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International Characters Support: Difference between revisions
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Fix "ANSI" vs "ASCII" and some typos, identify "surrogate pair"
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AndrewJanke (talk | contribs) (Fix "ANSI" vs "ASCII" and some typos, identify "surrogate pair") |
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=ANSI= | =ANSI= | ||
The first widely used character set was 7- | The first widely used character set was 7-bit ASCII, with values ranging from 0 to 127. Being developed for English, it uses a Latin character set, but without accents or other punctuation signs (diacritical marks). | ||
In the '80s, extensions were provided by using 8- | In the '80s, extensions were provided by using 8-bit character tables, whose code numbers 128 to 255 were used to encode the missing values. But there were so many characters that those additional 128 values were not enough. So a number of maps (code pages) where defined. For instance, ISO-8859-1 for Western European Languages, with letters for French: é, German: ä, Nordic languages: Ø, a few math symbols: °, µ, ½, and so on. | ||
Typical computer support consisted | Typical computer support consisted of loading the adequate character map beforehand; then glyphs were rendered correctly. | ||
The first issue with this approach is about conversion. To view some text in Greek or Cyrillic language on a display configured for Western European requires switching back and forth between code pages. | The first issue with this approach is about conversion. To view some text in Greek or Cyrillic language on a display configured for Western European requires switching back and forth between code pages. | ||
=Unicode= | =Unicode= | ||
Unicode is a standard and an effort to encode symbols from every language existing or having existed on Earth. There are actually | Unicode is a standard and an effort to encode symbols from every language existing or having existed on Earth. There are actually 190,000 signs from 93 languages. Unicode is equivalent to ISO/CEI 10646. Unicode consists of: | ||
* a table of symbols, each with an unique name, like "GREEK SMALL LETTER ALPHA" for α | * a table of symbols, each with an unique name, like "GREEK SMALL LETTER ALPHA" for α | ||
* encoding norms: UTF-16, UTF-32, UTF-8 | * encoding norms: UTF-16, UTF-32, UTF-8 | ||
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=Storage and binary representation= | =Storage and binary representation= | ||
* UTF-32 stores each | * UTF-32 stores each symbol in 4 bytes according to two schemes: Big Endian and Little Endian | ||
* UTF-16 stores most of its symbols | * UTF-16 stores most of its symbols in 2 bytes; rarely used values are stored using a sequence of "prefix-value" called a "surrogate pair". Two schemes: Big Endian and Little Endian | ||
* UTF-8 was designed to be mostly compatible with ASCII; | * UTF-8 was designed to be mostly compatible with ASCII; symbol storage is either 1, 2, 3, 4 bytes. This scheme is defined sequentially; there is no ambiguity linked to its endianness. | ||
=C and C++ support= | =C and C++ support= | ||
There are three types of "char" in C and C++: plain (unqualified), signed and unsigned. The two latters were added to have similar behavior as 'int', as chars may be used to store small numbers. The standard says: | There are three types of "char" in C and C++: plain (unqualified), signed and unsigned. The two latters were added to have similar behavior as 'int', as chars may be used to store small numbers. The [http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1905.pdf C++ standard] says: | ||
3.9.1 Fundamental types [basic.fundamental] | 3.9.1 Fundamental types [basic.fundamental] | ||
Objects declared as characters char) shall be large enough to store any member of the implementation's basic character set. If a character from this set is stored in a character object, the integral value of that character object is equal to the value of the single character literal form of that character. It is implementation-defined whether a char object can hold negative values. Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types. A char, a signed char, and an unsigned char occupy the same amount of storage and have the same alignment requirements (basic.types); that is, they have the same object representation. For character types, all bits of the object representation participate in the value representation. For unsigned character types, all possible bit patterns of the value representation represent numbers. These requirements do not hold for other types. In any particular implementation, a plain char object can take on either the same values as a signed char or an unsigned char; which one is implementation-defined. | <q>Objects declared as characters (char) shall be large enough to store any member of the implementation's basic character set. If a character from this set is stored in a character object, the integral value of that character object is equal to the value of the single character literal form of that character. It is implementation-defined whether a char object can hold negative values. Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types. A char, a signed char, and an unsigned char occupy the same amount of storage and have the same alignment requirements (basic.types); that is, they have the same object representation. For character types, all bits of the object representation participate in the value representation. For unsigned character types, all possible bit patterns of the value representation represent numbers. These requirements do not hold for other types. In any particular implementation, a plain char object can take on either the same values as a signed char or an unsigned char; which one is implementation-defined.</q> | ||
What is important here is that usual characters should be declared as "chars" or "signed chars". "Unsigned char" means they MAY be submitted to truncation of the eighth bit, this is implementation-dependant. | What is important here is that usual characters should be declared as "chars" or "signed chars". "Unsigned char" means they MAY be submitted to truncation of the eighth bit, this is implementation-dependant. | ||
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In order to support wide-characters, the two-byte storage wchar_t was added to the C standard. Functions whose argument is wchar instead of char are generally prefixed by "w". | In order to support wide-characters, the two-byte storage wchar_t was added to the C standard. Functions whose argument is wchar instead of char are generally prefixed by "w". | ||
= | =Character functions= | ||
Some of the basic functions about characters strings are listed below: | Some of the basic functions about characters strings are listed below: | ||
* length: how many symbols ? | * length: how many symbols ? | ||
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=Cons= | =Cons= | ||
* UTF-8: not all sequences are valid | * UTF-8: not all sequences are valid | ||
* UTF-16: not all sequences are valid. | * UTF-16: not all sequences are valid. Compared to ASCII, memory waste is 50%. While transmitting strings containing Western text, around half of the chars are typically zeros. | ||
* UTF-32: not all sequences are valid. | * UTF-32: not all sequences are valid. Compared to ASCII, memory waste is 75%. While transmitting strings containing Western text, around three-quarters of the chars are typically zeros. | ||
=Octave and international support= | =Octave and international support= | ||
== Graphs == | == Graphs == | ||
Adding special symbols to graph title, labels, legend, tick marks and inside graphs (e.g. text) | Adding special symbols to graph title, labels, legend, tick marks and inside graphs (e.g. text). | ||
== Locales == | == Locales == | ||
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* short term: tests to ensure every string processing is 8-bit clean | * short term: tests to ensure every string processing is 8-bit clean | ||
* middle and long term: there are a number of options to fully support whatever symbol existing in Unicode: | * middle and long term: there are a number of options to fully support whatever symbol existing in Unicode: | ||
** make use of C | ** make use of C wchar type | ||
** make use of ICU [http://site.icu-project.org/], an open-source lib with various Unicode support functions | ** make use of ICU [http://site.icu-project.org/], an open-source lib with various Unicode support functions | ||
[[Category:Development]] | [[Category:Development]] | ||