Identifier - AbsoluteAstronomy.com

An identifier is a name

Name

A name is a word or term used for identification. Names can identify a class or category of things, or a single thing, either uniquely, or within a given context. A personal name identifies a specific unique and identifiable individual person, and may or may not include a middle name...

that identifies (that is, labels the identity

Identity

-Philosophical topics:* Identity , also called sameness, is whatever makes an entity definable and recognizable* Law of identity, principle of logic stating that an object is the same as itself...

of) either a unique object or a unique class of objects, where the "object" or class may be an idea, physical [countable] object (or class thereof), or physical [noncountable] substance (or class thereof). The abbreviation ID often refers to identity, identification (the process of identifying), or an identifier (that is, an instance of identification). An identifier may be a word, number, letter, symbol, or any combination of those.

The words, numbers, letters, or symbols may follow an encoding system

Code

A code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....

(wherein letters, digits, words, or symbols stand for (represent) ideas or longer names) or they may simply be arbitrary. When an identifier follows an encoding system, it is often referred to as a code or ID code. Identifiers that do not follow any encoding scheme are often said to be arbitrary IDs; they are arbritrarily assigned and have no greater meaning. (Sometimes identifiers are called "codes" even when they are actually arbitrary, whether because the speaker believes that they have deeper meaning or simply because he is speaking casually and imprecisely.)

ID codes inherently carry metadata

Metadata

The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...

along with them. (For example, when you know that the food package in front of you has the identifier "2011-09-25T15:42Z-MFR5-P02-243-45", you not only have that data, you also have the metadata that tells you that it was packaged on September 25, 2011, at 3:42pm UTC, manufactured by Licensed Vendor Number 5, at the Peoria, IL, USA plant, in Building 2, and was the 243rd package off the line in that shift, and was inspected by Inspector Number 45.) Arbritrary identifiers carry no metadata. (For example, if your food package just says 100054678214, its ID may not tell you anything except identity—no date, manufacturer name, production sequence rank, or inspector number.)

A unique identifier

Unique identifier

With reference to a given set of objects, a unique identifier is any identifier which is guaranteed to be unique among all identifiers used for those objects and for a specific purpose...

(UID) is an identifier that refers to only one instance—only one particular object in the universe. A part number

Part number

A part number is an identifier of a particular part design used in a particular industry. Its purpose is to simplify referencing to that part...

is an identifier, but it is not a unique identifier—for that, a serial number

Serial number

A serial number is a unique number assigned for identification which varies from its successor or predecessor by a fixed discrete integer value...

is needed, to identify each instance of the part design. Thus the identifier "Model T" identifies the class (model) of automobiles that Ford's Model T

Ford Model T

The Ford Model T is an automobile that was produced by Henry Ford's Ford Motor Company from September 1908 to May 1927...

comprises; whereas the unique identifier "Model T Serial Number 159,862" identifies one specific member of that class—that is, one particular Model T car, owned by one specific person.

The concepts of name and identifier are denotatively

Denotation

This word has distinct meanings in other fields: see denotation . For the opposite of Denotation see Connotation.*In logic, linguistics and semiotics, the denotation of a word or phrase is a part of its meaning; however, the part referred to varies by context:** In grammar and literary theory, the...

equal, and the terms are thus denotatively synonym

Synonym

Synonyms are different words with almost identical or similar meanings. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy. The word comes from Ancient Greek syn and onoma . The words car and automobile are synonyms...

ous; but they are not always connotatively

Connotation

A connotation is a commonly understood subjective cultural or emotional association that some word or phrase carries, in addition to the word's or phrase's explicit or literal meaning, which is its denotation....

synonymous, because code name
Code name
A code name or cryptonym is a word or name used clandestinely to refer to another name or word. Code names are often used for military purposes, or in espionage...

s and ID numbers are often connotatively distinguished from names in the sense of traditional natural language

Natural language

In the philosophy of language, a natural language is any language which arises in an unpremeditated fashion as the result of the innate facility for language possessed by the human intellect. A natural language is typically used for communication, and may be spoken, signed, or written...

naming. For example, both "Jane Smith" and "Employee Number 8547" are identifiers for the same specific human being; but normal English-language connotation may consider "Jane Smith" a "name" and not an "identifier", whereas it considers "Employee Number 8547" an "identifier" but not a "name". This is an emic distinction rather than an etic one.

Metadata

In metadata

Metadata

, an identifier is a language-independent label, sign or token that uniquely identifies an object within an identification scheme

Identification scheme

In metadata, an identification scheme is used to identify unique records in a set.If a data element is used to identify a record within a data set, the data element uses the Identifier representation term....

.

