{{Short description|Type of streaming algorithm}} {{Refimprove|date=April 2021}}
In computing, a '''one-pass algorithm''' or '''single-pass algorithm''' is a streaming algorithm which reads its input exactly once.<ref name="frankfurt"/> It does so by processing items in order, without unbounded buffering; it reads a block into an input buffer, processes it, and moves the result into an output buffer for each step in the process.<ref name="sjsu"/> A one-pass algorithm generally requires ''O''(''n'') (see 'big O' notation) time and less than ''O''(''n'') storage (typically ''O''(1)), where ''n'' is the size of the input.<ref name="eds"/> An example of a one-pass algorithm is the Sondik partially observable Markov decision process.<ref name="pomdp"/>
==Example problems solvable by one-pass algorithms== Given any list as an input: * Count the number of elements.
Given a list of numbers: * Find the ''k'' largest or smallest elements, ''k'' given in advance. * Find the sum, mean, variance and standard deviation of the elements of the list. See also Algorithms for calculating variance.
Given a list of symbols from an alphabet of ''k'' symbols, given in advance. * Count the number of times each symbol appears in the input. * Find the most or least frequent elements. * Sort the list according to some order on the symbols (possible since the and after number of symbols is limited). * Find the maximum gap between two appearances of a given symbol.
==Example problems not solvable by one-pass algorithms== Given any list as an input: * Find the ''n''th element from the end (or report that the list has fewer than ''n'' elements). * Find the middle element of the list. However, this is solvable with two passes: Pass 1 counts the elements and pass 2 picks out the middle one.
Given a list of numbers: * Find the median. * Find the modes (This is not the same as finding the most frequent symbol from a limited alphabet). * Sort the list. * Count the number of items greater than or less than the mean. However, this can be done in constant memory with two passes: Pass 1 finds the average and pass 2 does the counting.
The two-pass algorithms above are still streaming algorithms but not one-pass algorithms.
== References== <references> <ref name="eds">{{Citation|last=Schweikardt|first=Nicole|title=One-Pass Algorithm|date=2009|url=https://doi.org/10.1007/978-0-387-39940-9_253|encyclopedia=Encyclopedia of Database Systems|pages=1948–1949|editor-last=LIU|editor-first=LING|editor-link=Ling Liu (computer scientist)|place=Boston, MA|publisher=Springer US|language=en|doi=10.1007/978-0-387-39940-9_253|isbn=978-0-387-39940-9|access-date=2021-04-13|editor2-last=ÖZSU|editor2-first=M. TAMER|url-access=subscription}}</ref> <ref name="frankfurt">{{Cite web|last=Schweikardt|first=Nicole|title=One-Pass Algorithm|url=http://www.tks.informatik.uni-frankfurt.de/schweika/downloads/EncycDBS_OnePassAlgos.pdf|access-date=2021-07-01}}</ref> <ref name="sjsu">{{Cite web|last=Pollett|first=Chris|date=2005-03-14|title=One and Two Pass Algorithms|url=http://www.cs.sjsu.edu/faculty/pollett/157b.12.05s/Lec14032005.pdf|access-date=2021-07-01}}</ref> <ref name="pomdp">{{Cite web|url=http://www.pomdp.org/tutorial/sondik.html|title=Sondik's One-Pass Algorithm|website=www.pomdp.org}}</ref> </references>
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{{DEFAULTSORT:One-Pass Algorithm}} Category:Streaming algorithms