# Nondeterministic Finite Automata

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1 Chapter, Part 2 Nondeterministic Finite Automata CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara

2 Fundamental set operations Let A and B be two languages. In addition to union and intersection, we consider the following set operations: Concatenation of A and B, A B, is {xy x A and y B}, Star of A, A, is {x x 2 x k k 0 and x,..., x k A}. Complement of A, A, is Σ A, i.e., {w w Σ w A}. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 2

3 Fundamental set operations Let A and B be two languages. In addition to union and intersection, we consider the following set operations: Concatenation of A and B, A B, is {xy x A and y B}, Star of A, A, is {x x 2 x k k 0 and x,..., x k A}. Complement of A, A, is Σ A, i.e., {w w Σ w A}. We will prove that the regular languages are closed under union, intersection, concatenation, star, and complement;, i.e., that if A and B are regular, then A B, A B, A B, A, and A are each regular. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 3

4 Nondeterministic Finite Automata The nondeterministic finite automaton is a variant of finite automaton with two characteristics: ǫ-transition: state transition can be made without reading a symbol; nondeterminism: zero or more than one possible value may exist for state transition. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 4

5 An Example Nondeterministic Finite Automaton An NFA that accepts all strings over {0,} that contain a either at the third position from the end or at the second position from the end. 0, q 0,,ε 0, q 2 q 3 q 4 There are two edges labeled coming out of q. There are no edges coming out of q 4. The edge from q 2 is labeled with ǫ, in addition to 0 and. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 5

6 What the Diagram Says If the node you are in has an outgoing edge labeled ǫ, you may choose to follow it. After receiving a symbol, if the node you are in has multiple outgoing edges labeled with the symbol, you nondeterministically choose one of them and cross it; if there are no choices, stop there. 0, q 0,,ε 0, q 2 q 3 q 4 CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 6

7 Nondeterministic Finite Automata, Formally A nondeterministic finite automaton is a 5-tuple N = (Q,Σ,δ,q 0,F), whereδ isamappingofq Σ ǫ to2 Q (alternatively, written P(Q)), and Σ ǫ = Σ {ǫ}. For each p Q and each a Σ ǫ, δ(s,a) = R means: upon reading an a the automaton N may transition from state s to any state in R. For ǫ in particular, δ(s,ǫ) = R means: without reading the symbol the automaton N may transition from state s to any state in R. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 7

8 Acceptance by Nondeterministic Finite Automata N accepts a word w = w w n,w,...,w n Σ if: there exist (p 0,..., p m ),p 0,...,p m Q and y,...,y m Σ ǫ such that w = y y m, p 0 = q 0, p m F, and for every i, i m, p i δ(p i,w i ). CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 8

9 Transition Function for the Previous Example 0, q 0,,ε 0, q 2 q 3 q 4 state symbol 0 ǫ q {q } {q,q 2 } q 2 {q 3 } {q 3 } { q 3 } q 3 {q 4 } {q 4 } q 4 CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 9

10 Transition Function for the Previous Example 0, q 0,,ε 0, q 2 q 3 q 4 state symbol 0 ǫ q {q } {q,q 2 } q 2 {q 3 } {q 3 } { q 3 } q 3 {q 4 } {q 4 } q 4 This NFA accepts y = with respect to state sequence (q,q 2,q 3,q 4 ) and decomposition y = ǫ. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 0

11 Nondeterministic Choices Nondeterministic computation can be thought of as a selfreproducing agent traveling in the state space.. At the start of computation the agent is in the initial state. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara

12 Nondeterministic Choices Nondeterministic computation can be thought of as a selfreproducing agent traveling in the state space.. At the start of computation the agent is in the initial state. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 2

13 Nondeterministic Choices 2. Both before and after receiving an input symbol, the agent follows each ǫ-labeled outgoing edge by producing its own clone and sending it along the edge. Thus, the original remains in the current location. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 3

