# Motivation. Automata = abstract computing devices. Turing studied Turing Machines (= computers) before there were any real computers

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2 Motivation Automata = abstract computing devices Turing studied Turing Machines (= computers) before there were any real computers We will also look at simpler devices than Turing machines (Finite State Automata, Pushdown Automata,... ), and specification means, such as grammars and regular expressions. NP-hardness = what cannot be efficiently computed 6

3 Finite Automata Finite Automata are used as a model for Software for designing digital circuits Lexical analyzer of a compiler Searching for keywords in a file or on the web. Software for verifying finite state systems, such as communication protocols. 7

4 Example: Finite Automaton modelling an on/off switch Push Start off on Push Example: Finite Automaton recognizing the string then Start t h e n t th the then 8

5 Structural Representations These are alternative ways of specifying a machine Grammars: A rule like E E + E specifies an arithmetic expression Lineup P erson.lineup says that a lineup is a person in front of a lineup. Regular Expressions: Denote structure of data, e.g. [A-Z][a-z]*[][A-Z][A-Z] matches Ithaca NY does not match Palo Alto CA Question: What expression would match Palo Alto CA 9

6 Central Concepts Alphabet: Finite, nonempty set of symbols Example: Σ = {0, 1} binary alphabet Example: Σ = {a, b, c,..., z} the set of all lower case letters Example: The set of all ASCII characters Strings: Finite sequence of symbols from an alphabet Σ, e.g Empty String: The string with zero occurrences of symbols from Σ The empty string is denoted ɛ 10

7 Length of String: Number of positions for symbols in the string. w denotes the length of string w 0110 = 4, ɛ = 0 Powers of an Alphabet: Σ k = the set of strings of length k with symbols from Σ Example: Σ = {0, 1} Σ 1 = {0, 1} Σ 2 = {00, 01, 10, 11} Σ 0 = {ɛ} Question: How many strings are there in Σ 3 11

8 The set of all strings over Σ is denoted Σ Σ = Σ 0 Σ 1 Σ 2 Also: Σ + = Σ 1 Σ 2 Σ 3 Σ = Σ + {ɛ} Concatenation: If x and y are strings, then xy is the string obtained by placing a copy of y immediately after a copy of x x = a 1 a 2... a i, y = b 1 b 2... b j xy = a 1 a 2... a i b 1 b 2... b j Example: x = 01101, y = 110, xy = Note: For any string x xɛ = ɛx = x 12

9 Languages: If Σ is an alphabet, and L Σ then L is a language Examples of languages: The set of legal English words The set of legal C programs The set of strings consisting of n 0 s followed by n 1 s {ɛ, 01, 0011, ,...} 13

10 The set of strings with equal number of 0 s and 1 s {ɛ, 01, 10, 0011, 0101, 1001,...} L P = the set of binary numbers whose value is prime {10, 11, 101, 111, 1011,...} The empty language The language {ɛ} consisting of the empty string Note: {ɛ} Note2: The underlying alphabet Σ is always finite 14

11 Problem: Is a given string w a member of a language L? Example: Is a binary number prime = is it a member in L P Is L P? What computational resources are needed to answer the question. Usually we think of problems not as a yes/no decision, but as something that transforms an input into an output. Example: Parse a C-program = check if the program is correct, and if it is, produce a parse tree. Let L X be the set of all valid programs in prog lang X. If we can show that determining membership in L X is hard, then parsing programs written in X cannot be easier. Question: Why? 15

12 Finite Automata Informally Protocol for e-commerce using e-money Allowed events: 1. The customer can pay the store (=send the money-file to the store) 2. The customer can cancel the money (like putting a stop on a check) 3. The store can ship the goods to the customer 4. The store can redeem the money (=cash the check) 5. The bank can transfer the money to the store 16

