CSE 135: Introduction to Theory of Computation Nondeterministic Finite Automata (cont )

Transcription

1 CSE 135: Introduction to Theory of Computation Nondeterministic Finite Automata (cont ) Sungjin Im University of California, Merced

2 Example II A ɛ B ɛ D F C E

3 Example II A ɛ B ɛ D F C E NFA accepting strings of s where the length is either a multiple 2 or 3 The NFA guesses at the begining whether it will see a multiple of 2 or 3, and then confirms that the guess was correct.

4 Example III, 1, 1 q ɛ q q 1 q p

5 Example III, 1, 1 q ɛ q q 1 q p NFA accepting strings with 1 as substring At some point the NFA guesses that the pattern 1 is starting and then checks to confirm the guess.

6 Nondeterministic Finite Automata (NFA) Formal Definition Definition A nondeterministic finite automaton (NFA) is M = (Q, Σ, δ, q, F ), where Q is the finite set of states Σ is the finite alphabet δ : q Q initial state F Q final/accepting states

7 Nondeterministic Finite Automata (NFA) Formal Definition Definition A nondeterministic finite automaton (NFA) is M = (Q, Σ, δ, q, F ), where Q is the finite set of states Σ is the finite alphabet δ : Q (Σ {ɛ}) P(Q), where P(Q) is the powerset of Q q Q initial state F Q final/accepting states

8 Example of NFA, 1, 1 q ɛ q q 1 q p Transition Diagram of NFA

9 Example of NFA, 1, 1 q ɛ q q 1 q p Transition Diagram of NFA Formally, the NFA is M 1 = ({q ɛ, q, q, q p }, {, 1}, δ, q ɛ, {q p }) where δ is given by δ(q ɛ, ) = {q ɛ, q } δ(q ɛ, 1) = {q ɛ } δ(q, ) = {q } δ(q, 1) = {q p } δ(q p, ) = {q p } δ(q p, 1) = {q p } δ is in all other cases.

10 Computation Definition For an NFA M = (Q, Σ, δ, q, F ), string w, and states q 1, q 2 Q, w we say q 1 M q 2 if there is one thread of computation on input w from state q 1 that ends in q 2.

11 Computation Definition For an NFA M = (Q, Σ, δ, q, F ), string w, and states q 1, q 2 Q, w we say q 1 M q 2 if there is one thread of computation on input w w from state q 1 that ends in q 2. Formally, q 1 M q 2 if there is a sequence of states r, r 1,... r k and a sequence x 1, x 2,... x k, where for each i, x i Σ {ɛ}, such that

12 Computation Definition For an NFA M = (Q, Σ, δ, q, F ), string w, and states q 1, q 2 Q, w we say q 1 M q 2 if there is one thread of computation on input w w from state q 1 that ends in q 2. Formally, q 1 M q 2 if there is a sequence of states r, r 1,... r k and a sequence x 1, x 2,... x k, where for each i, x i Σ {ɛ}, such that r = q 1,

13 Computation Definition For an NFA M = (Q, Σ, δ, q, F ), string w, and states q 1, q 2 Q, w we say q 1 M q 2 if there is one thread of computation on input w w from state q 1 that ends in q 2. Formally, q 1 M q 2 if there is a sequence of states r, r 1,... r k and a sequence x 1, x 2,... x k, where for each i, x i Σ {ɛ}, such that r = q 1, for each i, r i+1 δ(r i, x i+1 ),

14 Computation Definition For an NFA M = (Q, Σ, δ, q, F ), string w, and states q 1, q 2 Q, w we say q 1 M q 2 if there is one thread of computation on input w w from state q 1 that ends in q 2. Formally, q 1 M q 2 if there is a sequence of states r, r 1,... r k and a sequence x 1, x 2,... x k, where for each i, x i Σ {ɛ}, such that r = q 1, for each i, r i+1 δ(r i, x i+1 ), r k = q 2, and

15 Computation Definition For an NFA M = (Q, Σ, δ, q, F ), string w, and states q 1, q 2 Q, w we say q 1 M q 2 if there is one thread of computation on input w w from state q 1 that ends in q 2. Formally, q 1 M q 2 if there is a sequence of states r, r 1,... r k and a sequence x 1, x 2,... x k, where for each i, x i Σ {ɛ}, such that r = q 1, for each i, r i+1 δ(r i, x i+1 ), r k = q 2, and w = x 1 x 2 x 3 x k

16 Example Computation, 1, 1 q ɛ q q 1 q p ɛ q ɛ 1 M q p because taking r = q ɛ, r 1 = q, r 2 = q, r 3 = q p, r 4 = q p, r 5 = q p, and x 1 =, x 2 = ɛ, x 3 = 1, x 4 =, x 5 =, we have x 1 x 2 x 5 = ɛ1 = 1 r i+1 δ(r i, x i+1 )

17 Defining ˆ Definition For an NFA M = (Q, Σ, δ, q, F ), string w, and state q 1 Q, we say ˆ (q 1, w) to denote states of all the active threads of computation on input w from q 1.