14 Nondeterministic Choices 2. Both before and after receiving an input symbol, the agent follows each ǫ-labeled outgoing edge by producing its own clone and sending it along the edge. Thus, the original remains in the current location. I'll go to p7. I'll go to p2. I'll go to p5. I'll stay here. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 4

15 Nondeterministic Choices 3. On receiving an input symbol, say a, (a) If there is only one a-labeled outgoing edge, the agent follows the edge. (b) If there is no a-labeled outgoing edge, the agent evaporates. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 5

16 Nondeterministic Choices 3. On receiving an input symbol, say a, (a) If there is only one a-labeled outgoing edge, the agent follows the edge. (b) If there is no a-labeled outgoing edge, the agent evaporates. I am gone. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 6

17 Nondeterministic Choices 3. On receiving an input symbol, say a, (c) If there are k 2 a-labeled outgoing edges, the agent produces k clones, make them cross k of the edges, and cross the remaining one by himself. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 7

18 Nondeterministic Choices 3. On receiving an input symbol, say a, (c) If there are k 2 a-labeled outgoing edges, the agent produces k clones, make them cross k of the edges, and cross the remaining one by himself. I'll go to p7. I'll go to p2. I'll go to p5. I'll go to p3. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 8

19 Nondeterministic Choices 4. When two agents collide in a state, they merge themselves into one. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 9

20 Nondeterministic Choices 4. When two agents collide in a state, they merge themselves into one. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 20

21 Why Use NFA? For some languages construction is much easier. Below is a DFA that accepts the same language by remembering the last three symbols. 0 0 q 00 0 q 00 0 q 0 q q 00 q 0 q 0 q CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 2

22 Comparison 0 0 q 00 0 q 00 0 q 0 q , q 0,,ε 0, q 2 q 3 q 4 q 00 q 0 q 0 q NFA DFA CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 22

23 Example An NFA for the language of all strings over {a,b,c} that end with one of ab, bc, and ca. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 23

24 Example An NFA for the language of all strings over {a,b,c} that end with one of ab, bc, and ca. a,b,c p c p2 b q0 a p3 b p4 c p5 a p6 CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 24

25 A DFA Version of the Same Language b c b a p a c b b p2 c a q0 a p3 b p4 c c a a b c p5 a c p6 b CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 25

26 Example 2 An NFA for the language of all strings over {0,} that end with one of 00, 00, and 00. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 26

27 Example 2 An NFA for the language of all strings over {0,} that end with one of 00, 00, and 00. 0, q 0 0 q,e,e 0 q 2 q 3 q 4 CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 27

28 Example 3 An NFA for the language of all strings over {a,b,c} for which one of (the number of occurrences of a), (the number of occurrences of b), and (the number of occurrences of c) is a multiple of 3. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 28

29 Example 3 An NFA for the language of all strings over {a,b,c} for which one of (the number of occurrences of a), (the number of occurrences of b), and (the number of occurrences of c) is a multiple of 3. b,c a a b,c a b,c c,a b b c,a b c,a a,b c c a,b c a,b CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 29

30 Example 4 An NFA for the language of all strings over {a,b} that contain ababb. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 30

31 Example 4 An NFA for the language of all strings over {a,b} that contain ababb. a b a a,b b b a,b CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 3

32 The FA Model Versus NFA Model The NFA simplifies computational design, but the use of nondeterministic selections and ǫ-transitions makes it look very different from FA. Is that really so? CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 32

33 The FA Model Versus NFA Model The NFA simplifies computational design, but the use of nondeterministic selections and ǫ-transitions makes it look very different from FA. Is that really so? No, the FA model is equivalent to the NFA model. That is, every language accepted by an DFA is accepted by an NFA, vice versa. Obviously, we have: Theorem. Every FA is already an NFA. We will show: Theorem. Every NFA can be converted to an equivalent FA. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 33

34 Proof The principle is that just before the first symbol is received and after each symbol is read, there will be at most one agent in any state. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 34

35 Proof The principle is that just before the first symbol is received and after each symbol is read, there will be at most one agent in any state. We will thus consider the set of all states an agent can be in. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 35

