Lecture 10

Implementing a DFA

int state = q_0
char c
while not EOF do
    read c
    case state of
    q_0: case c of
        a: state = ...
        b: state = ...
    q_1: case c of
        a: state = ...
        b: state = ...
    q_n: case c of
        a: state = ...
        b: state = ...
end while
return state in A

Or: use a lookup table

a|   next
t|  states

DFAs with actions

Can attach computations to the arcs of a DFA

e.g. \(L=\{binary\enspace numbers\enspace with\enspace no\enspace leading\enspace 0s\}\), compute the value of the number - \(1(0|1)^* | 0\)



What do we gain by making DFAs more complex?

e.g. \(L=\{w=\{a,b\}^* | w\enspace ends\enspace with\enspace abb\}\) \((a|b)^* abb\)



Non-deterministic Finite Automaton

What if we allowed more than one arc with the same character from the same state? What would this mean?

Machine chooses one direction or the other (.e. the machine is non-deterministic). Accept if some set of choices leads to an accepting state.

With non-determinism, the machine becomes:



Machine "guesses" to stay in the first state until it reaches the final "abb", the transitions to accepting.

NFAs often simpler than DFA.

Formally NFA is a 5-tuple \((\Sigma, Q, q, A, \delta)\) where

Non-deterministic want to accept if some path through the NFA leads to acceptance for the given word. (reject if none do)

\(\delta ^*\) for NFAs: set of states \(\times\Sigma\) -> set of states

\(\delta ^*(qs, \epsilon)=qs\)

\(\delta ^((qs, cw)=\delta ^*(\cup_{q\in qs}(\delta(q, c)), w)\)

Then accept w if \(\delta ^*(\{q_0\}, w)\cap A=\emptyset\)

NFA simulation precedure:

states <- {q_0}
while not EOF do
    read ch
    states = U_{q in states) delta(q, ch)
end do
return states intersect A is not empty
Already read input Unread input States
\(\epsilon\) \(baabb\) \(\{1\}\)
\(b\) \(aabb\) \(\{1\}\)
\(ba\) \(abb\) \(\{1, 2\}\)
\(baa\) \(bb\) \(\{1, 2\}\)
\(baab\) \(b\) \(\{1, 3\}\)
\(baabb\) \(\epsilon\) \(\{1, 4\}\)

\(\{1,4\}\cup\{4\}=\{4\}\neq\emptyset\) accept

If you give each set of states a name, and call these the states, every NFA become a DFA.