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{{pp-semi-indef|small=yes}}
In ], a '''code''' is a rule for converting a piece of information (for example,
{{short description|System of rules to convert information into another form or representation}}
a letter, word, or phrase) into another object or action, not necessarily of the same sort.
{{hatgrp|
One reason for this is to enable communication in places where ordinary spoken or written language
{{Other uses}}
is difficult or impossible.
{{Redirect|Encoding}}
For example, a cable code replaces words (eg, ship, invoice, ...) into shorter words, allowing the same information to be sent with fewer characters, more quickly, and most important, less expensively.
}}
Another example is the use of ], where the configuration of flags
{{technical reasons|prefix=y|Code#|the EPs by Ladies' Code|Code 01 Bad Girl|and|Code 02 Pretty Pretty}}
held by a signaller or the arms of a semaphore tower encodes parts of the message, typically individual letters and numbers.
{{More citations needed|date=March 2010}}
Another person standing a great distance away can interpret the flags and reproduce the words sent.
In ]s and ], '''code''' is a system of rules to convert ]—such as a ], ], sound, image, or ]—into another form, sometimes ] or ], for communication through a ] or storage in a ]. An early example is an invention of ], which enabled a person, through ], to communicate what they thought, saw, heard, or felt to others. But speech limits the range of communication to the distance a voice can carry and limits the audience to those present when the speech is uttered. The invention of ], which converted spoken language into ] ]s, extended the range of communication across space and ].


The process of '''encoding''' converts information from a ] into symbols for communication or storage. '''Decoding''' is the reverse process, converting code symbols back into a form that the recipient understands, such as English or/and Spanish.
==Codes versus ciphers==


One reason for coding is to enable communication in places where ordinary ], spoken or written, is difficult or impossible. For example, ], where the configuration of ] held by a signaler or the arms of a ] encodes parts of the message, typically individual letters, and numbers. Another person standing a great distance away can interpret the flags and reproduce the words sent.
The term ''code'' is very often confused with the term ''cipher''.


== Theory ==
These can be destinguished best by the scope of the transformation.
{{main|Coding theory}}
A code is just a set of rules to represent meaningful information in another way - this doesn't necessarily imply secrecy. When the information should not be
In ] and ], a code is usually considered as an ] that uniquely represents ] from some source ], by ''encoded'' strings, which may be in some other target alphabet. An extension of the code for representing sequences of symbols over the source alphabet is obtained by concatenating the encoded strings.
known to somebody else but the intended recipient, there has to
be some secret knowledge not known to anybody else to decode it.
A code which requires such secret knowledge (or ''is'' this
secret knowledge) is a cryptographic code and the secret information is a codebook.


Before giving a mathematically precise definition, this is a brief example. The mapping
A ] by contrast, does not work at the level of meaningful information. While a code might transform "attack" into FRGPL or "fruit pie", a cipher transforms elements below the semantic level, ie, below the level of meaning. The "a" in attack might be converted to "Q", the first "t" to "f", the second "t" to "3", and so on. Cyphers are more convenient than codes in some situations, there being no need for a codebook. Codes on the other hand, were believed to be more secure than cyphers, there being (if one's codebook constructor did a good job) no pattern of transformation to discover. Witht the advent of automatic processors (ie, in recent times the electronic computer), cyphers have come to dominate cryptography.
:<math>C = \{\, a\mapsto 0, b\mapsto 01, c\mapsto 011\,\}</math>
is a code, whose source alphabet is the set <math>\{a,b,c\}</math> and whose target alphabet is the set <math>\{0,1\}</math>. Using the extension of the code, the encoded string 0011001 can be grouped into codewords as 0 011 0 01, and these in turn can be decoded to the sequence of source symbols ''acab''.


