalence relation]]) and <code>Hash</code> (hashability) traits and it stores entries in an unspecified order, and the <code>BTreeMap</code> requires the <code>Ord</code> ([[total order]]) trait for its keys and it stores entries in an order defined by the key type. The order is reflected by the default iterators.
<syntaxhighlight lang="rust">
use std::collections::HashMap;
let mut phone_book = HashMap::new();
phone_book.insert("Sally Smart", "555-9999");
phone_book.insert("John Doe", "555-1212");
phone_book.insert("J. Random Hacker", "555-1337");
</syntaxhighlight>
The default iterators visit all entries as tuples. The <code>HashMap</code> iterators visit entries in an unspecified order and the <code>BTreeMap</code> iterator visits entries in the order defined by the key type.
<syntaxhighlight lang="rust">
for (name, number) in &phone_book {
println!("{} {}", name, number);
}
</syntaxhighlight>
There is also an iterator for keys:
<syntaxhighlight lang="rust">
for name in phone_book.keys() {
println!("{}", name);
}
</syntaxhighlight>
===S-Lang===
[[S-Lang]] has an associative array type:
<syntaxhighlight lang="text">
phonebook = Assoc_Type[];
phonebook["Sally Smart"] = "555-9999"
phonebook["John Doe"] = "555-1212"
phonebook["J. Random Hacker"] = "555-1337"
</syntaxhighlight>
You can also loop through an associated array in a number of ways
<
foreach name (phonebook) {
vmessage ("%s %s", name, phonebook[name]);
}
</syntaxhighlight>
To print a sorted-list, it is better to take advantage of S-lang's strong
support for standard arrays:
<
keys = assoc_get_keys(phonebook);
i = array_sort(keys);
vals = assoc_get_values(phonebook);
array_map (Void_Type, &vmessage, "%s %s", keys[i], vals[i]);
</syntaxhighlight>
===Scala===
[[Scala (programming language)|Scala]] provides an immutable <code>Map</code> class as part of the <code>scala.collection</code> framework:
<
val phonebook = Map("Sally Smart" -> "555-9999",
"John Doe" -> "555-1212",
"J. Random Hacker" -> "553-1337")
</syntaxhighlight>
Scala's [[type inference]] will
<
phonebook.get("Sally Smart")
</syntaxhighlight>
This returns an <code>Option</code> type, Scala's equivalent of
===Smalltalk===
In [[Smalltalk]] a
<
phonebook := Dictionary new.
phonebook at: 'Sally Smart' put: '555-9999'.
phonebook at: 'John Doe' put: '555-1212'.
phonebook at: 'J. Random Hacker' put: '553-1337'.
</syntaxhighlight>
To access an entry the message <code>#at:</code> is sent to the dictionary object
<
phonebook at: 'Sally Smart'
</syntaxhighlight>
Which gives:
<
'555-9999'
</syntaxhighlight>
A dictionary hashes, or compares, based on equality and marks both key and value as
[[Garbage collection (computer science)#Strong and weak references|strong references]]. Variants exist in which hash/compare on identity (IdentityDictionary) or keep [[Garbage collection (computer science)#Strong and weak references|weak references]] (WeakKeyDictionary / WeakValueDictionary).
Because every object implements #hash, any object can be used as key (and of course also as value).
===SNOBOL===
[[SNOBOL]] is one of the first (if not the first) programming languages to use associative arrays. Associative arrays in SNOBOL are called Tables.
<
PHONEBOOK = TABLE()
PHONEBOOK['Sally Smart'] = '555-9999'
PHONEBOOK['John Doe'] = '555-1212'
PHONEBOOK['J. Random Hacker'] = '553-1337'
</syntaxhighlight>
===Standard ML===
The SML'97 standard of the [[Standard ML]] programming language does not provide any associative containers. However, various implementations of Standard ML do provide associative containers.
