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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 or representation, sometimes shortened or secret, for communication through a channel or storage in a medium. An early example is the invention of language, which enabled a person, through speech, to communicate what he or she saw, heard, felt, or thought 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 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 signaller 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 == (詳細はinformation theory and computer science, a code is usually considered as an algorithm which 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, we give 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 0011001011 can be grouped into codewords as 0 011 0 01 011, and these in turn can be decoded to the sequence of source symbols ''acabc''. 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 is a total function mapping each symbol from S to a sequence of symbols over T, and the extension of to a homomorphism of into , which naturally maps each sequence of source symbols to a sequence of target symbols, is referred to as its extension. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「code」の詳細全文を読む スポンサード リンク
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