13 e-commerce The protocol for each participant: Start a pay b redeem d transfer f (a) Store c ship redeem e ship transfer g ship cancel pay cancel redeem transfer Start Start (b) Customer (c) Bank 17

14 Completed protocols: cancel pay,cancel pay,cancel pay,cancel Start a b d f pay redeem transfer ship ship ship (a) Store redeem transfer c e g pay,cancel pay,cancel pay,cancel pay, ship ship. redeem, transfer, pay, cancel cancel 2 pay,redeem, cancel, ship pay,redeem, cancel, ship pay, ship redeem transfer Start Start (b) Customer (c) Bank 18

15 The entire system as an Automaton: Start a b c d e f g P P P P P P P S S S R C C C C C C C R P S S S P P P P P P R P,C P,C P S R S S C P,C P,C P,C T P R T S S S R P,C C P,C P,C P,C P,C P,C P,C 19

16 Example: Recognizing Strings Ending in ing Not i or g Not i Not i or n i nothing i Saw i n Saw in g Saw ing Start

17 Automata to Code In C/C++, make a piece of code for each state. This code: 1. Reads the next input. 2. Decides on the next state. 3. Jumps to the beginning of the code for that state.

18 Example: Automata to Code 2: /* i seen */ c = getnextinput(); if (c == n ) goto 3; else if (c == i ) goto 2; else goto 1; 3: /* in seen */...

19 Example: Protocol for Sending Data Ready data in Sending timeout Start ack

20 Extended Example Thanks to Jay Misra for this example. On a distant planet, there are three species, a, b, and c. Any two different species can mate. If they do: 1. The participants die. 2. Two children of the third species are born.

21 Strange Planet (2) Observation: the number of individuals never changes. The planet fails if at some point all individuals are of the same species. Then, no more breeding can take place. State = sequence of three integers the numbers of individuals of species a, b, and c.

22 Strange Planet Questions In a given state, must the planet eventually fail? In a given state, is it possible for the planet to fail, if the wrong breeding choices are made?

23 Questions (2) These questions mirror real ones about protocols. Can the planet fail? is like asking whether a protocol can enter some undesired or error state. Must the planet fail is like asking whether a protocol is guaranteed to terminate. Here, failure is really the good condition of termination.

24 Strange Planet Transitions An a-event occurs when individuals of species b and c breed and are replaced by two a s. Analogously: b-events and c-events. Represent these by symbols a, b, and c, respectively.

25 Strange Planet with 2 Individuals a 011 b 101 c 110 Notice: all states are must-fail states.

26 Strange Planet with 3 Individuals a 111 b c a a c b b c Notice: four states are must-fail states. The others are can t-fail states. State 111 has several transitions.

27 Strange Planet with 4 Individuals 211 a b c 004 a c 103 b 130 b a 310 c 013 c b 031 a 301 b 022 c c 202 a a 220 b Notice: states 400, etc. are must-fail states. All other states are might-fail states.

28 Taking Advantage of Symmetry The ability to fail depends only on the set of numbers of the three species, not on which species has which number. Let s represent states by the list of counts, sorted by largest-first. Only one transition symbol, x.

29 The Cases 2, 3, x 400 x 200 x 300 x x 310 x x Notice: for the case n = 4, there is nondeterminism : different transitions are possible from 211 on the same input.

30 5 Individuals Notice: 500 is a must-fail state; all others are might-fail states.

31 6 Individuals Notice: 600 is a must-fail state; 510, 420, and 321 are can t-fail states; 411, 330, and 222 are might-fail states.

32 7 Individuals Notice: 700 is a must-fail state; All others are might-fail states.

33 Questions for Thought 1. Without symmetry, how many states are there with n individuals? 2. What if we use symmetry? 3. For n individuals, how do you tell whether a state is must-fail, mightfail, or can t-fail?

