cdt314 faber formal languages, automata and models of computation lecture 6 mälardalen university
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CDT314
FABER
Formal Languages, Automata and Models of Computation
Lecture 6
Mälardalen University
2010
2
Content
Midterm againContext-Free LanguagesPush-Down Automata, PDANPDA: Non-Deterministic PDAFormal Definitions for NPDAs NPDAs Accept Context-Free LanguagesConverting NPDA to Context-Free Grammar
Midterm
• 5 out of 13 passed…• Many failed on basic definitions
• Is something wrong with the course format?• Scheduled exercises• Don’t hesitate to ask for help!• Do labs, exercises etc.• Review lecture material before lectures• Strongly recommended to get a book if you
don’t have one• It doesn’t get easier…
3
4
JFLAP demo
Concerning examination in the course:Exercises are voluntaryLabs are voluntaryMidterms are voluntaryLectures are voluntary…All of them are recommended!
http://www.cs.duke.edu/csed/jflap/movies
5
Regular Languages
}{ nnba }{ Rww
Context-Free Languages
Non-regular languages
}0:{ ! nan}0,:{ lncba lnln
6
Context-Free Languages
7
Context-Free Languages
Pushdown
Automata
Context-Free
Grammars
stack
automaton
(CF grammars are
defined as generalized Regular Grammars)
8
Definition: Context-Free Grammars
Grammar
Productions of the form:xA
x is string of variables and terminals
),,,( PSTVG
Variables Terminal
symbols
Start
variables
9
Pushdown AutomataPDAs
10
Pushdown Automaton - PDA
Input String
Stack
States
11
The Stack
The stack allows pushdown automata to recognize some non-regular languages.
All access to the stack only on the top!
(Stack top is written leftmost in the string, e.g. yxz)
A PDA can write symbols on stack and read them later on.
A stack is valuable as it can hold an unlimited amount of (certain kind of) information.
zxy
POP reading symbol PUSH writing symbol
12
The States
Input
symbol
Pop old- reading
stack symbol
Push new- writing
stack symbol
q1 q2cba /,
13
a
b top
input
stack
a
Replace
eh
$eh
$
c
q1 q2cba /,
(An alternative is to start and finish with empty stack or a stack bottom symbol such as $)
14
a a
input
Pushb
eh
$eh
$
bc
top
stack
q1 q2ca /,
15
a a input
Popb
eh
$eh
$
top
stack
q1 q2/,ba
16
a a input
No Changeb
eh
$eh
$
btop
stack
q1 q2 /,a
17
NPDAsNon-deterministic Push-Down Automata
18
Non-Determinism
q1
q2cba /,
q3
q1 q2
transitioncba /,
cb /,
19
A string is accepted if:
• All the input is consumed• The last state is a final state• Stack is in the initial condition (either: empty (when we started with empty stack), or: bottom symbol reached, or similar)
20
}0:{ nbaL nn
is the language accepted by the NPDA:
Example NPDA
q0 q1 q2 q3 /, /, ab $/$,
/, abaa /,
},{ ba
symbol)stack initial :($
21
/,
/,
bb
aa
q1 q2 /,q0
NPDA M
}{)( RwwML
bb
aa
/,
/,
$/$,
(Even-length palindromes)
},{ ba
Example NPDA
22
Pushing Strings
q1 q2wba /,
Input
symbol
Pop
symbolPush
string
23
q1 q2cdfba /,
a
b top
input
stack
a
Push
eh h
e
cdf
pushed
string
Example
$ $
24
Another NPDA example
$0/$,a00/0,a/1,a
$1/$,b11/1,b/0,b
$/$,q1 q2
NPDA M
}:{)( ba nnwML
},{ ba
25
Time 0
Input
a ab b ba
$
Stack
Current state
q1 q2
Execution Example
$0/$,a00/0,a/1,a
$1/$,b11/1,b/0,b
$/$,
26
Input
a ab b ba
Stack
$0
q1 q2
$0/$,a00/0,a/1,a
$1/$,b11/1,b/0,b
$/$,
Time 1
27
Input
a bb b a
Stack
a
q1 q2
$0
$0/$,a00/0,a/1,a
$1/$,b11/1,b/0,b
$/$,
Time 2
28
Input
a bb b a
Stack
a
$1
q1 q2
$0/$,a00/0,a/1,a
$1/$,b11/1,b/0,b
$/$,
Time 3
29
Input
a bb b a
Stack
a
$11
q1 q2
$0/$,a00/0,a/1,a
$1/$,b11/1,b/0,b
$/$,
Time 4
30
Input
a bb b a
Stack
a
q1 q2
$11
$0/$,a00/0,a/1,a
$1/$,b11/1,b/0,b
$/$,
Time 5
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Input
a bb b a
Stack
a
q1 q2
$1
$0/$,a00/0,a/1,a
$1/$,b11/1,b/0,b
$/$,
Time 6
32
Input
a bb b a a
$
Stack
q1 q2
accept
$0/$,a00/0,a/1,a
$1/$,b11/1,b/0,b
$/$,
Time 7
33
Formal Definitions for NPDAs
34
q1 q2wba /,
)},{(),,( 21 wqbaq
Transition function
35
Transition function
q1
q2wba /,
q3
wba /,
current state
)},(),,{(),,( 321 wqwqbaq
current input symbol
current stack top
new state
new stack top
An unspecified transition function is to the null set and represents a dead configuration for the NPDA.