The suffix identifier is also used as a representation term

Representation term

A representation term is a word, or a combination of words, that semantically represent the data type of a data element. A representation term is commonly referred to as a class word by those familiar with data dictionaries...

when naming a data element

Data element

In metadata, the term data element is an atomic unit of data that has precise meaning or precise semantics. A data element has:# An identification such as a data element name# A clear data element definition# One or more representation terms...

In computer science

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

, identifiers (IDs) are lexical

Lexical (semiotics)

In the lexicon of a language, lexical words or nouns refer to things. These words fall into three main classes:*proper nouns refer exclusively to the place, object or person named, i.e...

tokens that name entities

Entity

An entity is something that has a distinct, separate existence, although it need not be a material existence. In particular, abstractions and legal fictions are usually regarded as entities. In general, there is also no presumption that an entity is animate.An entity could be viewed as a set...

. The concept is analogous

Analogy

Analogy is a cognitive process of transferring information or meaning from a particular subject to another particular subject , and a linguistic expression corresponding to such a process...

to that of a "name." Identifiers are used extensively in virtually all information processing systems. Naming entities makes it possible to refer to them, which is essential for any kind of symbolic processing.

In computer languages

In computer languages, identifiers are tokens (also called symbol

Symbol

A symbol is something which represents an idea, a physical entity or a process but is distinct from it. The purpose of a symbol is to communicate meaning. For example, a red octagon may be a symbol for "STOP". On a map, a picture of a tent might represent a campsite. Numerals are symbols for...

s) which name language entities. Some of the kinds of entities an identifier might denote include variable

Variable (programming)

In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...

s, types, label

Label (programming language)

A label in a programming language is a sequence of characters that identifies a location within source code. In most languages labels take the form of an identifier, often followed by a punctuation character . In many high level programming languages the purpose of a label is to act as the...

s, subroutine

Subroutine

In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....

s, and packages

Modular programming

Modular programming is a software design technique that increases the extent to which software is composed of separate, interchangeable components called modules by breaking down program functions into modules, each of which accomplishes one function and contains everything necessary to accomplish...

.

In most languages, some character sequences have the lexical form of an identifier but are known as keywords

Keyword (computer programming)

In computer programming, a keyword is a word or identifier that has a particular meaning to the programming language. The meaning of keywords — and, indeed, the meaning of the notion of keyword — differs widely from language to language....

. In a few languages, e.g., PL/1, the distinction is not clear.

Computer languages usually place restrictions on what characters may appear in an identifier. For example, in early versions of the C

C (programming language)

C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

and C++

C++

C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...

languages, identifiers are restricted to being a sequence of one or more ASCII

ASCII

The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...

letters, digits (these may not appear as the first character), and underscores. Later versions of these languages, along with many other modern languages support almost all Unicode

Unicode

Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...

characters in an identifier (a common restriction is not to permit white space characters and language operators).

For implementations of programming languages that are using a compiler

Compiler

A compiler is a computer program that transforms source code written in a programming language into another computer language...

, identifiers are often only compile time

Compile time

In computer science, compile time refers to either the operations performed by a compiler , programming language requirements that must be met by source code for it to be successfully compiled , or properties of the program that can be reasoned about at compile time.The operations performed at...

entities. That is, at runtime the compiled program contains references to memory addresses and offsets rather than the textual identifier tokens (these memory addresses, or offsets, having been assigned by the compiler to each identifier).

Implementations of programming languages that offer interactive evaluation of source code (using an interpreter or an incremental compiler) present identifiers at runtime, sometimes even as first-class object

First-class object

In programming language design, a first-class citizen , in the context of a particular programming language, is an entity that can be constructed at run-time, passed as a parameter, returned from a subroutine, or assigned into a variable...

s that can be freely manipulated and evaluated. In Lisp, these are called symbols

Symbol (Lisp)

A symbol in computer programming is a primitive datatype whose instances have a unique human-readable form. Symbols can be used as identifiers. In some programming languages, they are called atoms....

.

Compilers and interpreters do not usually assign any semantic meaning to an identifier based on the actual character sequence used. However, there are exceptions.

For example:

In Perl
Perl
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...

a variable is indicated using a prefix called a sigil
Sigil (computer programming)
In computer programming, a sigil is a symbol attached to a variable name, showing the variable's datatype or scope. In 1999 Philip Gwyn adopted the term "to mean the funny character at the front of a Perl variable".- Historical context:...

, which specifies aspects of how the variable is interpreted in expressions
Expression (programming)
An expression in a programming language is a combination of explicit values, constants, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces another value...

.
In Ruby a variable is automatically considered immutable
Immutable object
In object-oriented and functional programming, an immutable object is an object whose state cannot be modified after it is created. This is in contrast to a mutable object, which can be modified after it is created...

if its identifier starts with a capital letter.
In Fortran
Fortran
Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...

, the first letter in a variable's name indicates whether by default it is created as an integer
Integer
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...

or floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...

variable.

