LIS512 lecture 3 numbers and characters

1. Thomas Krichel 2010 – 02 – 17 LIS512 lecture 3numbers and characters

2. structure • numbers • numeric information • character information • the ASCII set • Unicode • encoding • coda • ligatures • collations • transliterations

3. introduction • We have seen that databases store records. • Records contain fields, fields have values. • Here we talk about fundamentally, how do we compose those values. • Numerical values are easy • String values are harder

4. literature • The library textbooks are hopelessly short and confused about this topic. • I have most of what I have here from my own experience. • I recommend Wikipedia, it has fascinating articles about these topics.

5. all gone to a number • In all modern information system, information is stored to be treated on a computer. • A computer can only deal with numbers. • As a consequence all information has to be converted into a number. • It's a huge job. • Let’s look at the ground, numbers.

6. a bit • A bit is the elementary unit of information. • It takes a binary value. We can label it true/false, black/white, +/-, etc. • Every piece of information in all modern information storage systems has to be reduced to a sequence of bits. • We will denote them 0/1 here.

7. byte • A byte is a sequence of 8 bits. '00000000' to '11111111'. There are 2 to the power 8, meaning 256 possibilities to write a byte. • If the byte is required to start with 0, then we can only write '0000000' to '01111111'. This leaves us with 2 to the power 7, meaning 128 possibilities.

8. hex numbers • Hex numbers contain the usual digits 0 to 9, as well as A to F. A means 10, B means 11, etc F means 15. • One hex number can represent 2 to the power 4, meaning 16 possibilities (0 to 15). • Two hex numbers can represent 2 to the power 8 possibilities.

9. bytes and hex numbers • Since two hex numbers convene the same number of possibilities as a byte a byte is often represented as two hex numbers. • Thus, for example • '00000000' in binary is 00 in hex, • '11111111' in binary is 'FF' in hex, • '01111111' in binary is ‘7F‘ in hex

10. converting information to numbers • A lot of problem in converting information comes from some part of the information encode in some form and some other part in some other from. • Example: “15 Julliet 1923” vs “July 17, 1923” • Often such inconsistencies require manual reformatting, which is very expensive.

11. numerical information • Some information can be converted to a number using a simple conversion. • Examples: • A recent point in time is often converted into a number by taking the number of seconds since the first of January 1970. • A date is often written as an ISO date in the form yyyymmdd. yyyy in the year, mm is the month and ddthe day with leading 0s.

12. numerizing • In the design of every information system, it is a good idea to convert information into something that is directly a number. • There are examples where it is possible directly use a number, such as • colours • times and dates • locations.

13. another hex number example • Colors on the world wide web follow the red/green/blue color model. • Each color is given as a number #rrggbb, where rr is the amount of red gg is the amount of green and bb in the amount of blue. All these numbers are hex numbers. Example • #FFFFFF white • #00FFFF aqua

14. non-numerical information • A lot of information is not numerical by its nature. For example • the name of a person • the title of an expression of a work • The information is of a character string nature. • To store character strings in an information system, each character has to be converted to a number.

15. character • A character is an indivsible unit of textual information. • Textual information is composed of characters, and nothing else.

16. characters and computer • Computers can not deal with characters directly. They can only deal with numbers. • There we need to associate a number with every character that we want to use in an information encoding system. • A character set combines characters with number.

17. ASCII • ASCII is an old character set developed in the United States. It is a seven bit character set. • In hex notation, it goes from '00' to '7F' • Because Anglo-Saxon cultural imperialism, the first 128 characters in Unicode are the same as in ASCII.

18. notable characters in ASCII decimal hex byte • 8 8 08 U+0008 backspace • 9 9 09 U+0009 horizontal tab • 10 A 0A U+000A line feed • 13 D 0D U+000D carriage return • 32 20 20 U+0020 space • 127 7F 7F U+007F delete

19. wikipedia notation • Wikipedia denotes every character in the BMP as U+hhhh where h is a hex digit 0-F. • We will follow this notation here.

20. UCS / Unicode • UCS is a universal character set. • It is maintained by the International Standards Organization. • Unicode is an industry standard for characters. It is better documented than UCS. • For what we discuss here, UCS and Unicode are the same.

21. Basic multilingual plane • This is a name for the first 65536 characters in Unicode. • Each of these characters fits into two bytes and is conveniently represented by four hex numbers. • Even for these characters, there are numerous complications associated with them.

22. dashes • figure dash ‒ U+2012 to link numbers without a range • en dash – U+2013 to link numbers with a range • em dash — U+2014 for interjections in a sentence • minus sign − U+2212 for mathematics

23. “smart” quotes • U+201c “ is the opening double quote • U+201d ” is the closing • U+2019 ’ is the apostrophe • The single quote of the ASCII character set is considered to be of mixed usage, it should be avoided when a specific use can be done. • Similarly, the double quote of the ASCII character set is imprecise.

24. spaces • non-breaking space, U+00A0 is used when you want to avoid a line break between the two spaced items. For example in hyperlink text, it is good practice to replace spaces with non-breaking spaces as to avoid there appearing to be two links. • In whitespace collapsing contents, it can also be use to add extra space.

25. beyond ascii, foreign languages • Everything becomes difficult. • As an example consider the characters • o • ő • ö • The latter two can be considered o with diarcitics or as separate characters.

26. most problematic: encoding • One issue is how to map characters to numbers. • This is complicated for languages other than English. • But assume UCS/Unicode has solved this. • But this is not the main problem that we have when working

27. encoding • The encoding determines how the numbers of each character should be put into bytes. • If you have a character set that is has one byte for each character, you have no encoding issue. • But then you are limited to 256 characters in your character set.

28. fixed-length encoding • If you have a fixed length encoding, all characters take the same number of bytes. • Say for the basic-multilingual plane of unicode, you need two bytes for each character, and then you are limited to that. • If you are writing only ASCII, it appears a waste.

29. variable length encoding • The most widely used scheme to encode Unicode is a variable length scheme, called UTF-8. • I will leave out the technical details on how this is. • But it is important to understand that the encoding needs to known and correct.

30. ligature • In fine traditional typography, certain characters appear to be linked to each other. • The most command examples in English usage are fi, ff, fl, ffi, ffl.

31. ligatures growing up • In certain cases, ligatures have become so common that they have become characters of their own. • A prominent example is the German sz ligature the esszet. It looks a bit like a beta because it is derived from the fraktur font of the characters. • Another example, apparently, is &.

32. collations • Collations are topic that is related to characters. • A collation is a sorting order of character strings. • You may think this is trivial, just follow the alphabetic order. • But in many languages, diacritics come to complicate matters.

33. example German • Here are the extra letter of German: Ä/ä, Ö/ö, Ü/ü, ß • In German, there are two collations. • DIN 5007-1 “dictionary collation” treats umlauted characters as if they did not have them, and ß as s. • DIN 5007-2 “phonebook collation” treats umlauted as letter and e (ex. ä --> ae), and ß as ss

34. transliterations • When non-English characters are supposed to be entered in a system used by English speaking people, a transliteration might be used. • This can also be the case if the original script may not be commonly understood. An example are Japanese road sign. • Wikipedia lists 20 different ways to do that for Russian, say. Library of Congress scheme is apparently the most widely used.

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