36
Formal Definition
Non-Deterministic Pushdown Automaton NPDA
),,,,,( FzQM States
Input
alphabetStack
alphabet
Transition
function
Final
states
Stack
start
symbol
37
Instantaneous Description
),,( suq
Current
state Remaining
input
Current
stack
contents
38q0 q1 q2 q3
Input
Stack
a a a b b b$
aaTime 4:
Example Instantaneous Description
$),,( 1 aaabbbq
a
/, /, ab $/$,
/, abaa /,
),,( stackinputremainingstateurrentc
39q0 q1 q2 q3
Input
Stack
a a a b b b$
aaTime 5:
Example Instantaneous Description
$),,( 2 aabbq
a
/, /, ab $/$,
/, abaa /,
),,( stackinputremainingstateurrentc
40
We write
$),,($),,( 21 aabbqaaabbbq Time 4 Time 5
),,( contentstackcurrentinputremainingstatecurrent
41
$),,($),,($),,($),,(
$),,($),,($),,(
$),,($),,(
3222
111
10
qqabqaabbq
aaabbbqaaabbbqaaabbbq
aaabbbqaaabbbq
A computation example
q0 q1 q2 q3 /, /, ab $/$,
/, abaa /,
),,( contentstackcurrentinputremainingstatecurrent
42
$),,($),,($),,($),,(
$),,($),,($),,(
$),,($),,(
3222
111
10
qqabqaabbq
aaabbbqaaabbbqaaabbbq
aaabbbqaaabbbq
q0 q1 q2 q3 /, /, ab $/$,
/, abaa /,
),,( contentstackcurrentinputremainingstatecurrentA computation example
43
$),,($),,($),,($),,(
$),,($),,($),,(
$),,($),,(
3222
111
10
qqabqaabbq
aaabbbqaaabbbqaaabbbq
aaabbbqaaabbbq
q0 q1 q2 q3 /, /, ab $/$,
/, abaa /,
),,( contentstackcurrentinputremainingstatecurrentA computation example
44
$),,($),,($),,($),,(
$),,($),,($),,(
$),,($),,(
3222
111
10
qqabqaabbq
aaabbbqaaabbbqaaabbbq
aaabbbqaaabbbq
q0 q1 q2 q3 /, /, ab $/$,
/, abaa /,
),,( contentstackcurrentinputremainingstatecurrentA computation example
45
$),,($),,($),,($),,(
$),,($),,($),,(
$),,($),,(
3222
111
10
qqabqaabbq
aaabbbqaaabbbqaaabbbq
aaabbbqaaabbbq
q0 q1 q2 q3 /, /, ab $/$,
/, abaa /,
),,( contentstackcurrentinputremainingstatecurrentA computation example
46
$),,($),,($),,($),,(
$),,($),,($),,(
$),,($),,(
3222
111
10
qqabqaabbq
aaabbbqaaabbbqaaabbbq
aaabbbqaaabbbq
q0 q1 q2 q3 /, /, ab $/$,
/, abaa /,
),,( contentstackcurrentinputremainingstatecurrentA computation example
47
$),,($),,($),,($),,(
$),,($),,($),,(
$),,($),,(
3222
111
10
qqabqaabbq
aaabbbqaaabbbqaaabbbq
aaabbbqaaabbbq
q0 q1 q2 q3 /, /, ab $/$,
/, abaa /,
),,( contentstackcurrentinputremainingstatecurrentA computation example
48
$),,($),,($),,($),,(
$),,($),,($),,(
$),,($),,(
3222
111
10
qqabqaabbq
aaabbbqaaabbbqaaabbbq
aaabbbqaaabbbq
q0 q1 q2 q3 /, /, ab $/$,
/, abaa /,
),,( contentstackcurrentinputremainingstatecurrentA computation example
49
For convenience we write
$),,($),,( 30 qaaabbbq
$),,($),,($),,($),,(
$),,($),,($),,(
$),,($),,(
3222
111
10
qqabqaabbq
aaabbbqaaabbbqaaabbbq
aaabbbqaaabbbq
),,( contentstackcurrentinputremainingstatecurrent
50
Formal Definition
Language of NPDA M
)}',,(),,(:{)( 0 sqswqwML f
Initial state Final state
51
Example ,$),(,$),( 30 qaaabbbq
q0 q1 q2 q3
NPDA M
)(MLaaabbb
/, /, ab $/$,
/, abaa /,
52
,$),(,$),( 30 qbaq nn
)(MLba nn
q0 q1 q2 q3 /, /, ab $/$,
/, abaa /,NPDA M
53
}0:{)( nbaML nnTherefore:
q0 q1 q2 q3 /, /, ab $/$,
/, abaa /,
NPDA M
54
NPDAs Accept Context-Free Languages
55
Context-Free
Languages
(Grammars)
Languages
Accepted by
NPDAs
Theorem
56
Context-Free
Languages
(Grammars)
Languages
Accepted by
NPDAs
Proof - Step 1:
Convert any context-free grammar G to a NPDA M
with L(G) = L(M)
57
Context-Free
Languages
(Grammars)
Languages
Accepted by
NPDAs
Proof - Step 2:
Convert any NPDA M to a context-free grammar G with L(M) = L(G)
58
Converting Context-Free Grammars
to NPDAs
59
An example grammar:
T
TaT
bS
aSTbS
What is the equivalent NPDA?