IDs versus UIDs

Many identifiers are not unique identifiers. Typical examples are:

One person with multiple names, nicknames, and forms of address (titles, salutations)
- For example: One specific person may be identified by all of the following identifiers: Jane Smith; Jane Elizabeth Meredith Smith; Jane E. M. Smith; Jane E. Smith; Janie Smith; Janie; Little Janie (as opposed to her mother or sister or cousin, Big Janie); Aunt Jane; Auntie Janie; Mom; Grandmom; Nana; Kelly's mother; Billy's grandmother; Ms. :Smith; Dr. Smith; Jane E. Smith, PhD; and Fuzzy (her jocular nickname at work).
One document with multiple versions
One substance with multiple names (for example, CAS
Chemical Abstracts Service
Chemical Abstracts is a periodical index that provides summaries and indexes of disclosures in recently published scientific documents. Approximately 8,000 journals, technical reports, dissertations, conference proceedings, and new books, in any of 50 languages, are monitored yearly, as are patent...

index names versus IUPAC names; INN
International Nonproprietary Name
An International Nonproprietary Name is the official nonproprietary or generic name given to a pharmaceutical substance, as designated by the World Health Organization...

generic drug names versus USAN
United States Adopted Name
United States Adopted Names are unique nonproprietary names assigned to pharmaceuticals marketed in the United States. Each name is assigned by the USAN Council, which is co-sponsored by the American Medical Association , the United States Pharmacopeial Convention , and the American Pharmacists...

generic drug names versus brand names)

Implicit context and namespace conflicts

Many code

Code

A code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....

s and nomenclatural

Nomenclature

Nomenclature is a term that applies to either a list of names or terms, or to the system of principles, procedures and terms related to naming - which is the assigning of a word or phrase to a particular object or property...

systems originate within a small namespace

Namespace

In general, a namespace is a container that provides context for the identifiers it holds, and allows the disambiguation of homonym identifiers residing in different namespaces....

. Over the years, some of them bleed into larger namespaces (as people interact in ways they formerly hadn't, e.g., cross-border trade, scientific collaboration, military alliance, and general cultural interconnection or assimilation). When such dissemination happens, the limitations of the original naming convention, which had formerly been latent and moot, become painfully apparent, often necessitating retronym

Retronym

A retronym is a type of neologism that provides a new name for an object or concept to differentiate the original form or version of it from a more recent form or version. The original name is most often augmented with an adjective to account for later developments of the object or concept itself...

y, synonym

Synonym

ity, translation/transcoding, and so on. Such limitations generally accompany the shift away from the original context to the broader one. Typically the system shows implicit context (context was formerly assumed, and narrow), lack of capacity (e.g., low number of possible IDs, reflecting the outmoded narrow context), lack of extensibility

Extensibility

In software engineering, extensibility is a system design principle where the implementation takes into consideration future growth. It is a systemic measure of the ability to extend a system and the level of effort required to implement the extension...

(no features defined and reserved against future needs), and lack of specificity and disambiguating capability (related to the context shift, where longstanding uniqueness encounters novel nonuniqueness). The story of the origination and expansion of the CODEN

CODEN

CODEN – according to ASTM standard E250 – is a six character, alphanumeric bibliographic code, that provides concise, unique and unambiguous identification of the titles of serials and non-serial publications from all subject areas....

system provides a good case example in a recent-decades, technical-nomenclature context. The capitalization variations seen with specific designators reveals an instance of this problem occurring in natural languages, where the proper noun/common noun distinction (and its complications) must be dealt with. A universe in which every object had a UID would not need any namespaces, which is to say that it would constitute one gigantic namespace; but human minds could never keep track of, or semantically interrelate, so many UIDs.

