Search methods
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Search algorithms can be used to retrieve information from data structures or search problem domains. They are classified based on their search mechanism, such as linear searches that check every record sequentially or binary searches that repeatedly target the center of the search space and divide it in half. Binary search algorithms have a maximum complexity of O(log n), meaning the number of operations needed to find the target is a logarithmic function of the search space size. Search algorithms are also used to find solutions to constraint satisfaction problems by assigning values to variables to satisfy equations or optimize functions. Local search methods view the search space as a graph and move between items along heuristic edges.
Search methods
- 2. Search Algorithm • A search algorithm is any algorithm which solves the search problem, namely, to retrieve information stored within some data structure(linked list, array data structure, or a search tree), or calculated in the search space of a problem domain. 2
- 3. Search Algorithms Classified • Search algorithms can be classified based on their mechanism of searching. • Linear search algorithms check every record for the one associated with a target key in a linear fashion. • 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. • Digital search algorithms work based on the properties of digits in data structures that use numerical keys. 3
- 4. Logarithmic Function • Search functions are evaluated on the basis of their complexity, or maximum theoretical run time. • Binary search functions, have a maximum complexity of O(log n), or logarithmic time. • the maximum number of operations needed to find the search target is a logarithmic function of the size of the search space. 4
- 5. Heuristics Function • Algorithms for searching 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 equations. • They are also used when the goal is to find a variable assignment that will maximize or minimize a certain function of those variables. • Algorithms for these problems include the basic brute-force search (uninformed search), and a variety of heuristics that try to exploit partial knowledge about the structure of this space, such as linear relaxation, constraint generation, and constraint propagation. 5
- 6. Local Search Methods • An important subclass are the local search methods, that view the elements of the search space as the 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 steepest descent or best- first criterion, or in a stochastic search. This category includes a great variety of general meta heuristic methods(simulated annealing, tabu search, A- teams, & genetic programming-combine arbitrary heuristics in specific ways). 6
- 7. Completeness • This class also includes various tree search algorithms, that view the elements as vertices of a tree, and traverse that tree in some special order. • Examples of the exhaustive methods such as depth- first search and breadth-first search, as well as various heuristic-based search tree pruning methods such as backtracking and branch and bound. • Unlike general meta heuristics, 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. 7
- 8. Game Tree • Another important sub-class consists of algorithms for exploring the game tree of multiple-player games, such as chess or backgammon, whose nodes consist of all possible game situations that could result from the current situation. • The goal in these problems is to find the move that provides the best chance of a win, taking into account all possible moves of the opponent(s). • Similar problems occur when humans or machines have to make successive decisions whose outcomes are not entirely under one's control, such as in robot guidance or in marketing, financial, or military strategy planning. • This kind of problem — combinatorial search — has been extensively studied in the context of artificial intelligence. Examples of algorithms for this class are the minimax algorithm, alpha–beta pruning, Informational search and the A* algorithm. 8
- 9. Combinatorial Search • is generally used for algorithms that look for a specific sub-structure of a given discrete structure (graph, string, or finite group). • The term combinatorial optimization is typically used when the goal is to find a sub-structure with a maximum/ minimum value of some parameter. • (Since the sub-structure is usually represented in the computer by a set of integer variables with constraints, these problems can be viewed as special cases of constraint satisfaction or discrete optimization; but they are usually formulated and solved in a more abstract setting where the internal representation is not explicitly mentioned.) 9
- 10. Graph Algorithms • An important and extensively studied subclass are the graph algorithms, in particular graph traversal algorithms, for finding specific sub-structures in a given graph — subgraphs, paths, circuits. • 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 algorithms, that search for patterns within strings. Two famous examples are the Boyer–Moore and Knuth–Morris–Pratt algorithms, and several algorithms based on the suffix tree data structure. 10