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Costas Busch - LSU 1 Deterministic Finite Automata And Regular Languages
| Date post: | 11-Apr-2017 |
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Costas Busch - LSU 1
Deterministic Finite Automata
And Regular Languages
Costas Busch - LSU 2
Deterministic Finite Automaton (DFA)
Input Tape
“Accept” or“Reject”
String
FiniteAutomaton
Output
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Transition Graph
initialstate
accepting state
statetransition
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,
Costas Busch - LSU 4
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,
},{ ba
For every state, there is a transitionfor every symbol in the alphabet
Alphabet
Costas Busch - LSU 5
Initial Configuration
1q 2q 3q 4qa b b a
5q
a a bb
ba,Input Stringa b b a
ba,0q
Initial state
Input Tapehead
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Scanning the Input
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
a b b a
ba,
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0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
a b b a
ba,
Costas Busch - LSU 8
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
a b b a
ba,
Costas Busch - LSU 9
0q 1q 2q 3q 4qa b b a accept
5q
a a bb
ba,
a b b a
ba,
Input finished
Last state determines the outcome
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1q 2q 3q 4qa b b a
5q
a a bb
ba,
a b a
ba,0q
A Rejection Case
Input String
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0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
a b a
ba,
Costas Busch - LSU 12
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
a b a
ba,
Costas Busch - LSU 13
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
reject
a b a
ba,
Input finished
Last state determines the outcome
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1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,0q
)(
Another Rejection CaseTape is empty
reject
Input Finished (no symbol read)
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Language Accepted: abbaL
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,
This automaton accepts only one string
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To accept a string:all the input string is scanned and the last state is accepting
To reject a string:all the input string is scanned and the last state is non-accepting
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Another Example
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,
abbaabL ,,
Acceptstate
Acceptstate
Acceptstate
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)(Empty Tape
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,
accept
Input Finished
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Another Example
a
b ba,
ba,
0q 1q 2q
Accept state
trap state
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a
b ba,
ba,
0q 1q 2q
a ba
Input String
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a
b ba,
ba,
0q 1q 2q
a ba
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a
b ba,
ba,
0q 1q 2q
a ba
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a
b ba,
ba,
0q 1q 2q
a ba
accept
Input finished
Costas Busch - LSU 24
a
b ba,
ba,
0q 1q 2q
ab b
A rejection case
Input String
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a
b ba,
ba,
0q 1q 2q
ab b
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a
b ba,
ba,
0q 1q 2q
ab b
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a
b ba,
ba,
0q 1q 2q
ab b
reject
Input finished
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Language Accepted: }0:{ nbaL n
a
b ba,
ba,
0q 1q 2q
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Another Example
0q 1q
1
1
}1{Alphabet:
Language Accepted:even} is and :{ * xxxEVEN
},111111,1111,11,{
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Formal Definition Deterministic Finite Automaton (DFA)
FqQM ,,,, 0
Q
0q
F
: set of states: input alphabet: transition function: initial state: set of accepting states
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Set of States
Q
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba, 543210 ,,,,, qqqqqqQ
ba,
Example
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Input Alphabet
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba, ba,
ba,
:the input alphabet never contains
Example
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Initial State
0q
1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,0q
Example
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Set of Accepting States
QF
0q 1q 2q 3qa b b a
5q
a a bb
ba, 4qF
ba,
4q
Example
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Transition Function
QQ :
q qx
qxq ),(
Describes the result of a transitionfrom state with symbolq x
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2q 3q 4qa b b a
5q
a a bb
ba,
ba,0q 1q
10 , qaq Example:
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50 , qbq
1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,0q
Costas Busch - LSU 38
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,
32 , qbq
Costas Busch - LSU 39
a b
0q
1q
2q
3q
4q
5q
1q 5q
5q 2q5q 3q
4q 5q
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,5q5q5q5q
Transition Table for st
ates
symbols
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Extended Transition Function QQ ** :
qwq ),(*
Describes the resulting state after scanning string from statew q
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20
* , qabq
3q 4qa b b a
5q
a a bb
ba,
ba,0q 1q 2q
Example:
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50* , qabbbaaq
1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,0q
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41* , qbbaq
0q 1q 2q 3q 4qa b b a
5q
a a bb
ba,
ba,
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Special case:
qq ,*
for any state q
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qwq ,*
q qw
q qkw 21
1 2 k
states may be repeated
In general:
implies that there is a walk of transitions
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Language accepted by DFA :
Language Accepted by DFA
it is denoted as and containsall the strings accepted by
MLM
M
MLWe also say that recognizesM
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For a DFA
Language accepted by :
FqQM ,,,, 0
M FwqwML ,: 0
**
0q qw Fq
Costas Busch - LSU 48
Language rejected by :
FwqwML ,: 0**
M
0q qw Fq
Costas Busch - LSU 49
0q
More DFA Examples
ba,},{ ba
*)( ML
0q
ba,
}{)( MLEmpty language All strings
Costas Busch - LSU 50
1q
ba,
},{ ba
0q ba,
}{)( MLLanguage of the empty string
Costas Busch - LSU 51
ML = { all strings with prefix }ab
a b
ba,
0q 1q 2q
accept
ba,3q
ab
},{ ba
Costas Busch - LSU 52
ML = { all binary strings containing substring }001
0 00 001
1
0
110
0 1,0
Costas Busch - LSU 53
ML = { all binary strings without substring }001
0 00 001
1
0
110
0 1,0
Costas Busch - LSU 54
*,:)( bawawaML
a
b
ba,
a
b
ba
0q 2q 3q
1q
Costas Busch - LSU 55
Regular LanguagesDefinition:A language is regular if there is a DFA that accepts it ( )
The languages accepted by all DFAs form the family of regular languages
LM LML )(
Costas Busch - LSU 56
abba abbaab,,}0:{ nban
{ all strings in {a,b}* with prefix }ab{ all binary strings without substring }001
Example regular languages:
There exist DFAs that accept theselanguages (see previous slides).
*,: bawawa
even} is and }1{:{ * xxx
}{ }{ *},{ ba
Costas Busch - LSU 57
There exist languages which are not Regular:
}0:{ nbaL nn
There are no DFAs that accept these languages
(we will prove this in a later class)
}n ,1z,1y,1x :{ mn
kmzyxADDITION k
