Nondeterministic Finite Automata
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Transcript Nondeterministic Finite Automata
Non-Deterministic
Finite Automata
1
Nondeterministic Finite Automaton (NFA)
Alphabet = {a}
a
q0
q1 a
q2
a
q3
2
Alphabet = {a}
Two choices
a
q0
q1 a
q2
a
q3
3
Alphabet = {a}
Two choices
a
q0
q1 a
q2
No transition
a
q3
No transition
4
First Choice
a a
a
q0
q1 a
q2
a
q3
5
First Choice
a a
a
q0
q1 a
q2
a
q3
6
First Choice
a a
a
q0
q1 a
q2
a
q3
7
First Choice
a a
All input is consumed
a
q0
q1 a
q2
“accept”
a
q3
8
Second Choice
a a
a
q0
q1 a
q2
a
q3
9
Second Choice
a a
a
q0
q1 a
q2
a
q3
10
Second Choice
a a
a
q0
q1 a
a
q3
q2
No transition:
the automaton hangs
11
Second Choice
a a
Input cannot be consumed
a
q0
q1 a
q2
a
q3
“reject”
12
An NFA accepts a string:
when there is a computation of the NFA
that accepts the string
There is a computation:
all the input is consumed and the automaton
is in an accepting state
13
Example
aa is accepted by the NFA:
“accept”
a
q0
q1 a
q2
a
q0
a
q3
because this
computation
accepts aa
q1 a
a
q3
q2
“reject”
14
Rejection example
a
a
q0
q1 a
q2
a
q3
15
First Choice
a
a
q0
q1 a
q2
a
q3
16
First Choice
a
“reject”
a
q0
q1 a
q2
a
q3
17
Second Choice
a
a
q0
q1 a
q2
a
q3
18
Second Choice
a
a
q0
q1 a
q2
a
q3
19
Second Choice
a
a
q0
q1 a
q2
a
q3
“reject”
20
An NFA rejects a string:
when there is no computation of the NFA
that accepts the string.
For each computation:
• All the input is consumed and the
automaton is in a non final state
OR
• The input cannot be consumed
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Three major differences
between FA & NFA
• Each entry in the table of NFA is a set.
• Allow as the second argument of .
This means that the NFA can make a
transition function without consuming an
input symbol.
• The set (qi , a) may be empty.
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Example
a
is rejected by the NFA:
“reject”
a
q0
q1 a
q2
a
q0
a
q3
“reject”
q1 a
q2
a
q3
All possible computations lead to rejection
23
Rejection example
a a a
a
q0
q1 a
q2
a
q3
24
First Choice
a a a
a
q0
q1 a
q2
a
q3
25
First Choice
a a a
a
q0
q1 a
a
q3
q2
No transition:
the automaton hangs
26
First Choice
a a a
Input cannot be consumed
a
q0
q1 a
q2
“reject”
a
q3
27
Second Choice
a a a
a
q0
q1 a
q2
a
q3
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Second Choice
a a a
a
q0
q1 a
q2
a
q3
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Second Choice
a a a
a
q0
q1 a
a
q3
q2
No transition:
the automaton hangs
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Second Choice
a a a
Input cannot be consumed
a
q0
q1 a
q2
a
q3
“reject”
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aaa
is rejected by the NFA:
“reject”
a
q0
q1 a
q2
a
q0
a
q3
q1 a
a
q3
q2
“reject”
All possible computations lead to rejection
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Language accepted:
a
q0
q1 a
L {aa}
q2
a
q3
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Lambda Transitions
q0 a
q1
q2 a
q3
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a a
q0 a
q1
q2 a
q3
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a a
q0 a
q1
q2 a
q3
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(read head does not move)
a a
q0 a
q1
q2 a
q3
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a a
q0 a
q1
q2 a
q3
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all input is consumed
a a
“accept”
q0 a
String
q1
q2 a
q3
aa is accepted
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Rejection Example
a a a
q0 a
q1
q2 a
q3
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a a a
q0 a
q1
q2 a
q3
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(read head doesn’t move)
a a a
q0 a
q1
q2 a
q3
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a a a
q0 a
q1
q2 a
q3
No transition:
the automaton hangs
43
Input cannot be consumed
