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| In ], a '''code''' is a rule for converting a piece of information (for example, | In ], a '''code''' is a rule for converting a piece of information (for example, | ||
| a letter, word, or phrase) into another object or action, not necessarily of the same sort. | a letter, word, or phrase) into another object or action, not necessarily of the same sort. | ||
| One reason for this is to enable communication in places where ordinary spoken or written language | One reason for this is to enable communication in places where ordinary spoken or written language | ||
| is difficult or impossible. | is difficult or impossible. | ||
| 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. | |||
| For example, ] converts patterns of dots and dashes typed on a telegraph key into | |||
| letters of the alphabet, allowing text to be sent over telegraph wires, flashing lights, or other channels | |||
| that cannot carry speech or written text directly. | |||
| Another example is the use of ], where the configuration of flags | Another example is the use of ], where the configuration of flags | ||
| held by a signaller or the arms of a semaphore tower encodes the message. | held by a signaller or the arms of a semaphore tower encodes the message. | ||
| Another person standing a great distance away can interpret the flags and reproduce the letters, | Another person standing a great distance away can interpret the flags and reproduce the words sent. Semaphore operations which use letters not words and Morse code are not codes, but ]s. | ||
| enabling the signaller to communicate with someone too far away to hear ordinary speech. | |||
| Codes can be used for brevity. | Codes can be used for brevity. | ||
| In the days when |
In the days when telegraph messages were the state of the art, elaborate commercial codes that encoded complete phrases | ||
| into single words (five-letter groups) were developed, so that telegraphers became conversant with | into single words (typically five-letter groups) were developed, so that telegraphers became conversant with | ||
| such "words" as BYOXO ("Are you trying to crawl out of it?"), LIOUY ("Why do you not answer my question?"), | such "words" as BYOXO ("Are you trying to crawl out of it?"), LIOUY ("Why do you not answer my question?"), | ||
| or AYYLU ("Not clearly coded, repeat more clearly."). | |||
| The purpose of these codes was to save on cable costs. | The purpose of these codes was to save on cable costs. | ||
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| Extensions to ASCII have included 8-bit characters (for letters of European languages and such things | 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 | as card suit symbols), and in fullest flowering have included glyphs from essentially all of the | ||
| world's writing systems (see ]). | world's writing systems (see ]). Stictly, however, ASCII is a cipher, as it converts printable characters into bit patterns without regard to their meaning. | ||
| Another use of codes is ], where a code or ] is used to disguise a message, | Another use of codes is ], where a code or ] is used to disguise a message, | ||
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| One simple method is to use a "codebook" with a list of common phrases replaced by phrases with | 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 | 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. | hear a conversation that is entirely unrelated to the real intended message. A cable code, of which only the sender and receipient have copies, would be one such. | ||
| ] and abbreviations can be considered codes, and in a sense all ] | ] and abbreviations can be considered codes, and in a sense all ] | ||
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| For example, the number "86" was once used as a code word in restaurants meaning "We're out of | For example, the number "86" was once used as a code word in restaurants meaning "We're out of | ||
| the requested item". | the requested item". | ||
| It is now commonly used to mean the removal or destruction of something. | 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'. | ||
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| A '''code''' is a also rule or a set of rules, such as code of honor, code of laws, or dress code. | A '''code''' is a also rule or a set of rules, such as code of honor, code of laws, or dress code. | ||
| This word has acquired a large number of subtly and grossly incompatible meanings. | This word has acquired a large number of subtly and grossly incompatible meanings, particularly in cryptographic contexts. | ||
| Use with care. | Use it with care. | ||
Revision as of 12:57, 7 June 2002
In communications, a code is a rule for converting a piece of information (for example, a letter, word, or phrase) into another object or action, not necessarily of the same sort. One reason for this is to enable communication in places where ordinary spoken or written language is difficult or impossible. 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 semaphore flags, where the configuration of flags held by a signaller or the arms of a semaphore tower encodes the message. Another person standing a great distance away can interpret the flags and reproduce the words sent. Semaphore operations which use letters not words and Morse code are not codes, but ciphers.
Codes can be used for brevity. In the days when telegraph messages were the state of the art, elaborate commercial codes that encoded complete phrases into single words (typically five-letter groups) were developed, so that telegraphers became conversant with such "words" as BYOXO ("Are you trying to crawl out of it?"), LIOUY ("Why do you not answer my question?"), or AYYLU ("Not clearly coded, repeat more clearly."). The purpose of these codes was to save on cable costs.
Probably the most widely known communications code in use today is ASCII. It is employed by nearly all personal computers, terminals, printers, and other communication equipment. It represents 128 characters 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 Unicode). Stictly, however, ASCII is a cipher, as it converts printable characters into bit patterns without regard to their meaning.
Another use of codes is cryptography, where a code or cipher 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. A cable code, of which only the sender and receipient have copies, would be one such.
Acronyms and abbreviations can be considered codes, and in a sense all languages 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 computer programming, the word code refers to instructions to a computer in a programming language. In this usage, the noun "code" typically stands for source code, 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".
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".
A code is a also rule or a set of rules, such as code of honor, code of laws, or dress code.
This word has acquired a large number of subtly and grossly incompatible meanings, particularly in cryptographic contexts. Use it with care.