Comparison of programming languages (associative array)

Language support

The following is a comparison of associative arrays (also "mapping", "hash", and "dictionary") in various programming languages.

Awk

Awk has built-in, language-level support for associative arrays.

For example:

You can also loop through an associated array as follows:

You can also check if an element is in the associative array, and delete elements from an associative array.

Multi-dimensional associative arrays can be simulated in standard Awk using concatenation and e.g. SUBSEP:

C

There is no standard implementation of an associative array in C, but a 3rd party library with BSD license is available here, archived here, with the source code available here. POSIX 1003.1-2001 describes the functions hcreate(), hdestroy() and hsearch().

Another 3rd party library, uthash, also creates associative arrays from C structures. A structure represents a value, and one of the structure fields acts as the key.

Finally, the Glib library also supports associative arrays, along with many other advanced data types and is the recommended implementation of the GNU Project.[1]

Similar to Glib, Apple's cross-platform Core Foundation framework provides several basic data types. In particular, there are reference counted CFDictionary and CFMutableDictionary.

C#

The dictionary can also be initialized with all entries during construction. This is called "object initialization".

A foreach loop can enumerate through the entire collection. There is no guarantee of order. If order matters the programmer could choose to use a SortedDictionary or use a .Sort linq extension method.

C++

C++ also has a form of associative array called std::map (see Standard Template Library#Containers). One could create a map with the same information as above using C++ with the following code:

Or less efficiently as it creates temporary std::string values:

With the extension of initialization lists in C++11, entries can be added during a map's construction as shown below:

You can iterate through the list with the following code (C++03):

The same task in new C++11:

In C++, the std::map class is templated which allows the data types of keys and values to be different for different map instances. For a given instance of the map class the keys must be of the same base type. The same must be true for all of the values. Although std::map is typically implemented using a self-balancing binary search tree, C++11 defines a second map called std::unordered_map with the algorithmic characteristics of a hash table. This is a common vendor extension to the STL as well, usually called hash_map, being available from such implementations as SGI and STLPort.

CFML

A structure in CFML is equivalent to an associative array:

Cobra

Initializing an empty dictionary and adding items:

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:

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:

for key, val in dic print "[key]'s phone number is [val]"

D

D offers direct support for associative arrays in the core language – they are implemented as a chaining hash table with binary trees. The equivalent example would be:

Keys and values can be any types, but all the keys in an associative array must be of the same type, and the same for values.

You can also loop through all properties and associated values, i.e. as follows:

A property can be removed as follows:

Delphi

Versions of Delphi prior to 2009 do not offer direct support for associative arrays. However, you can simulate associative arrays using TStrings object. Here's an example:

Delphi 2009 introduced support for generics as well as several standard generic containers, including TDictionary.

Erlang

Erlang offers many approaches to represent mappings, two of the most common in the standard library are keylists and dictionaries.

Keylists are lists of tuples, 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 lists module.

Accessing an element of the keylist can be done with the lists:keyfind/3 function:

Dictionaries are implemented in the dict of the standard library. A new dictionary is created using the dict:new/0 function and new key/value pairs are stored using the dict:store/3 function:

Such a serial initialization would be more idiomatically represented in Erlang with the appropriate function:

The dictionary can be accessed using the dict:find/2 function:

In both cases, any Erlang term can be used as the key. Variations include the orddict module, implementing ordered dictionaries, and gb_trees, implementing general balanced trees.

F#

F# constructs maps from lists, sequences or arrays of tuples, using functions provided by the Map module. F# represents a tuple as two or more elements separated by a comma, and a list as a sequence of elements enclosed in square brackets, separated by semi-colons.

Values can be looked up via one of the Map object functions, such as TryFind. This returns an option type with a value of Some, for a successful lookup, or None, for an unsuccessful one. Pattern matching can then be used to extract the raw value, or a default, from the result.

In this example, the sallyNumber value will now contain the string "555-9999".

