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Random access

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Definition and Comparison
– Random access is the ability to access any element of a sequence in equal time.
– It allows for accessing data from a population of addressable elements efficiently.
– It contrasts with sequential access, which retrieves data in the order it was stored.
– Random access can be applied to single or multiple dimensions of data storage.
– The choice of datum is arbitrary as long as the address is known.

Terminology and History
– The term ‘random access’ was initially used because records could be found in any sequence.
– ‘Direct access’ gained favor as it emphasized the ability to retrieve any record directly.
– The key attribute is the immediate access to any required record.
– Sequential access takes longer to access remote elements.
– The distinction can be illustrated with examples like ancient scrolls vs. books, or cassette tapes vs. CDs.

Applications and Data Structures
– Direct access is crucial in algorithms like binary search and integer sorting.
– Arrays and related structures provide direct access to any entry in constant time.
– Linked lists sacrifice direct access for efficient inserts, deletes, and re-ordering.
– Self-balancing binary search trees offer a compromise with logarithmic retrieval time.
– Direct access is valuable in data structures that require efficient element retrieval.

– National Computer Conference and Exposition (1957) Proceedings.
– Introduction to IBM Direct-access Storage Devices and Organization Methods by IBM (1966).
– Random and Sequential Data Access in ‘The Art of Computer Programming’ by D. E. Knuth (1969).
– These references provide additional information on random access.
– They can be consulted for further study and verification.

Related Concepts
Data stream is a related concept to random access.
– Random-access machine is another term associated with random access.
– Random-access memory (RAM) is no longer truly random access due to cache and virtual memory.
– Locality of reference is an important concept related to efficient data access.
– These concepts are worth exploring to deepen understanding of random access.

Random access (Wikipedia)

Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any other, no matter how many elements may be in the set. In computer science it is typically contrasted to sequential access which requires data to be retrieved in the order it was stored.

Random access compared to sequential access

For example, data might be stored notionally in a single sequence like a row, in two dimensions like rows and columns on a surface, or in multiple dimensions. However, given all the coordinates, a program can access each record about as quickly and easily as any other. In this sense, the choice of datum is arbitrary in the sense that no matter which item is sought, all that is needed to find it is its address, i.e. the coordinates at which it is located, such as its row and column (or its track and record number on a magnetic drum). At first, the term "random access" was used because the process had to be capable of finding records no matter in which sequence they were required. However, soon the term "direct access" gained favour because one could directly retrieve a record, no matter what its position might be. The operative attribute, however, is that the device can access any required record immediately on demand. The opposite is sequential access, where a remote element takes longer time to access.

A typical illustration of this distinction is to compare an ancient scroll (sequential; all material prior to the data needed must be unrolled) and the book (direct: can be immediately flipped open to any arbitrary page). A more modern example is a cassette tape (sequential — one must fast forward through earlier songs to get to later ones) and a CD (direct access — one can skip to the track wanted, knowing that it would be the one retrieved).

In data structures, direct access implies the ability to access any entry in a list in constant time (independent of its position in the list and of the list's size). Very few data structures can make this guarantee other than arrays (and related structures like dynamic arrays). Direct access is required, or at least valuable, in many algorithms such as binary search, integer sorting, or certain versions of sieve of Eratosthenes.

Other data structures, such as linked lists, sacrifice direct access to permit efficient inserts, deletes, or re-ordering of data. Self-balancing binary search trees may provide an acceptable compromise, where access time is not equal for all members of a collection, but the maximum time to retrieve a given member grows only logarithmically with its size.

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