Code

In communications and information processing, a 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 of a code is language, which enables a person, through speech, to communicate what they think, see, hear, or feel 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.

One role of 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.

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

{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, is a homomorphism of

A character encoding describes how character-based data (text) is encoded. Antiquated encoding systems used a fixed number of bits, ranging from 4 to 7, but modern systems use one or more 8-bit bytes for each character. ASCII, the dominant system for decades, uses one byte for each character, and therefore, can encode up to 256 different characters. To support natural languages with more characters, other systems were invented that use more than one byte or a variable number of bytes for each character. A writing system with a large character set such as Chinese, Japanese and Korean can be represented with a multibyte encoding. Early multibyte encodings were fixed-length, meaning that each character is represented by the same number of bytes, making them suitable for decoding via a lookup table. On the other hand, a variable-width encoding is more complex to decode since it cannot be decoded via a single lookup table and must be processed sequentially, but it supports a more efficient representation of a large character set by using a smaller representation for more commonly used characters. Today, UTF-8, an encoding of the Unicode character set, is the most common text encoding used on the Internet.

Genetic code

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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