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[[File:Hash table 3 1 1 0 1 0 0 SP.svg|thumb|upright=1.2|Visual representation of a [[hash table]], a [[data structure]] that allows for fast retrieval of information
In [[computer science]], a '''search algorithm''' is an [[algorithm]] designed to solve a [[search problem]]. Search algorithms work to retrieve information stored within particular [[data structure]], or calculated in the [[Feasible region|search space]] of a problem ___domain, with [[Continuous or discrete variable|either discrete or continuous values]].
Algorithms are although [[Search engine (computing)|search engines]] use search algorithms, they belong to the study of [[information retrieval]], not algorithmics.▼
▲
The appropriate search algorithm often depends on the data structure being searched, and may also include prior knowledge about the data. Search algorithms can be made faster or more efficient by specially constructed database structures, such as [[search tree]]s, [[hash map]]s, and [[database index]]es.{{Sfn|Beame|Fich|2002|p=39}}{{full citation needed|date=April 2021}}{{Sfn|Knuth|1998|loc=§6.5 ("Retrieval on Secondary Keys")}}▼
▲The appropriate search algorithm to use often depends on the data structure being searched, and may also include prior knowledge about the data. Search algorithms can be made faster or more efficient by specially constructed database structures, such as [[search tree]]s, [[hash map]]s, and [[database index]]es.{{Sfn|Beame|Fich|2002|p=39
Search algorithms can be classified based on their mechanism of searching into three types of algorithms: linear, binary, and hashing. [[Linear search]] algorithms check every record for the one associated with a target key in a linear fashion.{{Sfn|Knuth|1998|loc=§6.1 ("Sequential Searching")}} [[Binary search algorithm|Binary, or half-interval, searches]] repeatedly target the center of the search structure and divide the search space in half. Comparison search algorithms improve on linear searching by successively eliminating records based on comparisons of the keys until the target record is found, and can be applied on data structures with a defined order.{{Sfn|Knuth|1998|loc=§6.2 ("Searching by Comparison of Keys")}} Digital search algorithms work based on the properties of digits in data structures by using numerical keys.{{Sfn|Knuth|1998|loc=§6.3 (Digital Searching)}} Finally, [[Hash table|hashing]] directly maps keys to records based on a [[hash function]].{{Sfn|Knuth|1998|loc=§6.4, (Hashing)}}
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* Finding a combination or password from the whole set of possibilities
* [[Factorization|Factoring]] an integer (an important problem in [[cryptography]])
* Search engine optimization (SEO) and content optimization for web crawlers
* Optimizing an industrial process, such as a [[chemical reaction]], by changing the parameters of the process (like temperature, pressure, and pH)
* Retrieving a record from a [[database]]
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Algorithms for searching virtual spaces are used in the [[constraint satisfaction problem]], where the goal is to find a set of value assignments to certain variables that will satisfy specific mathematical [[equation]]s and [[inequation]]s / equalities. They are also used when the goal is to find a variable assignment that will [[discrete optimization|maximize or minimize]] a certain function of those variables. Algorithms for these problems include the basic [[brute-force search]] (also called "naïve" or "uninformed" search), and a variety of [[heuristic function|heuristic]]s that try to exploit partial knowledge about the structure of this space, such as linear relaxation, constraint generation, and [[Local consistency|constraint propagation]].
An important subclass are the [[Local search (optimization)|local search]] methods, that view the elements of the search space as the [[vertex (graph theory)|vertices]] of a graph, with edges defined by a set of heuristics applicable to the case; and scan the space by moving from item to item along the edges, for example according to the [[gradient descent|steepest descent]] or [[best-first search|best-first]] criterion, or in a [[Stochastic optimization|stochastic search]]. This category includes a great variety of general [[metaheuristic]] methods, such as [[simulated annealing]], [[tabu search]], [[A-teams]] <ref>{{Cite journal |last=Talukdar |first=Sarosh |last2=Baerentzen |first2=Lars |last3=Gove |first3=Andrew |last4=De Souza |first4=Pedro |date=1998-12-01 |title=Asynchronous Teams: Cooperation Schemes for Autonomous Agents |url=https://doi.org/10.1023/A:1009669824615 |journal=Journal of Heuristics |language=en |volume=4 |issue=4 |pages=295–321 |doi=10.1023/A:1009669824615 |issn=1572-9397|url-access=subscription }}</ref>, and [[genetic programming]], that combine arbitrary heuristics in specific ways. The opposite of local search would be global search methods. This method is applicable when the search space is not limited and all aspects of the given network are available to the entity running the search algorithm.<ref>{{Cite journal|last1=Hunter|first1=A.H.|last2=Pippenger|first2=Nicholas|date=4 July 2013|title=Local versus global search in channel graphs|journal=Networks: An International Journey|arxiv=1004.2526}}</ref>
This class also includes various [[Tree traversal|tree search algorithm]]s, that view the elements as vertices of a [[tree (graph theory)|tree]], and traverse that tree in some special order. Examples of the latter include the exhaustive methods such as [[depth-first search]] and [[breadth-first search]], as well as various heuristic-based [[Pruning (decision trees)|search tree pruning]] methods such as [[backtracking]] and [[branch and bound]]. Unlike general metaheuristics, which at best work only in a probabilistic sense, many of these tree-search methods are guaranteed to find the exact or optimal solution, if given enough time. This is called "[[Completeness (logic)|completeness]]".
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===For sub-structures of a given structure===
An important and extensively studied subclass are the [[List of algorithms#Graph algorithms|graph algorithm]]s, in particular [[graph traversal]] algorithms, for finding specific sub-structures in a given graph — such as [[Glossary of graph theory#Subgraphs|subgraphs]], [[path (graph theory)|paths]], circuits, and so on. Examples include [[Dijkstra's algorithm]], [[Kruskal's algorithm]], the [[nearest neighbour algorithm]], and [[Prim's algorithm]].
Another important subclass of this category are the [[string searching algorithm]]s, that search for patterns within strings. Two famous examples are the [[Boyer–Moore string
===Search for the maximum of a function===
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===For quantum computers===
There are also search methods designed for [[Quantum computing|quantum computer]]s, like [[Grover's algorithm]], that are theoretically faster than linear or brute-force search even without the help of data structures or heuristics. While the ideas and applications behind quantum computers are still entirely theoretical, studies have been conducted with algorithms like Grover's that accurately replicate the hypothetical physical versions of quantum computing systems.<ref>{{Cite journal|last1=López|first1=G V|last2=Gorin|first2=T|last3=Lara|first3=L|date=26 February 2008|title=Simulation of Grover's quantum search algorithm in an Ising-nuclear-spin-chain quantum computer with first- and second-nearest-neighbour couplings|journal=Journal of Physics B: Atomic, Molecular and Optical Physics|volume=41|issue=5|page=055504|doi=10.1088/0953-4075/41/5/055504|arxiv=0710.3196|bibcode=2008JPhB...41e5504L|s2cid=18796310}}</ref>
==See also==
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===Bibliography===
====Books====
{{sfn whitelist|CITEREFKnuth1998}}
*{{TAOCP|volume=3|edition=2}}
====Articles====
*{{
*{{Cite journal|last1=Schmittou|first1=Thomas|last2=Schmittou|first2=Faith E.|date=2002-08-01|title=Optimal Bounds for the Predecessor Problem and Related Problems|journal=Journal of Computer and System Sciences|volume=65|issue=1|pages=38–72|doi=10.1006/jcss.2002.1822|doi-access=free |ref=none}}
==External links==
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