Because F# is a .NET language, it also has access to all of the features of the .NET Framework, including the Dictionary objects and HashTable objects that are used for the same purpose in both C# and Visual Basic. These objects may be preferred when writing code that is intended by be linked to from other languages on the .NET framework.

FoxPro

Visual FoxPro implements mapping with the Collection Class.

GetKey returns 0 if the key is not found.

See Collection in FoxPro Help for all the details.

Go

Go has built-in, language-level support for associative arrays, called maps. A map's key type may only be a boolean, numeric, string, array, struct, pointer, interface, or channel type. A map type is written like this: map[keytype]valuetype.

Adding elements one at a time:

A map literal:

Iterating over a map:

Haskell

The Haskell programming language's report only provides one kind of associative container: a list of pairs:

output:

Just "555-1212"

Note that the lookup function returns a "Maybe" value, which is "Nothing" if not found, or "Just result" when found.

GHC, the most commonly used implementation of Haskell, provides two more types of associative containers. Other implementations might also provide these.

One is polymorphic functional maps (represented as immutable balanced binary trees):

output:

Just "555-1212"

A specialized version for integer keys also exists as Data.IntMap.

Finally, a polymorphic hash table:

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 IO monad.

Java

In Java associative arrays are implemented as "maps"; they are part of the Java collections framework. Since J2SE 5.0 and the introduction of generics into Java, collections can have a type specified; for example, an associative array mapping strings to strings might be specified as follows:

The get method is used to access a key; for example, the value of the expression phoneBook.get("Sally Smart") is "555-9999".

This code above uses a hash map to store the associative array, by calling the constructor of the HashMap class; however, since the code only uses methods common to the interface Map, one could also use a self-balancing binary tree by calling the constructor of the TreeMap class (which implements the subinterface SortedMap), without changing the definition of the phoneBook variable or the rest of the code, or use a number of other underlying data structures that implement the Map interface.

The hash function in Java, used by HashMap and HashSet, is provided by the method Object.hashCode(). Since every class in Java inherits from Object, every object has a hash function. A class can override the default implementation of hashCode() to provide a custom hash function based on the properties of the object.

The Object class also contains the method equals(Object) that tests the object for equality with another object. Hashed data structures in Java rely on objects maintaining the following contract between their hashCode() and equals() methods:

For two objects a and b,

In order to maintain this contract, a class that overrides equals() must also override hashCode(), and maybe vice versa, so that hashCode() is based on the same properties (or a subset of the properties) as equals().

A further contract that hashed data structures has with the object is that the results of the hashCode() and equals() methods will not change once the object has been inserted into the map. For this reason, it is generally a good practice to base the hash function on immutable properties of the object.

Analogously, TreeMap, and other sorted data structures, requires that an ordering be defined on the data type. Either the data type must already have defined its own ordering, by implementing the Comparable interface; or a custom Comparator must be provided at the time the map is constructed. As with HashMap above, the relative ordering of keys in a TreeMap should not change once they have been inserted into the map.

JavaScript

JavaScript (and its standardized version: ECMAScript) is a prototype-based object-oriented language. In JavaScript an object is a mapping from property names to values—that is, an associative array with one caveat: since property names are strings, only string and (coerced) integer keys are allowed. Other than that difference, objects also include one feature unrelated to associative arrays: a prototype link to the object they inherit from. Doing a lookup for a property will forward the lookup to the prototype if the object does not define the property itself.

An object literal is written as { property1 : value1, property2 : value2, ... }. For example:

If the property name is a valid identifier, the quotes can be omitted, e.g.:

Lookup is written using property access notation, either square brackets, which always works, or dot notation, which only works for identifier keys:

You can also loop through all enumerable properties and associated values as follows:

A property can be removed as follows:

As mentioned before, properties are strings. However, since every native object and primitive can be implicitly converted to a string, you can do:

Any object, including built-in objects such as Array, can be dynamically extended with new properties. For example:

In modern JavaScript it's considered bad form to use the Array type as an associative array. Consensus is that the Object type is best for this purpose. The reasoning behind this is that if Array is extended via prototype and Object is kept pristine, 'for(in)' loops will work as expected on associative 'arrays'. This issue has been drawn into focus by the popularity of JavaScript frameworks that make heavy and sometimes indiscriminate use of prototype to extend JavaScript's inbuilt types.