Identifiers in various disciplines

Identifier	Namespace(s)
atomic number Atomic number In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element... , corresponding one-to-one with element name	[international namespace Namespace In general, a namespace is a container that provides context for the identifiers it holds, and allows the disambiguation of homonym identifiers residing in different namespaces.... (via ISV International Scientific Vocabulary International scientific vocabulary comprises scientific and specialized words whose language of origin may or may not be certain, but which are in current use in several modern languages. The name "International Scientific Vocabulary" was first used by Philip Gove in Webster’s Third New... )]
Australian Business Number Australian Business Number The Australian Business Number, or ABN, is a unique identifier issued by the Australian Business Register which is operated by the Australian Taxation Office...	[Australian namespace]
CAGE code Commercial and Government Entity The Commercial And Government Entity Code, or CAGE Code, is a unique identifier assigned to suppliers to various government or defense agencies, as well as to government agencies themselves and also various organizations...	[U.S. and NATO namespaces]
CAS registry number CAS registry number CAS Registry Numbersare unique numerical identifiers assigned by the "Chemical Abstracts Service" toevery chemical described in the...	[originated in U.S. namespace; today international (via ISV International Scientific Vocabulary International scientific vocabulary comprises scientific and specialized words whose language of origin may or may not be certain, but which are in current use in several modern languages. The name "International Scientific Vocabulary" was first used by Philip Gove in Webster’s Third New... )]
CODEN CODEN CODEN – according to ASTM standard E250 – is a six character, alphanumeric bibliographic code, that provides concise, unique and unambiguous identification of the titles of serials and non-serial publications from all subject areas....	[originated in U.S. namespace; today international]
Digital object identifier Digital object identifier A digital object identifier is a character string used to uniquely identify an object such as an electronic document. Metadata about the object is stored in association with the DOI name and this metadata may include a location, such as a URL, where the object can be found... (DOI, doi)	[international namespace]
DIN Deutsches Institut für Normung is the German national organization for standardization and is that country's ISO member body. DIN is a Registered German Association headquartered in Berlin... standard number	[originated in German namespace; today international]
E number E number E numbers are number codes for food additives that have been assessed for use within the European Union . They are commonly found on food labels throughout the European Union. Safety assessment and approval are the responsibility of the European Food Safety Authority...	[originated in E.U. namespace; may be seen internationally]
EC number EC number The Enzyme Commission number is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze....
Employer Identification Number Employer identification number Applicable to the United States, an Employer Identification Number or EIN is the corporate equivalent to a Social Security Number, although it is issued to anyone, including individuals, who has to pay withholding taxes on employees.-Other names:Also known as the Tax Identification Number ,... (EIN)	[U.S. namespace]
Global Trade Item Number Global Trade Item Number Global Trade Item Number is an identifier for trade items developed by GS1...	[international namespace]
Group identifier Group identifier In Unix-like systems, multiple users can be categorized into groups. POSIX and conventional Unix file system permissions are organized into three classes, user, group, and others. The use of groups allows additional abilities to be delegated in an organized fashion, such as access to disks,...	[many namespaces, e.g., specific computer systems]
International Chemical Identifier International Chemical Identifier The IUPAC International Chemical Identifier is a textual identifier for chemical substances, designed to provide a standard and human-readable way to encode molecular information and to facilitate the search for such information in databases and on the web...	[international namespace]
International Standard Book Number International Standard Book Number The International Standard Book Number is a unique numeric commercial book identifier based upon the 9-digit Standard Book Numbering code created by Gordon Foster, Emeritus Professor of Statistics at Trinity College, Dublin, for the booksellers and stationers W.H... (ISBN)	[international namespace]
International Standard Serial Number International Standard Serial Number An International Standard Serial Number is a unique eight-digit number used to identify a print or electronic periodical publication. Periodicals published in both print and electronic form may have two ISSNs, a print ISSN and an electronic ISSN... (ISSN)	[international namespace]
ISO International Organization for Standardization The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial... standard number, e.g., ISO 8601 ISO 8601 ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times is an international standard covering the exchange of date and time-related data. It was issued by the International Organization for Standardization and was first published in 1988...	[international namespace]
Library of Congress Control Number Library of Congress Control Number The Library of Congress Control Number or LCCN is a serially based system of numbering cataloging records in the Library of Congress in the United States...	[U.S. namespace, with some international bibliographic usefulness]
personal identification number Personal identification number A personal identification number is a secret numeric password shared between a user and a system that can be used to authenticate the user to the system. Typically, the user is required to provide a non-confidential user identifier or token and a confidential PIN to gain access to the system...	[many namespaces, e.g., banks, governments]
Personal identification number (Denmark) Personal identification number (Denmark) The Danish Personal Identification number is a national identification number, which is part of the personal information stored in the Civil Registration System ....	[Denmark namespace]
Product batch number
Serial Item and Contribution Identifier Serial Item and Contribution Identifier The Serial Item and Contribution Identifier is a code used to uniquely identify specific volumes, articles or other identifiable parts of a periodical...	[U.S. namespace, with some international bibliographic usefulness]
Serial number Serial number A serial number is a unique number assigned for identification which varies from its successor or predecessor by a fixed discrete integer value...	[many namespaces, e.g., company-specific, government-specific)
Service batch number
Social Security Number Social Security number In the United States, a Social Security number is a nine-digit number issued to U.S. citizens, permanent residents, and temporary residents under section 205 of the Social Security Act, codified as . The number is issued to an individual by the Social Security Administration, an independent...	[U.S. namespace]
Tax File Number Tax File Number Tax File Number is an 8 or 9 digit number issued by the Australian Taxation Office to each taxpayer to identify that taxpayer's Australian tax dealings. When it was introduced in 1988, individuals received a 9 digit TFN and non-individuals were issued an 8 digit TFN. Now both are issued 9 digit...	[Australian namespace]

Metadata

In computer science

In computer languages

IDs versus UIDs

Implicit context and namespace conflicts

Identifiers in various disciplines

See also