An associative array is a collection indexed by arbitrary data types, not just small integers. Whereas an array is typically implemented as many same-sized items stored in a contiguous block of memory, an associative array must be implemented via a more complex data structure, such as a hash table, a list, or some other type of map.
The terminology and semantics of these vary among different programming languages. For example in Perl and Ruby they are called “hashes” (from abbreviating “hash table”, the underlying implementation) while in Python, Objective-C, and Smalltalk they are called “dictionaries” (by analogy to the normal English usage of the term: an indexed reference by which keys (words) are associated with values (definitions)). In Lua they are called “tables”. In Java, C++, and Go they are called “maps”. In Pike they are called “mappings”.
The semantics differ as well. While all of these allow the programmer to associate some sort of key with some sort of value they can differ considerably in how they evaluate the key and what sorts of values can be stored.
For example in awk and Perl the keys are evaluated (as “scalars” in Perl terminology). Thus the the keys "1" (a string) and 1 (an integer) and 1.0 (a real or floating point number) would all evaluate into equivalent keys. By contrast these would each be distinct in Python. In a Python dictionary any immutable object (strings, integer, floats) and any object/class which implements the __hash__ special method can be used as a key. Values can be references to any objects (including functions, classes, class methods which are all "first class objects" in that language). In Lua, a table is a complex data structure which can be used to implement arrays, objects and associative arrays (integer key values are implicitly treated like indices into a virtual array, those with values that reference functions are methods, those which reference other types of objects are attributes or members).
- See also
- Associative array: Creation, Iteration
- Compound data type
- Doubly-linked list: Definition, Element definition, Element insertion, List Traversal, Element Removal
- Linked list
- Queue: Definition, Usage
- Singly-linked list: Element definition, Element insertion, List Traversal, Element Removal
- Free On-line Dictionary of Computing, http://foldoc.org/, Editor Denis Howe