Incommunications andinformation processing,code is a system of rules to convertinformation—such as aletter,word, sound, image, orgesture—into another form, sometimesshortened orsecret, for communication through acommunication channel or storage in astorage medium. An early example is an invention oflanguage, which enabled a person, throughspeech, 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 ofwriting, which converted spoken language intovisualsymbols, extended the range of communication across space andtime.
The process ofencoding converts information from asource 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 ordinaryplain language, spoken or written, is difficult or impossible. For example,semaphore, where the configuration offlags held by a signaler or the arms of asemaphore 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.
Ininformation theory andcomputer science, a code is usually considered as analgorithm that uniquely representssymbols from some sourcealphabet, byencoded 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
is a code, whose source alphabet is the set and whose target alphabet is the set. 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 symbolsacab.
Using terms fromformal language theory, the precise mathematical definition of this concept is as follows: let S and T be two finite sets, called the source and targetalphabets, respectively. Acode is atotal function mapping each symbol from S to asequence of symbols over T. Theextension of, is ahomomorphism of into, which naturally maps each sequence of source symbols to a sequence of target symbols.
In this section, we consider codes that encode each source (clear text) character by acode word from some dictionary, andconcatenation of such code words give us an encoded string. Variable-length codes are especially useful when clear text characters have different probabilities; see alsoentropy encoding.
Aprefix code is a code with the "prefix property": there is no valid code word in the system that is aprefix (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 arecountry calling codes, the country and publisher parts ofISBNs, and the Secondary Synchronization Codes used in theUMTSWCDMA 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.
A cable code replaces words (e.g.ship orinvoice) with shorter words, allowing the same information to be sent with fewercharacters, more quickly, and less expensively.
Codes can be used for brevity. Whentelegraph messages were the state of the art in rapid long-distance communication, elaborate systems ofcommercial codes that encoded complete phrases into single mouths (commonly five-minute groups) were developed, so that telegraphers became conversant with such "words" asBYOXO ("Are you trying to weasel out of our deal?"),LIOUY ("Why do you not answer my question?"),BMULD ("You're a skunk!"), orAYYLU ("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 AmericanBlack Chamber run byHerbert 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 fordata compression predates the computer era; an early example is the telegraphMorse code where more-frequently used characters have shorter representations. Techniques such asHuffman coding are now used by computer-basedalgorithms to compress large data files into a more compact form for storage or transmission.
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, andvariable-width (also called variable-length) encodings. The earliest character encodings were single-byte, the best-known example of which isASCII. ASCII remains in use today, for example inHTTP headers. However, single-byte encodings cannot model character sets with more than 256 characters. Scripts that require large character sets such asChinese, 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 includesUTF-8, an encoding of theUnicode character set; UTF-8 is the most common encoding of text media on the Internet.
Biological organisms contain genetic material that is used to control their function and development. This isDNA, which contains units namedgenes from whichmessenger RNA is derived. This in turn producesproteins through agenetic code in which a series of triplets (codons) of four possiblenucleotides can be translated into one of twenty possibleamino acids. A sequence of codons results in a corresponding sequence of amino acids that form a protein molecule; a type of codon called astop codon signals the end of the sequence.
There are codes using colors, liketraffic lights, thecolor code employed to mark the nominal value of theelectrical resistors or that of the trashcans devoted to specific types of garbage (paper, glass, organic, etc.).
Inmarketing,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 thecornet 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 assign language for deaf people andbraille for blind people, are based on movement or tactile codes.
Specific games have their own code systems to record the matches, e.g.chess notation.
Cryptography
In thehistory of cryptography,codes were once common for ensuring the confidentiality of communications, althoughciphers are now used instead.
Secret codes intended to obscure the real messages, ranging from serious (mainlyespionage 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:
Encoding (incognition) - 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.
Text encoding uses amarkup language to tag the structure and other features of a text to facilitate processing by computers. (See alsoText Encoding Initiative.)
Semantics encoding 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.
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 injournalism to mean "end of story", and has been used inother contexts to signify "the end".[1][2]
Codes and Abbreviations for the Use of the International Telecommunication Services (2nd ed.). Geneva, Switzerland: International Telecommunication Union. 1963.OCLC13677884.