60
Grammar NPDA
/,
/,
/,
/,
T
TaT
bS
aSTbS
T
TaT
bS
aSTbS
/,
/,
bb
aa
q0 q1 2qS/, $/$,
61
The NPDA simulates
leftmost derivations of the grammar
L(Grammar) = L(NPDA)
62
Grammar:
T
TaT
bS
aSTbS
A leftmost derivation:
abababTabbTabTbSaS
63
NPDA execution:
0q q1 2qS/, $/$,
/,
/,
/,
/,
T
TaT
bS
aSTbS
/,
/,
bb
aa
Input
Stack
$
a ab
Time 0
b
Start
64
q0 q1 2q
Input
Stack
a ab
Time 1
b
$S
S/, $/$,
/,
/,
/,
/,
T
TaT
bS
aSTbS
/,
/,
bb
aa
65
q0 2q
Input
Stack
a ab
Time 2
b
$
a
b
ST
q1S/, $/$,
/,
/,
/,
/,
T
TaT
bS
aSTbS
/,
/,
bb
aa
66
q0 2q
Input
Stack
a ab
Time 3
b
$
a
b
ST
q1S/, $/$,
/,
/,
/,
/,
T
TaT
bS
aSTbS
/,
/,
bb
aa
67
q0 2q
Input
Stack
a ab
Time 4
b
$bTb
q1S/, $/$,
/,
/,
/,
/,
T
TaT
bS
aSTbS
/,
/,
bb
aa
68
q0 2q
Input
Stack
a ab
Time 5
b
$bTb
q1S/, $/$,
/,
/,
/,
/,
T
TaT
bS
aSTbS
/,
/,
bb
aa
69
q0 2qS, , $ $
bb
aa
,
,
Input
Stack
a ab
Time 6
b
T
TaT
bS
aSTbS
,
,
,
, $b
Ta
q1
70
q0 2q
Input
Stack
a ab
Time 7
b
$b
Ta
q1S/, $/$,
/,
/,
/,
/,
T
TaT
bS
aSTbS
/,
/,
bb
aa
71
q0 2q
Input
Stack
a ab
Time 8
b
$ba
q1S/, $/$,
/,
/,
/,
/,
T
TaT
bS
aSTbS
/,
/,
bb
aa
72
q0 2q
Input
Stack
a ab
Time 9
b
$b
q1S/, $/$,
/,
/,
/,
/,
T
TaT
bS
aSTbS
/,
/,
bb
aa
73
q0 q1 2q
Input
Stack
$
a ab
Time 10
b
acceptS/, $/$,
/,
/,
/,
/,
T
TaT
bS
aSTbS
/,
/,
bb
aa
74
In general
Given any grammar Gwe can construct a NPDA M with
)()( MLGL
75
q0 q1 2qS/, $/$,
wA /, /, aa
For any productionwA
For any terminala
Constructing NPDA M from grammar GTop-down parser
76
Grammar G generates string w
if and only if
NPDA M accepts w
)()( MLGL
77
For any context-free language
there is an NPDA
that accepts the same language
78
Which means
Languages
Accepted by
NPDAs
Context-
FreeLanguages(Grammars)
79
Converting NPDAsto
Context-Free Grammars
80
For any NPDA M
we will construct
a context-free grammar G with
)()( GLML
81
in NPDA M
abcABCabcS
Input processed Stack contents
terminals variables
GA derivation in Grammar
The grammar simulates the machine
82
Some Simplifications
First we modify the NPDA so that
• It has a single final state qf and
• It empties the stack
when it accepts the input
Original NPDA Empty Stack
fq
83
Second we modify the NPDA transitions.
