Random access

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Random access compared to sequential access

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.

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.[1] However, soon the term "direct access" gained favour because one could directly retrieve a record, no matter what its position might be.[2] 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.[3]

A typical illustration of this distinction is to compare an ancient

CD
(direct access — one can skip to the track wanted, knowing that it would be the one retrieved).

In

binary search, integer sorting, or certain versions of sieve of Eratosthenes.[4]

Other data structures, such as

logarithmically
with its size.

References

  1. ^ National Computer Conference and Exposition (1957). Proceedings. Retrieved 2 October 2013.
  2. ^ Introduction to IBM Direct-access Storage Devices and Organization Methods. International Business Machines Corporation. 1966. pp. 3–. Retrieved 2 October 2013.
  3. ^ "Random and Sequential Data Access".
  4. . Retrieved 2 October 2013.

See also