The library of the popular [[Standard ML of New Jersey]] (SML/NJ) implementation provides a signature (somewhat like an "interface"), <code>ORD_MAP</code>, which defines a common interface for ordered functional (immutable) associative arrays. There are several general
<syntaxhighlight lang="sml">
- structure StringMap = BinaryMapFn (struct
type ord_key = string
Line 1,154 ⟶ 1,840:
- StringMap.find (m, "John Doe");
val it = SOME "555-1212" : string option
</syntaxhighlight>
SML/NJ also provides a polymorphic hash table:
<syntaxhighlight lang="sml">
- exception NotFound;
exception NotFound
Line 1,177 ⟶ 1,863:
- HashTable.lookup m "John Doe"; (* raises the exception if not found *)
val it = "555-1212" : string
</syntaxhighlight>
Monomorphic hash tables are also supported, using the <code>HashTableFn</code> functor.
Another Standard ML implementation, [[Moscow ML]], also provides some associative containers. First, it provides polymorphic hash tables in the <code>Polyhash</code> structure. Also, some functional maps from the SML/NJ library above are available as <code>Binarymap</code>, <code>Splaymap</code>, and <code>Intmap</code> structures.
===Tcl===
====array====
<syntaxhighlight lang=Tcl>
set {phonebook(Sally Smart)} 555-9999
set john {John Doe}
set phonebook($john) 555-1212
set
</syntaxhighlight>
If there is a space character in the variable name, the name must be grouped using either curly brackets (no substitution performed) or double quotes (substitution is performed).
Alternatively, several array elements can be set
<
array set phonebook [list {Sally Smart} 555-9999 {John Doe} 555-1212 {J. Random Hacker} 553-1337]
</syntaxhighlight>
To access one array entry and put it
<
puts
</syntaxhighlight>
Which returns this result:
<
555-9999
</syntaxhighlight>
To retrieve the entire array as a dictionary:
<
array get phonebook
</syntaxhighlight>
The result can be (order of keys is unspecified, not because the dictionary is unordered, but because the array is):
<
{Sally Smart} 555-9999 {J. Random Hacker} 553-1337 {John Doe} 555-1212
</syntaxhighlight>
===
<syntaxhighlight lang=Tcl>
set phonebook [dict create {Sally Smart} 555-9999 {John Doe} 555-1212 {J. Random Hacker} 553-1337]
</syntaxhighlight>
To look up an item:
<syntaxhighlight lang=Tcl>
dict get $phonebook {John Doe}
</syntaxhighlight>
To iterate through a dict:
<syntaxhighlight lang=Tcl>
foreach {name number} $phonebook {
puts "name: $name\nnumber: $number"
}
</syntaxhighlight>
=== Visual Basic ===
[[Visual Basic]] can use the Dictionary class from the [[Windows Scripting Host|Microsoft Scripting Runtime]] (which is shipped with Visual Basic 6). There is no standard implementation common to all versions:
<syntaxhighlight lang="vbnet">
' Requires a reference to SCRRUN.DLL in Project Properties
Dim phoneBook As New Dictionary
Line 1,241 ⟶ 1,943:
MsgBox name & " = " & phoneBook(name)
Next
</syntaxhighlight>
===Visual Basic .NET===
[[Visual Basic .NET]] ====Creation====
The following code demonstrates the creation and population of a dictionary (see [[#C#|the C# example on this page]] for additional information):
<syntaxhighlight lang=VBNet>
Dim dic As New System.Collections.Generic.Dictionary(Of String, String)
dic.Add("Sally Smart", "555-9999")
dic("John Doe") = "555-1212"
dic.Item("J. Random Hacker") = "553-1337"
</syntaxhighlight>
An alternate syntax would be to use a ''collection initializer'', which compiles down to individual calls to <code>Add</code>:
<syntaxhighlight lang=VBNet>
Dim dic As New System.Collections.Dictionary(Of String, String) From {
{"Sally Smart", "555-9999"},
{"John Doe", "555-1212"},
{"J. Random Hacker", "553-1337"}
}
</syntaxhighlight>
====Access by key====
Example demonstrating access (see [[#C# access|C# access]]):
<syntaxhighlight lang=VBNet>
Dim sallyNumber = dic("Sally Smart")
' or
Dim sallyNumber = dic.Item("Sally Smart")
</syntaxhighlight>
<syntaxhighlight lang=VBNet>
Dim result As String = Nothing
Dim sallyNumber = If(dic.TryGetValue("Sally Smart", result), result, "n/a")
</syntaxhighlight>
====Enumeration====
Example demonstrating enumeration (see [[#C# enumeration]]):
<syntaxhighlight lang=VBNet>
' loop through the collection and display each entry.