18 Defining ˆ Definition For an NFA M = (Q, Σ, δ, q, F ), string w, and state q 1 Q, we say ˆ (q 1, w) to denote states of all the active threads of computation on input w from q 1. Formally, ˆ (q 1, w) = {q Q q 1 w M q}

19 Example, 1, 1 q ɛ q q 1 q p ɛ Example NFA

20 Example, 1, 1 q ɛ q ɛ q 1 q p Example NFA q ɛ q q ɛ q ɛ q p X q ɛ q p q ɛ q ɛ q q p q q ɛ q X ɛ q Computation on 1

21 Example, 1, 1 q ɛ q ɛ q 1 q p Example NFA ˆ (q ɛ, 1) = {q, q p, q, q ɛ } q ɛ q q ɛ q ɛ q p X q ɛ q p q ɛ q ɛ q q p q q ɛ q X ɛ q Computation on 1

22 Acceptance/Recognition Definition For an NFA M = (Q, Σ, δ, q, F ) and string w Σ, we say M accepts w iff ˆ (q, w) F

23 Acceptance/Recognition Definition For an NFA M = (Q, Σ, δ, q, F ) and string w Σ, we say M accepts w iff ˆ (q, w) F Definition The language accepted or recognized by NFA M over alphabet Σ is L(M) = {w Σ M accepts w}.

24 Acceptance/Recognition Definition For an NFA M = (Q, Σ, δ, q, F ) and string w Σ, we say M accepts w iff ˆ (q, w) F Definition The language accepted or recognized by NFA M over alphabet Σ is L(M) = {w Σ M accepts w}. A language L is said to be accepted/recognized by M if L = L(M).

25 Observations about NFAs Observation 1 For NFA M, string w and state q 1 it could be that

26 Observations about NFAs Observation 1 For NFA M, string w and state q 1 it could be that ˆ (q 1, w) =

27 Observations about NFAs Observation 1 For NFA M, string w and state q 1 it could be that ˆ (q 1, w) = ˆ (q 1, w) has more than one element

28 Observations about NFAs Observation 1 For NFA M, string w and state q 1 it could be that ˆ (q 1, w) = ˆ (q 1, w) has more than one element Observation 2 However, the following proposition about DFAs continues to hold for NFAs

29 Observations about NFAs Observation 1 For NFA M, string w and state q 1 it could be that ˆ (q 1, w) = ˆ (q 1, w) has more than one element Observation 2 However, the following proposition about DFAs continues to hold for NFAs For NFA M, strings u and v, and state q, ˆ (q, uv) = ˆ (q, v) q ˆ (q,u)

30 Using Nondeterminism When designing an NFA for a language

31 Using Nondeterminism When designing an NFA for a language You follow the same methodology as for DFAs, like identifying what needs to be remembered

32 Using Nondeterminism When designing an NFA for a language You follow the same methodology as for DFAs, like identifying what needs to be remembered But now, the machine can guess at certain steps

33 Back to the Future Problem For Σ = {, 1, 2, #}, let L = {w#c w {, 1, 2}, c {, 1, 2}, and c occurs in w} So 111# L but 111#2 L. Design an NFA recognizing L.

34 Back to the Future Problem For Σ = {, 1, 2, #}, let L = {w#c w {, 1, 2}, c {, 1, 2}, and c occurs in w} So 111# L but 111#2 L. Design an NFA recognizing L. Solution Read symbols of w, i.e., portion of input before # is seen Guess at some point that current symbol in w is going to be the same as c ; store this symbol in the state Read the rest of w On reading #, check that the symbol immediately after is the one stored, and that the input ends immediately after that.

35 Back to the Future The Automaton, 1, 2, 1, 2 q s 2 1 q, 1, 2 q 1, 1, 2 # # q q q f q 2 # q 2 L(M) = {w#c c occurs in w}

36 Halving a Language Definition For a language L, define 1 2L as follows. 1 L = {x y. x = y and xy L} 2 In other words, 1 2L consists of the first halves of strings in L

37 Halving a Language Definition For a language L, define 1 2L as follows. 1 L = {x y. x = y and xy L} 2 In other words, 1 2L consists of the first halves of strings in L Example If L = {1,, 1, 111} then 1 2 L =.

38 Halving a Language Definition For a language L, define 1 2L as follows. 1 L = {x y. x = y and xy L} 2 In other words, 1 2L consists of the first halves of strings in L Example If L = {1,, 1, 111} then 1 2L = {,, 11}.

39 Recognizing Halves of Regular Languages Proposition If L is recognized by a DFA M then there is a NFA N such that L(N) = 1 2 L.

