Data structure

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A data structure in computer science is a way of storing data in a computer so that it can be used efficiently. It is an organization of mathematical and logical concepts of data. Often a carefully chosen data structure will allow the most efficient algorithm to be used.

[edit] Overview

The choice of the data structure often begins from the choice of an abstract data type. A well-designed data structure allows a variety of critical operations to be performed, using as few resources, both execution time and memory space, as possible. Data structures are implemented by a programming language as data types and the references and operations they provide.

Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to certain tasks. For example, B-trees are particularly well-suited for implementation of databases, while networks of machines rely on routing tables to function.

In the design of many types of computer program, the choice of data structures is a primary design consideration. Experience in building large systems has shown that the difficulty of implementation and the quality and performance of the final result depends heavily on choosing the best data structure. After the data structures are chosen, the algorithms to be used often become relatively obvious. Sometimes things work in the opposite direction — data structures are chosen because certain key tasks have algorithms that work best with particular data structures. In either case, the choice of appropriate data structures is crucial.

This insight has given rise to many formalized design methods and programming languages in which data structures, rather than algorithms, are the key organizing factor. Most languages feature some sort of module system, allowing data structures to be safely reused in different applications by hiding their verified implementation details behind controlled interfaces. Object-oriented programming languages such as C++ and Java in particular use classes for this purpose.

Since data structures are so crucial, many of them are included in standard libraries of modern programming languages and APIs, such as C++'s containers, the Java Collections Framework, and the Microsoft .NET Framework.

The fundamental building blocks of most data structures are arrays, records, discriminated unions, and references. For example, the nullable reference, a reference which can be null, is a combination of references and discriminated unions, and the simplest linked data structure, the linked list, is built from records and nullable references.

Data structures represent implementations or interfaces: A data structure can be viewed as an interface between two functions or as an implementation of methods to access storage that is organized according to the associated data type.

[edit] Active data structures

Active data structure is a data structure with an associated thread or process that performs internal operations to give the external behavior of another, usually more general, data structure.

For example, a queue is usually considered to be unbounded. However, actual queues provided by the hardware or operating system may be significantly limited. Changing the writing and reading processes to use a bounded queue makes those applications more complicated. However, an active queue can accept input from the writer through a system queue, and save items in memory or on disk if the other system queue for the reader is full. When the reader's queue has space, items can be retrieved and put back in the queue. Although there are now three components, rather than just the writer and reader, the high level abstraction is very simple and clear.