For Each kvp As KeyValuePair(Of String, String) In dic
Console.WriteLine("Phone number for {0} is {1}", kvp.Key, kvp.Value)
Next
</syntaxhighlight>
=== Windows PowerShell ===
Unlike many other [[command line interpreter]]s, [[Windows PowerShell]] has built-in, language-level support for defining associative arrays:
<
$phonebook = @{
'Sally Smart' = '555-9999';
Line 1,266 ⟶ 2,000:
'J. Random Hacker' = '553-1337'
}
</syntaxhighlight>
<
$myOtherObject = @{ foo = 42; bar = $false }
</syntaxhighlight>
Entries can be separated by either a semicolon or a newline
<
$myOtherObject = @{ foo = 42
bar = $false ;
zaz = 3
}
</syntaxhighlight>
Keys and values can be any [[.NET Framework|.NET]] object type
<
$now = [DateTime]::Now
$tomorrow = $now.AddDays(1)
Line 1,292 ⟶ 2,026:
(Get-Process calc) = $tomorrow
}
</syntaxhighlight>
It is also possible to create an empty associative array and add single entries, or even other associative arrays, to it later on
<
$phonebook = @{}
$phonebook += @{ 'Sally Smart' = '555-9999' }
$phonebook += @{ 'John Doe' = '555-1212'; 'J. Random Hacker' = '553-1337' }
</syntaxhighlight>
New entries can also be added by using the array index operator, the property operator, or the <code>Add()</code> method of the underlying .NET object:
<
$phonebook = @{}
$phonebook['Sally Smart'] = '555-9999'
$phonebook.'John Doe' = '555-1212'
$phonebook.Add('J. Random Hacker', '553-1337')
</syntaxhighlight>
To dereference assigned objects, the array index operator, the property operator, or the parameterized property <code>Item()</code> of the .NET object can be used:
<
$phonebook['Sally Smart']
$phonebook.'John Doe'
$phonebook.Item('J. Random Hacker')
</syntaxhighlight>
You can loop through an associative array as follows:
<
$phonebook.Keys | foreach { "Number for {0}: {1}" -f $_,$phonebook.$_ }
</syntaxhighlight>
An entry can be removed using the <code>Remove()</code> method of the underlying .NET object:
<
$phonebook.Remove('Sally Smart')
</syntaxhighlight>
Hash tables can be added
<
$hash1 = @{ a=1; b=2 }
$hash2 = @{ c=3; d=4 }
$hash3 = $hash1 + $hash2
</syntaxhighlight>
==Data serialization formats support==
Line 1,345 ⟶ 2,079:
===JSON===
In [[JSON]], associative arrays are also referred to as objects. Keys can only be strings.
<syntaxhighlight lang=JavaScript>
{
"Sally Smart": "555-9999",
Line 1,352 ⟶ 2,087:
"J. Random Hacker": "555-1337"
}
</syntaxhighlight>
=== TOML ===
[[TOML]] is designed to map directly to a hash map. TOML refers to associative arrays as tables. Tables within TOML can be expressed in either an "unfolded" or an inline approach. Keys can only be strings.<syntaxhighlight lang="toml">[phonebook]
"Sally Smart" = "555-9999"
"John Doe" = "555-1212"
"J. Random Hacker" = "555-1337"</syntaxhighlight><syntaxhighlight lang="toml">
phonebook = { "Sally Smart" = "555-9999", "John Doe" = "555-1212", "J. Random Hacker" = "555-1337" }
</syntaxhighlight>
===YAML===
[[YAML]] associative arrays are also called map elements or key-value pairs. YAML places no restrictions on the types of keys; in particular, they are not restricted to being scalar or string values.
<
Sally Smart: 555-9999
John Doe: 555-1212
J. Random Hacker: 555-1337
</syntaxhighlight>
== References ==
{{Reflist|2}}
[[Category:
[[Category:Articles with example Julia code]]
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