34 Deterministic Finite Automata A DFA is a quintuple A = (Q, Σ, δ, q 0, F ) Q is a finite set of states Σ is a finite alphabet (=input symbols) δ is a transition function (q, a) p q 0 Q is the start state F Q is a set of final states 20

35 Example: An automaton A that accepts L = {x01y : x, y {0, 1} } The automaton A = ({q 0, q 1, q 2 }, {0, 1}, δ, q 0, {q 1 }) as a transition table: 0 1 q 0 q 2 q 0 q 1 q 1 q 1 q 2 q 2 q 1 The automaton as a transition diagram: Start q 0 q 2 q 1 0, 1 21

36 An FA accepts a string w = a 1 a 2 a n if there is a path in the transition diagram that 1. Begins at a start state 2. Ends at an accepting state 3. Has sequence of labels a 1 a 2 a n Example: The FA 1 Start q 0 0 q q 1 2 accepts e.g. the string

37 The transition function δ can be extended to ˆδ that operates on states and strings (as opposed to states and symbols) Basis: ˆδ(q, ɛ) = q Induction: ˆδ(q, xa) = δ(ˆδ(q, x), a) Now, fomally, the language accepted by A is L(A) = {w : ˆδ(q 0, w) F } The languages accepted by FA:s are called regular languages 23

38 Example: DFA accepting all and only strings with an even number of 0 s and an even number of 1 s Start q q q 1 q Tabular representation of the Automaton 0 1 q 0 q 2 q 1 q 1 q 3 q 0 q 2 q 0 q 3 q 3 q 1 q 2 24

39 Example Marble-rolling toy from p. 53 of textbook A B x 1 x 3 x 2 C D 25

40 A state is represented as sequence of three bits followed by r or a (previous input rejected or accepted) For instance, 010a, means left, right, left, accepted Tabular representation of DFA for the toy A B 000r 100r 011r 000a 100r 011r 001a 101r 000a 010r 110r 001a 010a 110r 001a 011r 111r 010a 100r 010r 111r 100a 010r 111r 101r 011r 100a 101a 011r 100a 110r 000a 101a 110a 000a 101a 111r 001a 110a 26

41 Nondeterministic Finite Automata An NFA can be in several states at once, or, viewded another way, it can guess which state to go to next Example: An automaton that accepts all and only strings ending in Start q 0 0 q q 1 2 Here is what happens when the NFA processes the input q 0 q 0 q 0 q 0 q 0 q 0 q 1 q 1 q 1 (stuck) q 2 (stuck) q

42 Formally, an NFA is a quintuple A = (Q, Σ, δ, q 0, F ) Q is a finite set of states Σ is a finite alphabet δ is a transition function from Q Σ to the powerset of Q q 0 Q is the start state F Q is a set of final states 28

43 Example: The NFA from the previous slide is ({q 0, q 1, q 2 }, {0, 1}, δ, q 0, {q 2 }) where δ is the transition function 0 1 q 0 {q 0, q 1 } {q 0 } q 1 {q 2 } q 2 29

44 Extended transition function ˆδ. Basis: ˆδ(q, ɛ) = {q} Induction: ˆδ(q, xa) = p ˆδ(q,x) δ(p, a) Example: Let s compute ˆδ(q 0, 00101) on the blackboard. How about (q,0010)? Now, fomally, the language accepted by A is L(A) = {w : ˆδ(q 0, w) F } 30

45 Let s prove formally that the NFA 1 Start q 0 0 q q 1 2 accepts the language {x01 : x Σ }. We ll do a mutual induction on the three statements below 0. w Σ q 0 ˆδ(q 0, w) 1. q 1 ˆδ(q 0, w) w = x0 2. q 2 ˆδ(q 0, w) w = x01 31

46 Basis: If w = 0 then w = ɛ. Then statement (0) follows from def. For (1) and (2) both sides are false for ɛ Induction: Assume w = xa, where a {0, 1}, x = n and statements (0) (2) hold for x. We will show on the blackboard in class that the statements hold for xa. 32

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