See JavaScript Array And Object Prototype Awareness Day for more information on the issue.

Julia

Declare dictionary:

Access element:

phonebook["Sally Smart"]

Add element:

phonebook["New Contact"] = "555-2222"

Delete element:

delete!(phonebook, "Sally Smart")

Get keys and values as iterables:

keys(phonebook) values(phonebook)

KornShell 93 (and compliant shells: ksh93, zsh, bash4...)

Definition:

Dereference:

Lisp

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").

The syntax (x . y) is used to indicate a consed pair. Keys and values need not be the same type within an alist. Lisp and Scheme provide operators such as assoc to manipulate alists in ways similar to associative arrays.

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 they are implemented in SRFI 69. Hash tables have greater overhead than alists, but provide much faster access when there are many elements.

It is easy to construct composite abstract data types in Lisp, using structures and/or the object-oriented programming features, in conjunction with lists, arrays, and hash tables.

LPC

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 ([ key_1 : value_1, key_2 : value_2 ]). Procedural use looks like:

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. if(member(phone_book, "John Smith")) write("John Smith is listed. ");

Deletion is accomplished using a function called either m_delete() or map_delete(), depending on the driver, used like: m_delete(phone_book, "Sally Smart");

LPC drivers of the "Amylaar" family implement multivalued mappings using a secondary, numeric index. (Drivers of the MudOS family do not support multivalued mappings.) Example syntax:

LPC drivers modern enough to support a foreach() construct allow iteration over their mapping types using it.

Lua

In Lua, table is a fundamental type that can be used either as array (numerical index, fast) or as 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 { value, key = value, [index] = value, ["non id string"] = value }. For example:

If the key is a valid identifier (not a keyword), the quotes can be omitted. They are case sensitive.

Lookup is written using either square brackets, which always works, or dot notation, which only works for identifier keys:

You can also loop through all keys and associated values with iterators or for loops:

An entry can be removed by setting it to nil:

Likewise, you can overwrite values or add them:

Mathematica and Wolfram Language

Mathematica uses the Association expression to represent associative arrays.

To access

If the keys are strings, the Key keyword is not necessary, so:

To list keys and values

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 arrays stored on disk which are available concurrently by multiple jobs. The name for globals is preceded by the circumflex "^" to distinguish it from local variable names.

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

To access the value of an element, simply requires using the name with the subscript:

WRITE "Phone Number :",^phonebook("Sally Smart"),!

You can also loop through an associated array as follows:

SET NAME="" FOR S NAME=$ORDER(^phonebook(NAME)) QUIT:NAME="" WRITE NAME," Phone Number :",^phonebook(NAME),!

Objective-C (Cocoa/GNUstep)

Cocoa (API) and GNUstep handle associative arrays using NSMutableDictionary (a mutable version of NSDictionary) class cluster. This class allows assignments between any two objects to be made. A copy of the key object is made before it is inserted into NSMutableDictionary, therefore the keys must conform to the NSCopying protocol. When being inserted to a dictionary, the value object receives a retain message to increase its reference count. The value object will receive the release message when it will be deleted from the dictionary (both explicitly or by adding to the dictionary a different object with the same key).

To access assigned objects this command may be used:

All keys or values can be simply enumerated using NSEnumerator

On Mac OS X 10.5+ and iPhone OS, dictionary keys can also be enumerated more concisely using this NSFastEnumeration construct:

What is even more practical, structured data graphs may be easily created using Cocoa, especially NSDictionary (NSMutableDictionary). This can be illustrated with this compact example:

And relevant fields can be quickly accessed using key paths:

OCaml

The OCaml programming language provides three different associative containers. The simplest is a list of pairs:

The second is a polymorphic hash table:

The code above uses OCaml's default hash function Hashtbl.hash, which is defined automatically for all types. If you wanted to use your own hash function, you can use the functor interface Hashtbl.Make to create a module, like with Map below.

