# lect_02 - £ £ Lecture 2 Strings Languages DFA s 24...

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Unformatted text preview: £ : £ Lecture 2: Strings, Languages, DFA s 24 January 2010 This lecture covers material on strings and languages from Sipser chapter 0. Also, this lecture covers an account of countable and uncountable sets, and shows that C-programs cannot decide all languages. 1 Alphabets, strings, and languages 1.1 Alphabets An alphabet is any nite set of characters. Here are some examples for such alphabets: (i) { , 1 } . (ii) { a , b , c } . (iii) { , 1 , # } . (iv) { a ,... z , A ,... Z } : all the letters in the English language. (v) ASCII - this is the standard encoding schemes used by computers mappings bytes (i.e., integers in the range .. 255 ) to characters. As such, a is 65 , and the space character is 32 . (vi) { moveforward , moveback , rotate90 , reset } . 1.2 Strings This section should be recapping stu already seen in discussion section 1. A string over an alphabet Σ is a nite sequence of characters from Σ . Some sample strings with alphabet (say) Σ = { a , b , c } are abc , baba , and aaaabbbbccc . The length of a string x is the number of characters in x , and it is denoted by | x | . Thus, the length of the string w = abcdef is | w | = 6 . The empty string is denoted by , and it (of course) has length . The empty string is the string containing zero characters in it. The concatenation of two strings x and w is denoted by xw , and it is the string formed by the string x followed by the string w . As a concrete example, consider x = cat , w = nip and the concatenated strings xw = catnip and wx = nipcat . Naturally, concatenating with the empty string results in no change in the string. For- mally, for any string x , we have that x = x . As such = . 1 For a string w , the string x is a substring of w if the string x appears contiguously in w . As such, for w = abcdef we have that bcd is a substring of w, but ace is not a substring of w. A string x is a su x of w if its a substring of w appearing in the end of w . Similarly, y is a pre x of w if y is a substring of w appearing in the beginning of w . As such, for w = abcdef we have that abc is a pre x of w, and def is a su x of w. Here is a formal de nition of pre x and substring. De nition 1.1 The string x is a pre x of a string w , if there exists a string z , such that w = xz . Similarly, x is a substring of w if there exist strings y and z such that w = yxz . 1.3 Languages A language is a set of strings. One special language is Σ * , which is the set of all possible strings generated over the alphabet Σ * . For example, if Σ = { a , b , c } then Σ * = { , a , b , c , aa , ab , ac , ba ,..., aaaaaabbbaababa ,... } . Namely, Σ * is the full language made of characters of Σ . Naturally, any language over Σ is going to be a subset of Σ * ....
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lect_02 - £ £ Lecture 2 Strings Languages DFA s 24...

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