Using terms from ], the precise mathematical definition of this concept is as follows: let S and T be two finite sets, called the source and target ], respectively. A '''code''' <math>C:\, S \to T^*</math> is a ] mapping each symbol from S to a ] over T. The '''extension''' <math>C'</math> of <math>C</math>, is a ] of <math>S^*</math> into <math>T^*</math>, which naturally maps each sequence of source symbols to a sequence of target symbols.
''The following paragraph needs to be rewritten: split it an put the pieces at the proper place''
Another use of codes is ], where a code or ] is used to disguise a message,
preventing those not in on the secret from understanding a transmission.
One simple method is to use a "codebook" with a list of common phrases replaced by phrases with
completely different meanings, so that people without the codebook who may intercept the message
hear a conversation that is entirely unrelated to the real intended message, or alternatively, complete gibberish. A cable code would be one such.


=== Variable-length codes ===
{{main|Variable-length code}}
In this section, we consider codes that encode each source (clear text) character by a ] from some dictionary, and ] of such code words give us an encoded string. Variable-length codes are especially useful when clear text characters have different probabilities; see also ].


A ''prefix code'' is a code with the "prefix property": there is no valid code word in the system that is a ] (start) of any other valid code word in the set. ] is the most known algorithm for deriving prefix codes. Prefix codes are widely referred to as "Huffman codes" even when the code was not produced by a Huffman algorithm. Other examples of prefix codes are ], the country and publisher parts of ]s, and the Secondary Synchronization Codes used in the ] ] 3G Wireless Standard.


] characterizes the sets of codeword lengths that are possible in a prefix code. Virtually any uniquely decodable one-to-many code, not necessarily a prefix one, must satisfy Kraft's inequality.
==Codes in communication used for brevity==


=== Error-correcting codes ===
Code can be used for brevity. When telegraph messages were the state of the art, elaborate commercial codes that encoded complete phrases
{{Main|Error detection and correction}}
into single words (typically five-letter groups) were developed, so that telegraphers became conversant with
{{See also|Block code}}
such "words" as BYOXO ("Are you trying to crawl out of it?"), LIOUY ("Why do you not answer my question?"),
Codes may also be used to represent data in a way more resistant to errors in transmission or storage. This so-called ] works by including carefully crafted redundancy with the stored (or transmitted) data. Examples include ]s, ], ], ], ], ], ], ], ]s, and ]s.
or AYYLU ("Not clearly coded, repeat more clearly."). Codebook publishers proliferated.
Error detecting codes can be optimised to detect ''burst errors'', or ''random errors''.
The purpose of these codes was to save on cable costs.
Nowadays there are "codes" for ],
e.g. ], which uses short codes for frequent
symbols and longer codes for seldom used symbols - the same principle
is used in the ].


== Examples ==
==An example: the ASCII code==
=== Codes in communication used for brevity ===
{{Main|Brevity code}}
A cable code replaces words (e.g. ''ship'' or ''invoice'') with shorter words, allowing the same information to be sent with fewer ], more quickly, and less expensively.


Codes can be used for brevity. When ] messages were the state of the art in rapid long-distance communication, elaborate systems of ] that encoded complete phrases into single mouths (commonly five-minute groups) were developed, so that telegraphers became conversant with such "words" as ''BYOXO'' ("Are you trying to weasel out of our deal?"), ''LIOUY'' ("Why do you not answer my question?"), ''BMULD'' ("You're a skunk!"), or ''AYYLU'' ("Not clearly coded, repeat more clearly."). ]s were chosen for various reasons: ], ], etc. Meanings were chosen to fit perceived needs: commercial negotiations, military terms for military codes, diplomatic terms for diplomatic codes, any and all of the preceding for espionage codes. Codebooks and codebook publishers proliferated, including one run as a front for the American ] run by ] between the First and Second World Wars. The purpose of most of these codes was to save on cable costs. The use of data coding for ] predates the computer era; an early example is the telegraph ] where more-frequently used characters have shorter representations. Techniques such as ] are now used by computer-based ]s to compress large data files into a more compact form for storage or transmission.
Probably the most widely known data communications code (aka character representation) in use today is ].
In one or another (somewhat compatible) version, it is used by nearly all personal ]s, ]s,
]s, and other communication equipment.
Its original version represents 128 ]s with seven-bit binary numbers--that is, as a string of seven 1s and 0s.
In ASCII a lowercase "a" is always 1100001, an uppercase "A" always 1000001, and so on.
Extensions to ASCII have included 8-bit characters (for letters of European languages and such things
as card suit symbols), and in fullest flowering have included glyphs from essentially all of the
world's writing systems (see ]).