Finally, functional maps (represented as immutable balanced binary trees):

Note that in order to use Map, you have to provide the functor Map.Make 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 PMap, where you provide the comparison function when creating the map.

Lists of pairs and functional maps both provide a purely functional interface. In contrast, hash tables provide an imperative interface. For many operations, hash tables are significantly faster than lists of pairs and functional maps.

OptimJ

The OptimJ programming language is an extension of Java 5. As java, Optimj provides maps. But, OptimJ also provides true associative arrays: java arrays are indexed with 0-based integers; associative arrays are indexed with any collection of keys.

Of course, it is possible to define multi-dimensional arrays, to mix java array and associative arrays, to mix maps and associative arrays.

Perl

Perl has built-in, language-level support for associative arrays. Modern Perl vernacular refers to associative arrays as hashes; the term associative array is found in older documentation, but is considered somewhat archaic. Perl hashes are flat: keys are strings and values are scalars. However, values may be references to arrays or other hashes, and the standard Perl module Tie::RefHash enables hashes to be used with reference keys.

A hash variable is marked by a % 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 => token, which is mostly semantically identical to the comma and makes the key-value association clearer:

Accessing a hash element uses the syntax $hash_name{$key} – the key is surrounded by curly braces and the hash name is prefixed by a $, indicating that the hash element itself is a scalar value, even though it is part of a hash. The value of $phone_book{'John Doe'} is '555-1212'. The % sigil is only used when referring to the hash as a whole, such as when asking for keys %phone_book.

The list of keys and values can be extracted using the built-in functions keys and values, respectively. So, for example, to print all the keys of a hash:

One can iterate through (key, value) pairs using the each function:

A hash reference, which is a scalar value that points to a hash, is specified in literal form using curly braces as delimiters, with syntax otherwise similar to specifying a hash literal:

Values in a hash reference are accessed using the dereferencing operator:

When the hash contained in the hash reference needs to be referred to as a whole, as with the keys function, the syntax is as follows:

PHP

PHP's built-in array type is in reality an associative array. Even when using numerical indexes, PHP internally stores it as an associative array. This is why one in PHP can have non-consecutive numerically indexed arrays. The keys have to be integer or string (floating point numbers are truncated to integer), 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:

PHP can loop through an associative array as follows:

PHP has an extensive set of functions to operate on arrays.

If you want an associative array that can use objects as keys instead of strings and integers, you can use the SplObjectStorage class from the Standard PHP Library (SPL).

Pike

Pike has built-in support for Associative Arrays, which are referred to as mappings. Mappings are created as follows:

Accessing and testing for presence in mappings is done using the indexing operator. So phonebook["Sally Smart"] would return the string "555-9999", and phonebook["John Smith"] would return 0.

Iterating through a mapping can be done using either foreach:

Or using an iterator object:

Elements of a mapping can be removed using m_delete, which returns the value of the removed index:

PostScript

In PostScript, associative arrays are called dictionaries. In Level 1 PostScript they must be created explicitly, but Level 2 introduced direct declaration using the double-brace syntax:

Dictionaries can be accessed directly using get or implicitly by placing the dictionary on the dictionary stack using begin:

Dictionary contents can be iterated through using forall, though not in any particular order:

May well output:

Dictionaries can be augmented (up to their defined size only in Level 1) or altered using put, and entries can be removed using undef:

Prolog

Some versions of Prolog include "dicts".

Python

In Python, associative arrays are called dictionaries. Dictionary literals are marked with curly braces:

To access an entry in Python simply use the array indexing operator. For example,

An example loop iterating through all the keys of the dictionary:

Iterating through (key, value) tuples:

Dictionary keys can be individually deleted using the del statement. The corresponding value can be returned before the key-value pair are deleted using the pop method of dict types:

Python 2.7 and 3.x also supports dictionary comprehensions, a compact syntax for generating a dictionary from any iterator:

REXX

In REXX, associative arrays are called Stem variables or Compound variables.

