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# FA - Harvard CS 121 and CSCI E-207 Lecture 4 Languages and...

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Harvard CS 121 and CSCI E-207 Lecture 4: Languages and Finite Automata Harry Lewis September 15, 2009 Reading: Sipser, § 1.1 and § 1.2.

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Harvard CS 121 & CSCI E-207 September 15, 2009 Languages A language L over alphabet Σ is a set of strings over Σ (i.e. L Σ * ) Computational problem: given x Σ * , is x L ? Any YES/NO problem can be cast as a language. 1
Harvard CS 121 & CSCI E-207 September 15, 2009 Examples of languages All words in the American Heritage Dictionary Σ * Σ { x Σ * : | x | = 3 } = { aaa, aab, aba, abb, baa, bab, bba, bbb } The set of strings x ∈ { a, b } * such that x has more a ’s than b ’s. The set of strings x ∈ { 0 , 1 } * such that x is the binary representation of a prime number. All ‘C’ programs that do not go into an infinite loop. 2

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Harvard CS 121 & CSCI E-207 September 15, 2009 The highly abstract and metaphorical term “language” A language can be either finite or infinite A language need not have any “internal structure” 3
Harvard CS 121 & CSCI E-207 September 15, 2009 Be careful to distinguish ε The empty string (a string) The empty set (a set, possibly a language) { ε } The set containing one element, which is the empty string (a language) {∅} The set containing one element, which is the empty set (a set of sets, maybe a set of languages) 4

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Harvard CS 121 & CSCI E-207 September 15, 2009 Deterministic Finite Automata (DFAs) Example: Home Stereo P = power button (ON/OFF) S = source button (CD/Radio/TV), only works when stereo is ON, but source remembered when stereo is OFF.
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