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Theory of Automata

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Theory of Automata
(CS402)
Theory of Automata
Lecture N0. 45
Reading Material
Introduction to Computer Theory
Chapter 19
Summary
Turing machine, examples, DELETE subprogram, example, INSERT subprogram, example.
Turing machine
The mathematical models (FAs, TGs, PDAs) that have been discussed so far can decide whether a string is
accepted or not by them i.e. these models are language identifiers. However, there are still some languages
which can't be accepted by them e.g. there does not exist any FA or TG or PDA accepting any non-CFLs.
Alan Mathison Turing developed the machines called Turing machines, which accept some non-CFLs as well,
in addition to CFLs.
Definition
A Turing machine (TM) consists of the following
An alphabet  of input letters.
An input TAPE partitioned into cells, having infinite many locations in one direction. The input string is placed
on the TAPE starting its first letter on the cell i, the rest of the TAPE is initially filled with blanks (D's).
Input TAPE
ii
iii
iv
i
D
µ
a
b
a
TAPE Head
A tape Head can read the contents of cell on the TAPE in one step. It can replace the character at any cell and
can reposition itself to the next cell to the right or to the left of that it has just read. Initially the TAPE Head is at
the cell i. The TAPE Head can't move to the left of cell i. the location of the TAPE Head is denoted by
.
An alphabet G of characters that can be printed on the TAPE by the TAPE Head. G may include the letters of Â.
Even the TAPE Head can print blank D, which means to erase some character from the TAPE.
Finite set of states containing exactly one START state and some (may be none) HALT states that cause
execution to terminate when the HALT states are entered.
A program which is the set of rules, which show that which state is to be entered when a letter is read form the
TAPE and what character is to be printed. This program is shown by the states connected by directed edges
labeled by triplet (letter, letter, direction). It may be noted that the first letter is the character the TAPE Head
reads from the cell to which it is pointing. The second letter is what the TAPE Head prints the cell before it
leaves. The direction tells the TAPE Head whether to move one cell to the right, R, or one cell to the left, L.
Note
It may be noted that there may not be any outgoing edge at certain state for certain letter to be read from the
TAPE, which creates nondeterminism in Turing machines. It may also be noted that at certain state, there can't
be more than one out going edges for certain letter to be read from the TAPE. The machine crashes if there is
not path for a letter to be read from the TAPE and the corresponding string is supposed to be rejected.
To terminate execution of certain input string successfully, a HALT state must be entered and the corresponding
string is supposed to be accepted by the TM. The machine also crashes when the TAPE Head is instructed to
move one cell to the left of cell i.
Following is an example of TM
Example
Consider the following Turing machine
147
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Theory of Automata
(CS402)
(a,a,R)
(b,b,R)
(a,a,R)
(D,D,R)
(b,b,R)
2
4 HALT
3
1 START
(b,b,R)
Let the input string aba be run over this TM
Input TAPE
ii
iii
iv
i
D
µ
a
b
a
TAPE Head
Starting from the START state, reading a form the TAPE and according to the TM program, a will be printed
i.e. a will be replaced by a and the TAPE Head will be moved one cell to the right.
Which can be seen as
Input TAPE
ii
iii
iv
i
D
µ
a
b
a
TAPE Head
This process can be expressed as
1
2
aba
aba
At state 2 reading b, state 3 is entered and the letter b is replaced by b, i.e.
1
3
2
aba
aba
aba
At state 3 reading a, will keep the state of the TM unchanged. Lastly, the blank D is read and D is replaced by D
and the HALT state is entered. Which can be expressed as
1
2
3
3
HALT
abaD
aba
aba
aba
Which shows that the string aba is accepted by this machine. It can be observed, from the program of the TM,
that the machine accepts the language expressed by (a+b)b(a+b)*.
Theorem
Every regular language is accepted by some TM.
(b,b,R)
Example
(D,D,R)
2
1 START
5 HALT
(b,b,R)
(a,a,R)
(a,a,R)
(a,a,R)
(a,a,R)
(b,b,R)
4
3
(b,b,R)
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Theory of Automata
(CS402)
Consider the EVEN-EVEN language. Following is a TM accepting the EVEN-EVEN language.
It may be noted that the above diagram is similar to that of FA corresponding to EVEN-EVEN language.
Following is another example
Example
Consider the following TM
(b,b,R)
(a,a,R)
(a,a,L)
(b,b,R)
4
3
2
(b,a,R)
(a,*,R)
(a,a,R)
(D,D,R)
5
9 HALT
1 START
(D,D,L)
(*,*,R)
(a,a,L)
(b,b,L)
(a,D,L)
(a,D,L)
6
7
8
The string aaabbbaaa can be observed to be accepted by the above TM. It can also be observed that the above
TM accepts the non-CFL {anbnan}.
