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Lecture 7 From NFA to DFAPowerPoint Presentation

Lecture 7 From NFA to DFA

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Lecture 7 From NFA to DFA. DFA. For every string x , there is a unique path from initial state and associated with x . x is accepted if and only if this path ends at a final state. . x. NFA.

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DFA

- For every string x, there is a unique path from initial state and associated with x.
x is accepted if and only if this path ends at a final state.

x

NFA

- For any string x, there may exist none or more than one path from initial state and associated with x.

NFA DFA

- Consider an NFA M=(Q, Σ, δ, s, F).
- For x in Σ*, define
[x] = {q in Q | there exists a path s q}

- Define DFA M’=(Q’, Σ, δ’, s’, F’}:
Q’ = { [x] | x in Σ* },

δ ([x], a) = [xa] for x in Σ* and a in Σ,

s’ = [ε],

F’ = { [x] | x in L(M) }

x

Construction of M’

Special Case: M has no ε-move.

- [ε] = {s}
- Suppose [x] is known. How to get [xa] for
a in Σ?

From [x] to [xa]

a

- [xa] = { p | there exists a path q p for
some q in [x] }

= { p | there exists q in [x],

q p }

= U

path

a

edge

δ(q,a)

q in [x]

Construction of M’

- F’ = {[x] | x in L(M)}
= {[x] | [x] ∩ F ≠ Ǿ }

Example 3

- Design DFA to accept 00(0+1)*11

0

0

0

1

1

t

s

p

q

r

0,1

Ǿ

1

1

1

0

1

q

1

q

1

0

0

s

r

q

p

t

r

0

0

Construction of M’

- For q in Q, define
ε-closure(q) = {p | there exists a path q p}

- [ε] = {q | there is a path s q}
= ε-closure(s)

- Suppose [x] is known. How to get [xa] for
a in Σ?

ε

path

ε

path

From [x] to [xa]

a

- [xa] = { p | there exists a path q p for
some q in [x] }

= { p | there exists q in [x],

q r p }

= { p | for some q in [x] and r in δ(q,a),

p in ε-closure(r) }

= U U ε-closure(r)

path

a

ε

edge

path

r in δ(q,a)

q in [x]

Construction of M’

- F’ = {[x] | x in L(M)}
= {[x] | [x] ∩ F ≠ Ǿ }

Example 7

- Convert the following NFA to DFA.

0

q

0

0

s

ε

q

r

0

1

ε

s

0

r

s

ε

p

p

r

p

1

1

1

0

0

q, r, p

1

0,1

0

0

0

0

1

1

1

1

e

s

a

b

c

d

1

0

0

0

0

0

s,a

s,a,b

s,a,b,c

s,a,b,c,d

s,a,b,c,d,e

s

1

5

s,b

2 =32

How many states?

1

s,c

4

How many final states?

2 = 16

1

s,d

1

No, it is minimum!

Can we simplify it?

s,e

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