=== Character encodings ===
==Codes with redundancy to correct certain errors==
{{Main|Character encoding}}
Character encodings are representations of textual data. A given character encoding may be associated with a specific character set (the collection of characters which it can represent), though some character sets have multiple character encodings and vice versa. Character encodings may be broadly grouped according to the number of bytes required to represent a single character: there are single-byte encodings, ] (also called wide) encodings, and ] (also called variable-length) encodings. The earliest character encodings were single-byte, the best-known example of which is ]. ASCII remains in use today, for example in ]. However, single-byte encodings cannot model character sets with more than 256 characters. Scripts that require large character sets such as ] must be represented with multibyte encodings. Early multibyte encodings were fixed-length, meaning that although each character was represented by more than one byte, all characters used the same number of bytes ("word length"), making them suitable for decoding with a lookup table. The final group, variable-width encodings, is a subset of multibyte encodings. These use more complex encoding and decoding logic to efficiently represent large character sets while keeping the representations of more commonly used characters shorter or maintaining backward compatibility properties. This group includes ], an encoding of the ] character set; UTF-8 is the most common encoding of text media on the Internet.


=== Genetic code ===
Codes may also be used to represent data in a way more resistant
{{Main|Genetic code}}
to errors in transmission or storage. Such a "code" is
] organisms contain genetic material that is used to control their function and development. This is ], which contains units named ]s from which ] is derived. This in turn produces ]s through a ] in which a series of triplets (]s) of four possible ] can be translated into one of twenty possible ]s. A sequence of codons results in a corresponding sequence of amino acids that form a protein molecule; a type of codon called a ] signals the end of the sequence.
called an ], and works by including carefully crafted redundancy with the stored (or transmitted) data.


==Codes and acronyms== === Gödel code ===
In ], a ] is the basis for the proof of ]'s ]. Here, the idea is to map ] to a ] (using a ]).


=== Other ===
] and abbreviations can be considered codes, and in a sense all ]
There are codes using colors, like ], the ] employed to mark the nominal value of the ]s or that of the trashcans devoted to specific types of garbage (paper, glass, organic, etc.).
and writing systems are codes for human thought.
Occasionally a code word achieves an independent existence (and meaning) while the original equivalent
phrase is forgotten or at least no longer has the precise meaning attributed to the code word.
For example, the number "86" was once used as a code word in restaurants meaning "We're out of
the requested item".
It is now commonly used to mean the removal or destruction of something. '30' was widely used in journalism to mean "end of story", and it is sometimes used in other contexts to signify "the end".


In ], ] codes can be used for a financial discount or rebate when purchasing a product from a (usual internet) retailer.
----

In military environments, specific sounds with the ] are used for different uses: to mark some moments of the day, to command the infantry on the battlefield, etc.

Communication systems for sensory impairments, such as ] for deaf people and ] for blind people, are based on movement or tactile codes.

] are the most common way to encode ].

Specific games have their own code systems to record the matches, e.g. ].

=== Cryptography ===
In the ], ] were once common for ensuring the confidentiality of communications, although ]s are now used instead.

Secret codes intended to obscure the real messages, ranging from serious (mainly ] in military, diplomacy, business, etc.) to trivial (romance, games) can be any kind of imaginative encoding: ], game cards, clothes, fans, hats, melodies, birds, etc., in which the sole requirement is the pre-agreement on the meaning by both the sender and the receiver.

== Other examples ==
Other examples of encoding include:
*Encoding (in ]) - a basic perceptual process of interpreting incoming stimuli; technically speaking, it is a complex, multi-stage process of converting relatively objective sensory input (e.g., light, sound) into a subjectively meaningful experience.
*A ] - a specific encoding format for converting a specific type of ] to ].
*Text encoding uses a ] to tag the structure and other features of a text to facilitate processing by computers. (See also ].)
*] of formal language A informal language B is a method of representing all terms (e.g. programs or descriptions) of language A using language B.
*] transforms a signal into a code optimized for ] or ], generally done with a ].
*] - the way in which information is represented in ]s.
*] - the process of converting sensations into memories.
*]: ], ] and ]

Other examples of decoding include:
* ]
* ], methods in communication theory for decoding codewords sent over a noisy channel
* ], the study of signals in a digital representation and the processing methods of these signals
* ], the use of analog circuit for decoding operations
* Word decoding, the use of ] to decipher print patterns and translate them into the sounds of language

==Codes and acronyms==
]s and abbreviations can be considered codes, and in a sense, all ]s and ]s are codes for human thought.