INSERT subprogram
Sometimes, a character is required to be inserted on the TAPE exactly at the spot where the TAPE Head is
pointing, so that the character occupies the required cell and the other characters on the TAPE are moved one
cell right. The characters to the left of the pointed cell are also required to remain as such.
In the situation stated above, the part of TM program that executes the process of insertion does not affect the
function that the TM is performing. The subprogram of insertion is independent and can be incorporated at any
time with any TM program specifying what character to be inserted at what location. The subprogram of
insertion can be expressed as
INSERT a
INSERT b
INSERT #
The above diagrams show that the characters a,b and # are to be inserted, respectively. Following is an example
showing how does the subprogram INSERT perform its function
Example
If the letter b is inserted at the cell where the TAPE Head is pointing as shown below
Dµ
µ
D
b
X
a
b
b
X
then, it is expressed as
Dµ
µ
D
b
X
a
b
b
X
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Theory of Automata
(CS402)
INSERT b
The function of subprogram INSERT b can be observed from the following diagram
Dµ
µ
D
b
X
b
a
b
b
X
Following is the INSERT subprogram
The subprogram INSERT
Keeping in view the same example of inserting b at specified location, to determine the required subprogram,
first Q will be inserted as marker at the required location, so that the TAPE Head must be able to locate the
proper cell to the right of the insertion cell. The whole subprogram INSERT is given as
(a,a,R)
2
(a,Q,R)
(b,a,R)
(a,b,R)
(D,a,R)
(b,b,R)
(b,Q,R)
(a,X,R)
1
3
In
(X,a,R)
(b,X,R)
(D, b,R)
(X,b,R)
(X,Q,R)
(D, X,R)
5
4
(D, b,R)
(D, D,L)
(X,X,R)
(b,b,L)
(Q, b,R)
(a,a,L)
6
7
(X,X,L)
Out
It is supposed that machine is at state 1, when b is to be inserted. All three possibilities of reading a, b or X are
considered by introducing the states 2,3 and 4 respectively. These states remember what letter displaced during
the insertion of Q.
Consider the same location where b is to be inserted
Dµ
µ
D
b
X
a
b
b
X
After reading a from the TAPE, the program replaces a by Q and the TAPE Head will be moved one step right.
Here the state 2 is entered. Reading b at state 2, b will be replaced by a and state 3 will be entered. At state 3, b
is read which is not replaced by any character and the state 3 will not be left.
At state 3, the next letter to be read is X, which will be replaced by b and the state 4 will be entered. At state 4,
D will be read, which will be replaced by X and state 5 will be entered. At state 5, D will be read and without
any change state 6 will be entered, while TAPE Head will be moved one step left. The state 6 makes no change
whatever (except Q) is read at that state. However at each step, the TAPE Head is moved one step left. Finally,
Q is read which is replaced by b and the TAPE Head is moved to one step right.
Hence, the required situation of the TAPE can be shown as
Dµ
µ
D
b
X
a
b
b
X
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Theory of Automata
(CS402)
DELETE subprogram
Sometimes, a character is required to be DELETED on the TAPE exactly at the spot where the TAPE Head is
pointing, so that the other characters on the right of the TAPE Head are moved one cell left. The characters to
the left of the pointed cell are also required to remain as such.
In the situation stated above, the part of TM program that executes the process of deletion does not affect the
function that the TM is performing. The subprogram of deletion is independent and can be incorporated at any
time with any TM program specifying what character to be deleted at what location. The subprogram of deletion
can be expressed as
Example
If the letter a is to be deleted from the string bcabbc, shown below
Dµ
µ
D
b
c
a
b
b
c
then, it is expressed as
Dµ
µ
D
b
c
a
b
b
X
DELETE
The function of subprogram DELETE can be observed from the following diagram
µ
◊◊
D
D
b
c
b
b
c
(a,a,R)
(b,b,R)
(c,c,R)
In
2
3
1
(c,D,R)
(D,D,L)
(b,D,R)
(a,D,R)
(c,D,L)
Following is the DELETE subprogram
(b, D,L)
(c,c,L)
(a,a,L)
(b,b,L)
(b,a,L)
(c,b,L)
6
5
4
(b,c,L)
(a,b,L)
(a,c,L)
(c,a,L)
(D,c,R)
(D,a,R)
(D,b,R)
Out
7
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Theory of Automata
(CS402)
The process of deletion of letter a from the string bcabbc can easily be checked, giving the TAPE situation as
shown below
µ
◊◊
D
D
b
c
b
b
c
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