Data structure

In, a data structure is a data organization, management, and storage format that enables access and modification. More precisely, a data structure is a collection of, the relationships among them, and the functions or operations that can be applied to the data.

Usage
Data structures serve as the basis for s (ADT). The ADT defines the logical form of the data type. The data structure implements the physical form of the data type.

Different types of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks. For example, relational databases commonly use indexes for data retrieval, while  implementations usually use s to look up identifiers.

Data structures provide a means to manage large amounts of data efficiently for uses such as large s and. Usually, efficient data structures are key to designing efficient s. Some formal design methods and s emphasize data structures, rather than algorithms, as the key organizing factor in software design. Data structures can be used to organize the storage and retrieval of information stored in both and.

Implementation
Data structures are generally based on the ability of a to fetch and store data at any place in its memory, specified by a —a bit string, representing a, that can be itself stored in memory and manipulated  by the program. Thus, the and  data structures are based on computing the addresses of data items with, while the s are based on storing addresses of data items within the structure itself.

The implementation of a data structure usually requires writing a set of that create and manipulate instances of that structure. The efficiency of a data structure cannot be analyzed separately from those operations. This observation motivates the theoretical concept of an, a data structure that is defined indirectly by the operations that may be performed on it, and the mathematical properties of those operations (including their space and time cost).

Examples
There are numerous types of data structures, generally built upon simpler s:
 * An is a number of elements in a specific order, typically all of the same type (depending on the language, individual elements may either all be forced to be the same type, or may be of almost any type). Elements are accessed using an integer index to specify which element is required. Typical implementations allocate contiguous memory words for the elements of arrays (but this is not always a necessity). Arrays may be fixed-length or resizable.
 * A  (also just called list) is a linear collection of data elements of any type, called nodes, where each node has itself a value, and points to the next node in the linked list. The principal advantage of a linked list over an array, is that values can always be efficiently inserted and removed without relocating the rest of the list. Certain other operations, such as to a certain element, are however slower on lists than on arrays.
 * A (also called tuple or struct) is an aggregate data structure. A record is a value that contains other values, typically in fixed number and sequence and typically indexed by names. The elements of records are usually called fields or members.
 * A is a data structure that specifies which of a number of permitted primitive types may be stored in its instances, e.g. float or long integer. Contrast with a, which could be defined to contain a float and an integer; whereas in a union, there is only one value at a time. Enough space is allocated to contain the widest member datatype.
 * A  (also called, variant record, discriminated union, or disjoint union) contains an additional field indicating its current type, for enhanced type safety.
 * An is a data structure that contains data fields, like a record does, as well as various  which operate on the data contents. An object is an in-memory instance of a class from a taxonomy. In the context of, records are known as s to distinguish them from objects.

In addition, and  are other commonly used data structures.

Language support
Most s and some, such as (Basic Combined Programming Language), lack built-in support for data structures. On the other hand, many s and some higher-level assembly languages, such as, have special syntax or other built-in support for certain data structures, such as records and arrays. For example, the (a direct descendant of BCPL) and  languages support  and records, respectively, in addition to vectors (one-dimensional ) and multi-dimensional arrays.

Most programming languages feature some sort of mechanism that allows data structure implementations to be reused by different programs. Modern languages usually come with standard libraries that implement the most common data structures. Examples are the, the , and the.

Modern languages also generally support, the separation between the of a library module and its implementation. Some provide s that allow clients to hide implementation details. s, such as, , and , typically use for this purpose.

Many known data structures have versions which allow multiple computing threads to access a single concrete instance of a data structure simultaneously.