]s are three-letter codes used to designate airports and used for ]s. ]s are similarly used on railways but are usually national, so the same code can be used for different stations if they are in different countries.
In ], the word '''code''' refers to instructions to a computer
in a ].
In this usage, the noun "code" typically stands for ],
and the verb "to code" means to write source code, to program. This usage may have originated
when the first symbolic languages were developed and were punched onto cards as "codes".


Occasionally, a code word achieves an independent existence (and meaning) while the original equivalent phrase is forgotten or at least no longer has the precise meaning attributed to the code word. For example, '30' was widely used in ] to mean "end of story", and has been used in ] to signify "the end".<ref>Kogan, Hadass {{webarchive|url=https://web.archive.org/web/20101212101705/http://ajr.org/Article.asp?id=4408 |date=2010-12-12 }} American Journalism Review. Retrieved 2012-07-03.</ref>
Engineers often use the word "code" to mean a single program. They might say "I wrote a code" or "I have two codes". No software engineer or computer scientist would say that. They would say "I wrote some code" or "I have two programs".
<ref>{{cite web
|title = Western Union "92 Code" & Wood's "Telegraphic Numerals"
|publisher = Signal Corps Association
|year = 1996
|url = http://www.civilwarsignals.org/pages/tele/wurules1866/92code.html
|access-date = 2012-07-03
|url-status = live
|archive-url = https://web.archive.org/web/20120509135118/http://www.civilwarsignals.org/pages/tele/wurules1866/92code.html
|archive-date = 2012-05-09
}}</ref>


== See also ==
----
{{Commons category|Codes}}
In ] a code is often just a mere synonym for a variety of a language (dialect, sociolect). The term has some technical connotation.
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]


== References ==
----
{{reflist}}
* {{cite journal |last1=Chevance |first1=Fabienne |title=Case for the genetic code as a triplet of triplets |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=114 |issue=18 |pages=4745–4750 |pmc=5422812 |year=2017 |pmid=28416671 |doi=10.1073/pnas.1614896114 |doi-access=free |bibcode=2017PNAS..114.4745C }}


==Further reading==
A '''code''' is also a rule or a set of rules, such as code of honor, code of laws, or dress code. See ].
* {{cite book |date=1963 |title=Codes and Abbreviations for the Use of the International Telecommunication Services |edition=2nd |location=Geneva, Switzerland |publisher=International Telecommunication Union |oclc=13677884}}


]
This word has acquired a large number of subtly, and grossly, incompatible meanings, particularly in cryptographic contexts.
]
Use it with care.

Latest revision as of 11:57, 26 December 2024

System of rules to convert information into another form or representation For other uses, see Code (disambiguation). "Encoding" redirects here. For other uses, see Encoding (disambiguation). For technical reasons, terms beginning with "Code#" redirect here. For the EPs by Ladies' Code, see Code 01 Bad Girl and Code 02 Pretty Pretty.
This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
Find sources: "Code" – news · newspapers · books · scholar · JSTOR (March 2010) (Learn how and when to remove this message)

In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication channel or storage in a storage medium. An early example is an invention of language, which enabled a person, through speech, to communicate what they thought, saw, heard, or felt to others. But speech limits the range of communication to the distance a voice can carry and limits the audience to those present when the speech is uttered. The invention of writing, which converted spoken language into visual symbols, extended the range of communication across space and time.

The process of encoding converts information from a source into symbols for communication or storage. Decoding is the reverse process, converting code symbols back into a form that the recipient understands, such as English or/and Spanish.

One reason for coding is to enable communication in places where ordinary plain language, spoken or written, is difficult or impossible. For example, semaphore, where the configuration of flags held by a signaler or the arms of a semaphore tower encodes parts of the message, typically individual letters, and numbers. Another person standing a great distance away can interpret the flags and reproduce the words sent.