a a a
“reject”
q0 a
String
aaa
q1
q2 a
q3
is rejected
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Language accepted:
q0 a
q1
L {aa}
q2 a
q3
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Another NFA Example
q0
a
b
q1
q2
q3
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a b
q0
a
b
q1
q2
q3
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a b
q0
a
b
q1
q2
q3
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a b
q0
a
b
q1
q2
q3
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a b
“accept”
q0
a
b
q1
q2
q3
50
Another String
a b a b
q0
a
b
q1
q2
q3
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a b a b
q0
a
b
q1
q2
q3
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a b a b
q0
a
b
q1
q2
q3
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a b a b
q0
a
b
q1
q2
q3
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a b a b
q0
a
b
q1
q2
q3
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a b a b
q0
a
b
q1
q2
q3
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a b a b
q0
a
b
q1
q2
q3
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a b a b
“accept”
q0
a
b
q1
q2
q3
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Language accepted
L ab, abab, ababab, ...
ab
q0
a
b
q1
q2
q3
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Another NFA Example
0
q0
1
q1
0, 1 q2
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Language accepted
L(M ) = {λ, 10, 1010, 101010, ...}
= {10} *
0
q0
1
q1
0, 1 q2
(redundant
state)
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Remarks:
•The symbol never appears on the
input tape
•Simple automata:
M1
q0
M2
L(M1) = {}
L(M 2 ) = {λ}
q0
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•NFAs are interesting because we can
express languages easier than FAs
NFA
q0
a
M1
FA
q2
q1
a
q0
L( M1) = {a}
a
M2
a
q1
L( M 2 ) = {a}
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Formal Definition of NFAs
M Q, , , q0 , F
Q : Set of states, i.e. q0 , q1, q2
: Input aplhabet, i.e. a, b
:
q0 :
Transition function
Initial state
F : Accepting states
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Transition Function
q0 , 1 q1
0
q0
1
q1
0, 1 q
2
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(q1,0) {q0 , q2}
0
q0
1
q1
0, 1 q
2
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Do a transition diagram!!
(q0 , ) {q0 , q2}
0
q0
1
q1
0, 1 q
2
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(q2 ,1)
0
q0
1
q1
0, 1 q
2
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Extended Transition Function *
* q0 , a q1
q5
q4
a
q0
a
a
b
q1
q2
q3
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* q0 , aa q4 , q5
q5
q4
a
q0
a
a
b
q1
q2
q3
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* q0 , ab q2 , q3 , q0
q5
q4
a
q0
a
a
b
q1
q2
q3
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Formally
q j * qi , w : there is a walk from qi to q j
with label
w
w
qi
qj
w 1 2 k
qi
1
2
k
qj
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The Language of an NFA M
F q0 ,q5
q5
q4
a
q0
a
a
b
q1
q2
q3
* q0 , aa q4 , q5
aa L(M )
F
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F q0 ,q5
q5
q4
a
q0
a
a
b
q1
q2
q3
* q0 , ab q2 , q3 , q0
F
ab LM
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F q0 ,q5
q5
q4
a
q0
a
a
b
q1
q2
q3
* q0 , abaa q4 , q5
aaba L(M )
F
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F q0 ,q5
q5
q4
a
q0
a
a
b
q1
q2
q3
* q0 , aba q1
F
aba LM
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q5
q4
a
q0
a
a
b
q1
q2
q3
LM ab* {aa}
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Formally
The language accepted by NFA
M is:
LM w1, w2 , w3 ,...
where
* (q0 , wm ) {qi , q j ,..., qk ,}
and there is some
qk F
(accepting state)
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w LM
* (q0 , w)
qi
w
q0
qk
w
w
qk F
qj
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