Lookup
Encyclopedia
In computing
, lookup usually refers to searching a data structure
for an item that satisfies some specified property. (Note that in grammatical usage, lookup is the noun form describing the verb form to look up.) For example, variable lookup performed by a (scripting) language interpreter
, virtual machine
or other similar engine usually consists of performing certain actions to dynamically find correspondence between variable identifier
and actual variable internal representation, usually involving symbol table
lookup. Symbol table lookup can be performed either during run-time (interpreter or scripting engine), or during compile time (compiler
). A hybrid scheme when lookup is performed both during translation
phase and then later during runtime is also possible (e.g. bytecode
compiler and virtual machine). In all of these cases, search item is a variable and the search property (or search criterion) is a variable name. Variable lookup is usually performed according to variable visibility rules that are specific to the (scripting) language in question.
Another example is DNS lookup. In DNS lookup, a DNS server is requested to find a host IP address
given host domain name
. Here the search criterion (property) is a domain name and the search result (item) is an IP address. A domain name may be associated with several IP addresses. Reverse DNS lookup
performs the reverse task: given IP address, it attempts to resolve domain name associated with the specified IP address.
Spreadsheet
software typically has functions such as LOOKUP, HLOOKUP, and VLOOKUP, to find either a value in a row or column that matches the argument, or in the case of range lookup, to find an interval in a set of adjacent intervals (given by their limits, specified in a row or column) which contains the argument. The result is the corresponding value in a specified parallel row or column.
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, lookup usually refers to searching a data structure
Data structure
In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...
for an item that satisfies some specified property. (Note that in grammatical usage, lookup is the noun form describing the verb form to look up.) For example, variable lookup performed by a (scripting) language interpreter
Interpreter (computing)
In computer science, an interpreter normally means a computer program that executes, i.e. performs, instructions written in a programming language...
, virtual machine
Virtual machine
A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...
or other similar engine usually consists of performing certain actions to dynamically find correspondence between variable identifier
Identifier
An identifier is a name that identifies either a unique object or a unique class of objects, where the "object" or class may be an idea, physical [countable] object , or physical [noncountable] substance...
and actual variable internal representation, usually involving symbol table
Symbol table
In computer science, a symbol table is a data structure used by a language translator such as a compiler or interpreter, where each identifier in a program's source code is associated with information relating to its declaration or appearance in the source, such as its type, scope level and...
lookup. Symbol table lookup can be performed either during run-time (interpreter or scripting engine), or during compile time (compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
). A hybrid scheme when lookup is performed both during translation
Translation
Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. Whereas interpreting undoubtedly antedates writing, translation began only after the appearance of written literature; there exist partial translations of the Sumerian Epic of...
phase and then later during runtime is also possible (e.g. bytecode
Bytecode
Bytecode, also known as p-code , is a term which has been used to denote various forms of instruction sets designed for efficient execution by a software interpreter as well as being suitable for further compilation into machine code...
compiler and virtual machine). In all of these cases, search item is a variable and the search property (or search criterion) is a variable name. Variable lookup is usually performed according to variable visibility rules that are specific to the (scripting) language in question.
Another example is DNS lookup. In DNS lookup, a DNS server is requested to find a host IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
given host domain name
Domain name
A domain name is an identification string that defines a realm of administrative autonomy, authority, or control in the Internet. Domain names are formed by the rules and procedures of the Domain Name System ....
. Here the search criterion (property) is a domain name and the search result (item) is an IP address. A domain name may be associated with several IP addresses. Reverse DNS lookup
Reverse DNS lookup
In computer networking, reverse DNS lookup or reverse DNS resolution is the determination of a domain name that is associated with a given IP address using the Domain Name System of the Internet....
performs the reverse task: given IP address, it attempts to resolve domain name associated with the specified IP address.
Spreadsheet
Spreadsheet
A spreadsheet is a computer application that simulates a paper accounting worksheet. It displays multiple cells usually in a two-dimensional matrix or grid consisting of rows and columns. Each cell contains alphanumeric text, numeric values or formulas...
software typically has functions such as LOOKUP, HLOOKUP, and VLOOKUP, to find either a value in a row or column that matches the argument, or in the case of range lookup, to find an interval in a set of adjacent intervals (given by their limits, specified in a row or column) which contains the argument. The result is the corresponding value in a specified parallel row or column.