Stem variables with numeric keys typically start at 1 and go up from there. The 0 key stem variable is used (by convention) as the count of items in the whole stem.

REXX has no easy way of automatically accessing the keys for a stem variable and typically the keys are stored in a separate associative array with numeric keys.

Ruby

In Ruby a hash is used as follows:

Ruby supports hash looping and iteration with the following syntax:

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.

S-Lang

S-Lang has an associative array type.

For example:

You can also loop through an associated array in a number of ways. Here is one

To print a sorted-list, it is better to take advantage of S-lang's strong support for standard arrays:

Scala

Scala provides an immutable Map class as part of the scala.collection framework:

Scala's type inference will work out that this is a Map[String, String]. To access the array:

This returns an Option type, Scala's equivalent of a the Maybe monad in Haskell.

Smalltalk

In Smalltalk a dictionary is used:

To access an entry the message #at: is sent to the dictionary object.

gives

Dictionary hashes/compares based on equality and holds strong references to both key and value. Variants exist which hash/compare on identity (IdentityDictionary) or keep 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.

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 implementation provides a signature (somewhat like an "interface"), ORD_MAP, which defines a common interface for ordered functional (immutable) associative arrays. There are several general functors, BinaryMapFn, ListMapFn, RedBlackMapFn, and SplayMapFn, that allow you to create the corresponding type of ordered map (the types are a self-balancing binary search tree, sorted association list, red-black tree, and splay tree, respectively) using a user-provided structure to describe the key type and comparator. The functor returns a structure that follows the ORD_MAP interface. In addition, there are two pre-defined modules for associative arrays with integer keys: IntBinaryMap and IntListMap.

SML/NJ also provides a polymorphic hash table:

Monomorphic hash tables are also supported using the HashTableFn functor.

Another Standard ML implementation, Moscow ML, also provides some associative containers. First, it provides polymorphic hash tables in the Polyhash structure. Also, some functional maps from the SML/NJ library above are available as Binarymap, Splaymap, and Intmap structures.

Tcl

There are two Tcl facilities that support associative array semantics. An array is a collection of variables. A dict is a full implementation of associative arrays.

dict

To lookup an item:

To iterate through a dict:

array

If there is a literal space character in the variable name, it must be grouped using either curly brackets (no substitution performed) or double quotes (substitution is performed).

Alternatively, several array elements can be set in a single command by providing their mappings as a list (words containing whitespace are braced):

To access one array entry and put it on standard output

The result is here

To retrieve the entire array as a dictionary:

The result can be (order of keys is unspecified, not because the dictionary is unordered, but because the array is):

Visual Basic

There is no standard implementation common to all dialects. Visual Basic can use the Dictionary class from the Microsoft Scripting Runtime (which is shipped with Visual Basic 6):

Visual Basic .NET relies on the collection classes provided by .NET Framework:

Windows PowerShell

Unlike many other command line interpreters, PowerShell has built-in, language-level support for defining associative arrays.

For example:

Like in JavaScript, if the property name is a valid identifier, the quotes can be omitted, e.g.:

Entries can be separated by either a semicolon or a newline, e.g.:

Keys and values can be any .NET object type, e.g.:

It is also possible to create an empty associative array and add single entries or even other associative arrays to it later on.

New entries can also be added by using the array index operator, the property operator or the Add() method of the underlying .NET object:

To dereference assigned objects the array index operator, the property operator or the parameterized property Item() of the .NET object can be used:

You can loop through an associative array as follows:

An entry can be removed using the Remove() method of the underlying .NET object:

Hash tables can be added, e.g.:

Data serialization formats support

Many data serialization formats also support associative arrays (see this table)

JSON

In JSON, associative arrays are called objects. Keys can only be strings.