All transitions will have form:
iq jq/, Ba
or
iq jqCDBa /,
which means that each move
increases/decreases stack by a single symbol.
84
Those simplifications do not affect generality of our argument.
It can be shown that for any NPDA there exists an equivalent one having above two properties
i.e.the equivalent NPDA with a single final state which empties
its stack when it accepts the input, and which for each move increases/decreases stack by a single symbol.
85
/1,
00/0,
$0/$,
a
a
a
/$,0q fq
}:{)( ba nnwML
Example of a NPDA in an appropriate form
symbol)stack initial :($/0,
11/1,
$1/$,
b
b
b},{ ba
86
The Grammar Construction
)( jiBqq
In grammar G
Terminals:
Input symbols of NPDA
states
Stack symbol
Variables:
87
iq jq/, BaFor each transition:
we add production: aBqq ji )(
88
For each transition:
we add production: ))(()( klljki DqqCqqaBqq
iq jqCDBa /,
for all states qk , ql
89
Start Variable )$( fo qq
Stack bottom symbol
Start state (Single) Final state
90
Example
aqq )1( 00Grammar production:
0q fq
/1,
00/0,
$0/$,
a
a
a
/$,
/0,
11/1,
$1/$,
b
b
biq jq/, Ba
aBqq ji )(
}:{)( ba nnwML },{ ba
91
)$)(1(|)$)(1()$(
)$)(1(|)$)(1()$(
00000
00000000
fffff
ff
qqqqbqqqqbqq
qqqqbqqqqbqq
Grammar productions:
0q fq
/1,
00/0,
$0/$,
a
a
a
/$,
/0,
11/1,
$1/$,
b
b
b iq jqCDBa /,
))(()( klljki DqqCqqaBqq
lk qqstatesallfor ,
92
Grammar production: )$( 0 fqq
0q fq
/1,
00/0,
$0/$,
a
a
a
/$,
/0,
11/1,
$1/$,
b
b
biq jq/, Ba
aBqq ji )(
93
0q fq
/1,
00/0,
$0/$,
a
a
a
/$,
/0,
11/1,
$1/$,
b
b
b
Resulting Grammar
ablestart vari:)$( 0 fqq
)$)(1(|)$)(1()$(
)$)(1(|)$)(1()$(
00000
00000000
fffff
ff
qqqqbqqqqbqq
qqqqbqqqqbqq
)1)(1(|)1)(1()1(
)1)(1(|)1)(1()1(
00000
00000000
fffff
ff
qqqqbqqqqbqq
qqqqbqqqqbqq
}:{)( ba nnwML },{ ba
))(()( klljki DqqCqqaBqq
lk qqstatesallfor ,
94
)$)(0(|)$)(0()$(
)$)(0(|)$)(0()$(
00000
00000000
fffff
ff
qqqqaqqqqaqq
qqqqaqqqqaqq
Resulting Grammar, cont.
)0)(0(|)0)(0()0(
)0)(0(|)0)(0()0(
00000
00000000
fffff
ff
qqqqaqqqqaqq
qqqqaqqqqaqq
0q fq
/1,
00/0,
$0/$,
a
a
a
/$,
/0,
11/1,
$1/$,
b
b
b }:{)( ba nnwML
95
bqq
aqq
)0(
)1(
00
00
)$( 0 fqq0q fq
/1,
00/0,
$0/$,
a
a
a
/$,
/0,
11/1,
$1/$,
b
b
b
Resulting Grammar, cont.
iq jq/, Ba
aBqq ji )(
}:{)( ba nnwML
96
Derivation of string abba
)$( 0 fqq )$)(0( 000 fqqqqa
)$( 0 fqqab
)$)(1( 000 fqqqqabb
)$( 0 fqqabba
abba
0q fq
/1,
00/0,
$0/$,
a
a
a
/$,
/0,
11/1,
$1/$,
b
b
b
iq jq/, Ba
aBqq ji )(
97
In general, in grammar:
wqq f
)$( 0
if and only if
w is accepted by the NPDA
98
Explanation
By construction of Grammar:
wAqq ji
)(
if and only if
in the NPDA going from qi to qj
the stack doesn’t change below
and A is removed from stack
iq jq/, Ba
aBqq ji )(
99
Context-Free
Languages
(Grammars)
Languages
Accepted by
NPDAs
We have shown the procedure to convert
any NPDA M to a context-free
grammar G with L(M) = L(G)which means
100
Context-Free
Languages
(Grammars)
Languages
Accepted by
NPDAs
Therefore
END OF PROOF
101
q0 q1
Example (Sudkamp 8.1.2)
/, Ab
/,
/, AbAa /,
}0:{}0:{)( ibaiaML iii
Language consisting solely of a’s or an equal number of a´s and b´s.
q2 /, A
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