36 Proof (cont d) Let N = (Q,Σ,δ,q 0,F) be an NFA. We will construct a DFA M = (S,Σ,γ,s 0,G). The state set S is P(Q). The initial state s 0 is the set consisting of q 0 and all the states reachable from q 0 by following only ǫ transitions. The final state set G is {A S A F }; i.e., the set of all subsets of Q containing an element of F. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 36

37 Transition Function The transition function γ is defined as follows: For each A S and for each b Σ, γ(a,b) = p A δ(p,ǫ bǫ ), the collection of all states r that can be reached from a state p in A by following any number of ǫ-arrows, a b-arrow, and then any number of ǫ-arrows. For all w over Σ, w is accepted by N if and only if the new DFA transitions from s 0 to a state in G on input w. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 37

38 Example An NFA that recognizes the language consisting of all strings over {0,} that contain a at either the third to last position or the second to last position. 0, q 0,,ε 0, q 2 q 3 q 4 CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 38

39 Conversion to DFA The state set consists of:, {q }, {q 2 }, {q 3 }, {q 4 }, {q,q 2 }, {q,q 3 }, {q,q 4 }, {q 2,q 3 }, {q 2,q 4 }, {q 3,q 4 }, {q,q 2,q 3 }, {q,q 2,q 4 }, {q,q 3,q 4 }, {q 2,q 3,q 4 }, {q,q 2,q 3,q 4 }. F consists of: {q 4 },{q,q 4 },{q 2,q 4 },{q 3,q 4 }, {q,q 2,q 4 },{q,q 3,q 4 },{q 2,q 3,q 4 },{q,q 2,q 3,q 4 }. The initial state is {q }. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 39

40 Transition State 0 {q } {q } {q,q 2,q 3 } {q,q 2,q 3 } {q,q 3,q 4 } {q,q 2,q 3,q 4 } {q,q 3,q 4 } {q,q 4 } {q,q 2,q 3,q 4 } {q,q 4 } {q } {q,q 2,q 3 } {q,q 2,q 3,q 4 } {q,q 3,q 4 } {q,q 2,q 3,q 4 } The other states are unreachable from the initial state. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 40

41 CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 4

42 A Greedy Conversion Algorithm Step For each state p Q and for each symbol a Σ, compute the set R(p,a) of all states that can be reached from state p by: (a) any number of ǫ transitions, (b) one transition labeled by a, and then (c) any number of ǫ transitions. Step 2 Initialize the state collection S as {p 0 }, where p 0 is the set of all states that can be reached from q 0 by following any number of ǫ transitions. Step 3 While S contains a state with no outgoing edges, select an arbitrary member, say r, of S, and do the following: For each symbol a, compute the state r a as q r R(q,a), add r a to S if r a is not already in it, and then draw an arc from r to r a. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 42

43 Use the previous NFA. Algorithm Execution Example 0, q 0,,ε 0, q 2 q 3 q 4 CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 43

44 Step state 0 q q q,q 2,q 3 q 2 q 3,q 4 q 3,q 4 q 3 q 4 q 4 q 4 Step 2 Initially S = {{q }}. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 44

45 Step 3 {q } on 0 changes the state to {q } {q } on changes the state to {q,q 2,q 3 }. New! {q,q 2,q 3 } on 0 changes the state to {q,q 3,q 4 }. New! {q,q 2,q 3 } on changes the state to {q,q 2,q 3,q 4 }. New! {q,q 3,q 4 } on 0 changes the state to {q,q 4 }. New! {q,q 2,q 3 } on changes the state to {q,q 2,q 3,q 4 }. {q,q 2,q 3,q 4 } on 0 changes the state to {q,q 3,q 4 }. {q,q 2,q 3,q 4 } on changes the state to {q,q 2,q 3,q 4 }. {q,q 4 } on 0 changes the state to {q 3}. {q,q 4 } on changes the state to {q,q 2,q 3 }. CSC527, Chapter, Part 2 c 202 Mitsunori Ogihara 45

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