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{{Short description|Sequence of data items available over time}}
[[Image:Stdstreams-notitle.svg|thumb|right|The standard streams for input, output, and error]]
In [[Computer science|computer science]], a '''stream''' is a [[Sequence|sequence]] of [[Data element|data element]]s made available over time. A stream can be thought of as items on a [[Engineering:Conveyor belt|conveyor belt]] being processed one at a time rather than in large batches.

Streams are processed differently from [[Software:Batch processing|batch data]] – normal functions cannot operate on streams as a whole, as they have potentially unlimited data, and formally, streams are ''codata'' (potentially unlimited), not data (which is finite). Functions that operate on a stream, producing another stream, are known as [[Software:Filter|filters]], and can be connected in [[Pipeline (computing)|pipelines]], analogously to [[Function composition (computer science)|function composition]]. Filters may operate on one item of a stream at a time, or may base an item of output on multiple items of input, such as a [[Moving average|moving average]].

==Examples==
The term "stream" is used in a number of similar ways:
* "Stream editing", as with [[Sed|sed]], awk, and [[Perl|perl]]. Stream editing processes a file or files, in-place, without having to load the file(s) into a user interface. One example of such use is to do a search and replace on all the files in a directory, from the command line.
* On [[Software:Unix|Unix]] and related systems based on the [[C (programming language)|C language]], a stream is a source or [[Sink (computing)|sink]] of data, usually individual bytes or [[Character (computing)|character]]s. Streams are an abstraction used when reading or writing files, or communicating over [[Network socket|network socket]]s. The [[Standard streams|standard streams]] are three streams made available to all programs.
* I/O devices can be interpreted as streams, as they produce or consume potentially unlimited data over time.
* In [[Object-oriented programming|object-oriented programming]], input streams are generally implemented as [[Iterator|iterator]]s.
* In the [[Scheme (programming language)|Scheme language]] and some others, a stream is a [[Lazy evaluation|lazily evaluated]] or ''delayed'' sequence of data elements. A stream can be used similarly to a list, but later elements are only calculated when needed. Streams can therefore represent infinite [[Sequence|sequence]]s and [[Series (mathematics)|series]].<ref>[http://srfi.schemers.org/srfi-41/srfi-41.html SRFI 41: Streams<!-- Bot generated title -->]</ref>
* In the [[Smalltalk]] [[Standard library|standard library]] and in other [[Programming language|programming language]]s as well, a stream is an external iterator. As in Scheme, streams can represent finite or infinite sequences.
* [[Stream processing]] — in [[Parallel computing|parallel processing]], especially in graphic processing, the term stream is applied to [[Computer hardware|hardware]] as well as [[Software|software]]. There it defines the quasi-continuous flow of data that is processed in a [[Dataflow programming|dataflow programming]] language as soon as the program state meets the starting condition of the stream.

==Applications==
{{section-expand|date=February 2014}}
Streams can be used as the underlying data type for [[Channel (programming)|channels]] in interprocess communication.
==Other uses==
The term "stream" is also applied to [[File system|file system]] ''[[Fork (file system)|forks]]'', where multiple sets of data are associated with a single filename. Most often, there is one main stream that makes up the normal file data, while additional streams contain [[Metadata Encoding and Transmission Standard|metadata]]. Here "stream" is used to indicate "variable size data", as opposed to fixed size metadata such as [[Extended file attributes|extended attributes]], but differs from "stream" as used otherwise, meaning "data available over time, potentially infinite".

==See also==
* [[Bitstream]]
* Codata
* [[Data stream]]
* [[Data stream mining]]
* Flow (computer networking)
* [[Network socket]]
* Streaming algorithm
* [[Engineering:Streaming media|Streaming media]]
* [[Stream processing]]

==References==
{{Reflist}}

==External links==
* [http://portal.acm.org/citation.cfm?id=796530 An Approximate L1-Difference Algorithm for Massive Data Streams, 1995 Feigenbaum et al.]

{{Data types}}

<!--Categories-->
[[Category:Computer data]]
[[Category:Inter-process communication]]

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