Theory

Main article: Coding theory

In information theory and computer science, a code is usually considered as an algorithm that uniquely represents symbols from some source alphabet, by encoded strings, which may be in some other target alphabet. An extension of the code for representing sequences of symbols over the source alphabet is obtained by concatenating the encoded strings.

Before giving a mathematically precise definition, this is a brief example. The mapping

C = { a ↦ 0 , b ↦ 01 , c ↦ 011 } {\displaystyle C=\{\,a\mapsto 0,b\mapsto 01,c\mapsto 011\,\}}

is a code, whose source alphabet is the set { a , b , c } {\displaystyle \{a,b,c\}} and whose target alphabet is the set { 0 , 1 } {\displaystyle \{0,1\}} . Using the extension of the code, the encoded string 0011001 can be grouped into codewords as 0 011 0 01, and these in turn can be decoded to the sequence of source symbols acab.

Using terms from formal language theory, the precise mathematical definition of this concept is as follows: let S and T be two finite sets, called the source and target alphabets, respectively. A code C : S → T ∗ {\displaystyle C:\,S\to T^{*}} is a total function mapping each symbol from S to a sequence of symbols over T. The extension C ′ {\displaystyle C'} of C {\displaystyle C} , is a homomorphism of S ∗ {\displaystyle S^{*}} into T ∗ {\displaystyle T^{*}} , which naturally maps each sequence of source symbols to a sequence of target symbols.

Variable-length codes

Main article: Variable-length code

In this section, we consider codes that encode each source (clear text) character by a code word from some dictionary, and concatenation of such code words give us an encoded string. Variable-length codes are especially useful when clear text characters have different probabilities; see also entropy encoding.

A prefix code is a code with the "prefix property": there is no valid code word in the system that is a prefix (start) of any other valid code word in the set. Huffman coding is the most known algorithm for deriving prefix codes. Prefix codes are widely referred to as "Huffman codes" even when the code was not produced by a Huffman algorithm. Other examples of prefix codes are country calling codes, the country and publisher parts of ISBNs, and the Secondary Synchronization Codes used in the UMTS WCDMA 3G Wireless Standard.

Kraft's inequality characterizes the sets of codeword lengths that are possible in a prefix code. Virtually any uniquely decodable one-to-many code, not necessarily a prefix one, must satisfy Kraft's inequality.

Error-correcting codes

Main article: Error detection and correction See also: Block code

Codes may also be used to represent data in a way more resistant to errors in transmission or storage. This so-called error-correcting code works by including carefully crafted redundancy with the stored (or transmitted) data. Examples include Hamming codes, Reed–Solomon, Reed–Muller, Walsh–Hadamard, Bose–Chaudhuri–Hochquenghem, Turbo, Golay, algebraic geometry codes, low-density parity-check codes, and space–time codes. Error detecting codes can be optimised to detect burst errors, or random errors.

Examples

Codes in communication used for brevity

Main article: Brevity code

A cable code replaces words (e.g. ship or invoice) with shorter words, allowing the same information to be sent with fewer characters, more quickly, and less expensively.

Codes can be used for brevity. When telegraph messages were the state of the art in rapid long-distance communication, elaborate systems of commercial codes that encoded complete phrases into single mouths (commonly five-minute groups) were developed, so that telegraphers became conversant with such "words" as BYOXO ("Are you trying to weasel out of our deal?"), LIOUY ("Why do you not answer my question?"), BMULD ("You're a skunk!"), or AYYLU ("Not clearly coded, repeat more clearly."). Code words were chosen for various reasons: length, pronounceability, etc. Meanings were chosen to fit perceived needs: commercial negotiations, military terms for military codes, diplomatic terms for diplomatic codes, any and all of the preceding for espionage codes. Codebooks and codebook publishers proliferated, including one run as a front for the American Black Chamber run by Herbert Yardley between the First and Second World Wars. The purpose of most of these codes was to save on cable costs. The use of data coding for data compression predates the computer era; an early example is the telegraph Morse code where more-frequently used characters have shorter representations. Techniques such as Huffman coding are now used by computer-based algorithms to compress large data files into a more compact form for storage or transmission.