40 Recognizing Halves of Regular Languages Proposition If L is recognized by a DFA M then there is a NFA N such that L(N) = 1 2 L. Proof Idea On input x, need to check if x is the first half of some string w = xy that is accepted by M. Run M on input x; let M be in state q i after reading all of x Guess a string y such that y = x Check if M reaches a final state on reading y from q i

41 Recognizing Halves of Regular Languages Proposition If L is recognized by a DFA M then there is a NFA N such that L(N) = 1 2 L. Proof Idea On input x, need to check if x is the first half of some string w = xy that is accepted by M. Run M on input x; let M be in state q i after reading all of x Guess a string y such that y = x Check if M reaches a final state on reading y from q i How do you guess a string y of equal length to x using finite memory? Seems to require remembering the length of x!

42 Fixing the Idea Problem How do you guess a string y of equal length to x using finite memory?

43 Fixing the Idea Problem and Fix(?) How do you guess a string y of equal length to x using finite memory? Guess one symbol of y as you read one symbol of x!

44 Fixing the Idea Problem and Fix(?) How do you guess a string y of equal length to x using finite memory? Guess one symbol of y as you read one symbol of x! How do you run M on y from q i, if you cannot store all the symbols of y?

45 Fixing the Idea Problem and Fix(?) How do you guess a string y of equal length to x using finite memory? Guess one symbol of y as you read one symbol of x! How do you run M on y from q i, if you cannot store all the symbols of y? Run M on y as you guess each symbol, without waiting to finish the execution on x!

46 Fixing the Idea Problem and Fix(?) How do you guess a string y of equal length to x using finite memory? Guess one symbol of y as you read one symbol of x! How do you run M on y from q i, if you cannot store all the symbols of y? Run M on y as you guess each symbol, without waiting to finish the execution on x! If we don t first execute M on x, how do we know the state q i from which we have to execute y from?

47 Fixing the Idea Problem and Fix(?) How do you guess a string y of equal length to x using finite memory? Guess one symbol of y as you read one symbol of x! How do you run M on y from q i, if you cannot store all the symbols of y? Run M on y as you guess each symbol, without waiting to finish the execution on x! If we don t first execute M on x, how do we know the state q i from which we have to execute y from? Guess it! And then check that running M on x does indeed end in q i, your guessed state.

48 New Algorithm On input x, NFA N 1. Guess state q i and place left finger on (initial state of M) q and right finger on q i 2. As characters of x are read, N moves the left finger along transitions dictated by x and simultaneously moves the right finger along nondeterministically chosen transitions labelled by some symbol

49 New Algorithm On input x, NFA N 1. Guess state q i and place left finger on (initial state of M) q and right finger on q i 2. As characters of x are read, N moves the left finger along transitions dictated by x and simultaneously moves the right finger along nondeterministically chosen transitions labelled by some symbol 3. Accept if after reading x, left finger is at q i (state initially guessed for right finger) and right finger is at an accepting state

50 New Algorithm On input x, NFA N 1. Guess state q i and place left finger on (initial state of M) q and right finger on q i 2. As characters of x are read, N moves the left finger along transitions dictated by x and simultaneously moves the right finger along nondeterministically chosen transitions labelled by some symbol 3. Accept if after reading x, left finger is at q i (state initially guessed for right finger) and right finger is at an accepting state Things to remember: initial guess for right finger, and positions of left and right finger.

51 Algorithm on Example 11 L and so x = L q 1 q 1 1 q q 3 DFA M

52 Algorithm on Example q 1 q 1 1 q q 3 11 L and so x = L NFA N execution on x = 1 is String Read Left Finger Right Finger ɛ q q 2 1 q 1 q 2 1 q 3 q 1 1 q 2 q 3 DFA M

53 Algorithm on Example q 1 q 1 1 q q 3 DFA M 11 L and so x = L NFA N execution on x = 1 is String Read Left Finger Right Finger ɛ q q 2 1 q 1 q 2 =? 1 q 3 1 q 2 q 1 q 3 accept?

54 Formal Construction of NFA N States and Initial State Given M = (Q, Σ, δ, q, F ) recognizing L define N = (Q, Σ, δ, q, F ) that recognizes 1 2 L Q = Q Q Q {s}, where s Q s is a new start state Other states are of the form left finger, initial guess, right finger ; initial guess records the initial guess for the right finger q = s

55 Formal Construction of NFA N Transitions and Final States Transitions δ (s, ɛ) = { q, q i, q i q i Q} Guess the state q i that the input will lead to δ ( q i, q j, q k, a) = { q l, q j, q m δ(q i, a) = q l, b Σ. δ(q k, b) = q m } b is the guess for the next symbol of y and initial guess does not change F = { q i, q i, q j q i Q, q j F }