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Reverse Lookup
Reverse lookup is a procedure of using a value to retrieve a unique key in an associative array. Applications of reverse lookup include * reverse DNS lookup, which provides the domain name associated with a particular IP address, * reverse telephone directory, which provides the name of the entity associated with a particular telephone number,. * reverse image search, which provides similar images to the one provided. See also * Inverse function In mathematics, the inverse function of a function (also called the inverse of ) is a function that undoes the operation of . The inverse of exists if and only if is bijective, and if it exists, is denoted by f^ . For a function f\colon ... * Reverse dictionary References {{information-science-stub Arrays ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Value (computer Science)
In computer science and software programming, a value is the representation of some entity that can be manipulated by a program. The members of a type are the values of that type. The "value of a variable" is given by the corresponding mapping in the environment. In languages with assignable variables, it becomes necessary to distinguish between the ''r-value'' (or contents) and the ''l-value'' (or location) of a variable. In declarative (high-level) languages, values have to be referentially transparent. This means that the resulting value is independent of the location of the expression needed to compute the value. Only the contents of the location (the bits, whether they are 1 or 0) and their interpretation are significant. Value category Despite its name, in the C++ language standards this terminology is used to categorize expressions, not values. Assignment: l-values and r-values Some languages use the idea of l-values and r-values, deriving from the typical mod ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Unique Key
In relational database management systems, a unique key is a candidate key. All the candidate keys of a relation can uniquely identify the records of the relation, but only one of them is used as the primary key of the relation. The remaining candidate keys are called unique keys because they can uniquely identify a record in a relation. Unique keys can consist of multiple columns. Unique keys are also called alternate keys. Unique keys are an alternative to the primary key of the relation. In SQL, the unique keys have a UNIQUE constraint assigned to them in order to prevent duplicates (a duplicate entry is not valid in a unique column). Alternate keys may be used like the primary key when doing a single-table select or when filtering in a ''where'' clause, but are not typically used to join multiple tables. Summary Keys provide the means for database users and application software to identify, access and update information in a database table. ''There may be several keys in any ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Associative Array
In computer science, an associative array, key-value store, map, symbol table, or dictionary is an abstract data type that stores a collection of (key, value) pairs, such that each possible key appears at most once in the collection. In mathematical terms, an associative array is a function with ''finite'' domain. It supports 'lookup', 'remove', and 'insert' operations. The dictionary problem is the classic problem of designing efficient data structures that implement associative arrays. The two major solutions to the dictionary problem are hash tables and search trees..Dietzfelbinger, M., Karlin, A., Mehlhorn, K., Meyer auf der Heide, F., Rohnert, H., and Tarjan, R. E. 1994"Dynamic Perfect Hashing: Upper and Lower Bounds". SIAM J. Comput. 23, 4 (Aug. 1994), 738-761. http://portal.acm.org/citation.cfm?id=182370 It is sometimes also possible to solve the problem using directly addressed arrays, binary search trees, or other more specialized structures. Many programmin ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Reverse DNS Lookup
In computer networks, a reverse DNS lookup or reverse DNS resolution (rDNS) is the querying technique of the Domain Name System (DNS) to determine the domain name associated with an IP address – the reverse of the usual "forward" DNS lookup of an IP address from a domain name. The process of reverse resolving of an IP address uses PTR records. rDNS involves searching domain name registry and registrar tables. The reverse DNS database of the Internet is rooted in the .arpa top-level domain. Although the informationaRFC 1912 (Section 2.1)recommends that "every Internet-reachable host should have a name" and that "for every IP address, there should be a matching PTR record," it is not an Internet Standard requirement, and not all IP addresses have a reverse entry. Historical usage The modern "reverse DNS lookup" should not be confused with the now-obsolete "inverse query" (IQUERY) mechanism specified in : The IQUERY message type was always "optional" and "never achieved wi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Reverse Telephone Directory
A reverse telephone directory (also known as a gray pages directory, criss-cross directory or reverse phone lookup) is a collection of telephone numbers and associated customer details. However, unlike a standard telephone directory, where the user uses customer's details (such as name and address) in order to retrieve the telephone number of that person or business, a reverse telephone directory allows users to search by a telephone service number in order to retrieve the customer details for that service. Reverse telephone directories are used by law enforcement and other emergency services in order to determine the origin of any request for assistance, however these systems include both publicly accessible (listed) and private (unlisted) services. As such, these directories are restricted to internal use only. Some forms of city directories provide this form of lookup for listed services by phone number, along with address cross-referencing. Publicly accessible reverse telephon ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Reverse Image Search
Reverse image search is a content-based image retrieval (CBIR) Query string, query technique that involves providing the CBIR system with a sample image that it will then base its search upon; in terms of information retrieval, the sample image is very useful. In particular, reverse image search is characterized by a lack of search terms. This effectively removes the need for a user to guess at keywords or terms that may or may not return a correct result. Reverse image search also allows users to discover content that is related to a specific sample image or the popularity of an image, and to discover manipulated versions and derivative works. A visual search engine is a search engine (computing), search engine designed to search for information on the World Wide Web through a reverse image search. Information may consist of web pages, locations, other images and other types of documents. This type of search engines is mostly used to search on the mobile Internet through an image ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Inverse Function
In mathematics, the inverse function of a function (also called the inverse of ) is a function that undoes the operation of . The inverse of exists if and only if is bijective, and if it exists, is denoted by f^ . For a function f\colon X\to Y, its inverse f^\colon Y\to X admits an explicit description: it sends each element y\in Y to the unique element x\in X such that . As an example, consider the real-valued function of a real variable given by . One can think of as the function which multiplies its input by 5 then subtracts 7 from the result. To undo this, one adds 7 to the input, then divides the result by 5. Therefore, the inverse of is the function f^\colon \R\to\R defined by f^(y) = \frac . Definitions Let be a function whose domain is the set , and whose codomain is the set . Then is ''invertible'' if there exists a function from to such that g(f(x))=x for all x\in X and f(g(y))=y for all y\in Y. If is invertible, then there is exactly one functi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Reverse Dictionary
A reverse dictionary is a dictionary alphabetized by the reversal of each entry: :stock (kcots) :diestock (kcotseid) :restock (kcotser) :livestock (kcotsevil) Before computers, reverse dictionaries were tedious to produce. The first computer-produced was Stahl and Scavnicky's ''A Reverse Dictionary of the Spanish Language'', in 1974. The first computer-produced reverse dictionary for a single text was Wisbey, R., ''Vollständige Verskonkordanz zur Wiener Genesis. Mit einem rückläufigen Wörterbuch zum Formenbestand'', Berlin, E. Schmidt, 1967. Definition In a reverse word dictionary, the entries are alphabetized by the last letter first, then next to last, and so on.Stahl, Fred A., Scavnicky, Gary E. A., ''A Reverse Dictionary of the Spanish Language'', University of Illinois Press, Urbana, IL,1974. In them, words with the same suffix appear together. This can be useful for linguists and poets looking for words ending with a particular suffix, or by an epigrapher or forensics ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