Character encodings

Main article: Character encoding

Character encodings are representations of textual data. A given character encoding may be associated with a specific character set (the collection of characters which it can represent), though some character sets have multiple character encodings and vice versa. Character encodings may be broadly grouped according to the number of bytes required to represent a single character: there are single-byte encodings, multibyte (also called wide) encodings, and variable-width (also called variable-length) encodings. The earliest character encodings were single-byte, the best-known example of which is ASCII. ASCII remains in use today, for example in HTTP headers. However, single-byte encodings cannot model character sets with more than 256 characters. Scripts that require large character sets such as Chinese, Japanese and Korean must be represented with multibyte encodings. Early multibyte encodings were fixed-length, meaning that although each character was represented by more than one byte, all characters used the same number of bytes ("word length"), making them suitable for decoding with a lookup table. The final group, variable-width encodings, is a subset of multibyte encodings. These use more complex encoding and decoding logic to efficiently represent large character sets while keeping the representations of more commonly used characters shorter or maintaining backward compatibility properties. This group includes UTF-8, an encoding of the Unicode character set; UTF-8 is the most common encoding of text media on the Internet.

Genetic code

Main article: Genetic code

Biological organisms contain genetic material that is used to control their function and development. This is DNA, which contains units named genes from which messenger RNA is derived. This in turn produces proteins through a genetic code in which a series of triplets (codons) of four possible nucleotides can be translated into one of twenty possible amino acids. A sequence of codons results in a corresponding sequence of amino acids that form a protein molecule; a type of codon called a stop codon signals the end of the sequence.

Gödel code

In mathematics, a Gödel code is the basis for the proof of Gödel's incompleteness theorem. Here, the idea is to map mathematical notation to a natural number (using a Gödel numbering).

Other

There are codes using colors, like traffic lights, the color code employed to mark the nominal value of the electrical resistors or that of the trashcans devoted to specific types of garbage (paper, glass, organic, etc.).

In marketing, coupon codes can be used for a financial discount or rebate when purchasing a product from a (usual internet) retailer.

In military environments, specific sounds with the cornet are used for different uses: to mark some moments of the day, to command the infantry on the battlefield, etc.

Communication systems for sensory impairments, such as sign language for deaf people and braille for blind people, are based on movement or tactile codes.

Musical scores are the most common way to encode music.

Specific games have their own code systems to record the matches, e.g. chess notation.

Cryptography

In the history of cryptography, codes were once common for ensuring the confidentiality of communications, although ciphers are now used instead.

Secret codes intended to obscure the real messages, ranging from serious (mainly espionage in military, diplomacy, business, etc.) to trivial (romance, games) can be any kind of imaginative encoding: flowers, game cards, clothes, fans, hats, melodies, birds, etc., in which the sole requirement is the pre-agreement on the meaning by both the sender and the receiver.

Other examples

Other examples of encoding include:

Other examples of decoding include:

Codes and acronyms

Acronyms and abbreviations can be considered codes, and in a sense, all languages and writing systems are codes for human thought.

International Air Transport Association airport codes are three-letter codes used to designate airports and used for bag tags. Station codes are similarly used on railways but are usually national, so the same code can be used for different stations if they are in different countries.

Occasionally, a code word achieves an independent existence (and meaning) while the original equivalent phrase is forgotten or at least no longer has the precise meaning attributed to the code word. For example, '30' was widely used in journalism to mean "end of story", and has been used in other contexts to signify "the end".

See also

References

  1. Kogan, Hadass "So Why Not 29" Archived 2010-12-12 at the Wayback Machine American Journalism Review. Retrieved 2012-07-03.
  2. "Western Union "92 Code" & Wood's "Telegraphic Numerals"". Signal Corps Association. 1996. Archived from the original on 2012-05-09. Retrieved 2012-07-03.

Further reading

  • Codes and Abbreviations for the Use of the International Telecommunication Services (2nd ed.). Geneva, Switzerland: International Telecommunication Union. 1963. OCLC 13677884.
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