# CS5371 Theory of Computation. Lecture 8: Automata Theory VI (PDA, PDA = CFG)

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1 CS5371 Theory of Computation Lecture 8: Automata Theory VI (PDA, PDA = CFG)

2 Objectives Introduce Pushdown Automaton (PDA) Show that PDA = CFG In terms of descriptive power

3 Pushdown Automaton (PDA) Roughly speaking, PDA = NFA + stack with unlimited size A stack is a last in, first out storage device How does a PDA operate? In each step, it can read a character from the input string, can pop (remove) a symbol from the stack Then, depending on the character and the symbol, the PDA enters another state and can push (place) a symbol to the stack

4 Stack is powerful Recall that an NFA cannot recognize the language {0 n 1 n n 0} However, a PDA can do so (informally): Read the characters from input. For any 0 it reads, push it onto the stack. As soon as 1s are seen**, pop a 0 off the stack for each 1 read. Accept the string if the stack is just empty after the last 1 is read; Reject the string otherwise ** at this point, if we read a 0 again, we reject the string immediately.

5 PDA (Formal Definition) A PDA is a 6-tuple (Q,,,, q 0, F) Q is a finite set of states is a finite set of characters is a finite set of stack symbols is the transition function of the form: : Q x x 2 Q x, where = [ { } and = [ { } q 0 is the start state F is the set of accept states

6 Acceptance by PDA A PDA M = (Q,,,, q 0, F) accepts the input string w if w can be written as w 1 w 2 w m where w i in, there exist states r 0, r 1,, r m in Q, and there exist strings s 0, s 1,, s m in * satisfying the following three conditions: (see next slide) Intuitively, r i denotes the sequence of states visited by the PDA, and s i denotes the corresponding contents in the stack (reading from top to bottom)

7 Acceptance by PDA (cont.) Condition 1: r 0 = q 0, s 0 = This ensures that PDA starts at q 0, with an empty stack Condition 2: For i = 0, 1,, m-1, we have (r i+1, b)2 (r i, w i+1, a), where s i = at, s i+1 = bt for some a, b in This ensures that PDA moves properly according to the state, the input character, and the stack Condition 3: r m 2 F This ensures PDA accepts only when the PDA is in an accept state after processing the whole input string

8 PDA (example 1) The following state diagram gives the PDA that recognizes {0 n 1 n n 0}., \$ 0, 0 1, 0, \$ 1, 0 The notation a, b c means that the machine reads a from input, replace b by c from the top of stack. That is, pop b then push c.

9 PDA (example 1) Some points to notice: The formal definition of PDA does not allow us to test if the stack is empty. The previous PDA tries to get the same effect by first placing \$ to the stack, so that if it ever sees \$ again, it knows the stack is empty Similarly, the PDA cannot test if the input has all been processed. The previous PDA can have the same effect because it can stay at the accept states only at the end of the input

10 PDA (example 1) We can also write the formal definition of the previous PDA, call it M, as follows: M = ({q 1,q 2,q 3,q 4 }, {0,1}, {0,\$},, q 1, {q 1,q 4 }), And is given by (q 1,, ) = { (q 2, \$) } (q 2, 0, ) = { (q 2, 0) } (q 2, 1, 0) = { (q 3, ) } (q 3, 1, 0) = { (q 3, ) } (q 3,, \$) = { (q 4, ) }

11 PDA (example 2) Give a PDA that recognizing the language { a i b j c k i,j,k 0 and i=j or i=k } How to construct it? First, for each a read, we should push it onto the stack (for later matching) Then, we guess whether a should be matched with b or matched with c We can guess because of the non-deterministic nature of PDA

12 PDA (example 2) b, a c,, \$,, \$,,, \$ a, a b, c, a

13 PDA (example 3) Give a PDA recognizing { ww R w in {0,1}* } How to construct it? First, the PDA should match the first character with the last character, the second character with the second last character, and so on So, we push each character that is read to the stack in case it will be matched later At each point, we guess the middle of the string has been reached. We match the remaining characters with those stored in the stack (how?)

14 CFG = PDA Theorem: (1) If a language is generated by a CFG, it can be recognized by some PDA. (2) If a language is recognized by a PDA, it can be generated by some CFG. Proof: We shall prove both statements by construction.

15 Proof of (1) (1) If a language is generated by a CFG, it can be recognized by some PDA. Let L be a language generated by a CFG G. We show how to convert G into an equivalent PDA. Our PDA will do the following: For an input string w, it can determine whether there is a derivation for w by G Recall that a derivation for w = a sequence of substitutions in the grammar that generates w

16 Proof of (1) The difficulty in testing whether there is a derivation for w is to figure out which substitutions to make PDA can do so by guessing the sequence of correct substitutions

17 Proof of (1) Informally, our PDA does the following: Place the mark symbol \$ and then the start variable S (of G) on the stack Repeat If the top of stack is a variable, say A, guess a rule A u and substitute A by the string u If the top of stack is a terminal, say a, read the next symbol from the input and compare it with a. If they match, repeat. Otherwise, reject this branch of non-determinism If the top of stack is \$, enter the accept state

18 An Example Run Input: 1100 CFG: S SS 1S Next char S \$ Next char S 0 \$ 100 Next char S 0 \$ At the beginning PDA uses the rule S 1S0 Input char is matched

19 An Example Run (cont.) Input: 1100 CFG: S SS 1S Next char 0 0 \$ 00 Next char 0 0 \$ 0 Next char 0 \$ PDA uses the rule S 10 Input char is matched Input char is matched

20 An Example Run (cont.) Input: 1100 CFG: S SS 1S0 10 Next char \$ Next char Whole input string is processed The \$ in the top of stack tells PDA the stack is empty. PDA accepts the string

21 Another Example Run Input: 1100 CFG: S SS 1S Next char S Next char 0 Next char 0 \$ \$ \$ At the beginning PDA uses the rule S 10 Input char is matched

22 Another Example Run (cont.) Input: 1100 CFG: S SS 1S Next char 0 \$ Input char cannot match top of stack. What will PDA do? It stops exploring this branch of computation

23 Implementation Details We can see that, an input string w is accepted by our PDA if and only there is a derivation from S to w What remains is to show that such a PDA can be constructed Pushing \$, pushing S, or matching input chars with terminals in the stack is easy Difficulty: How to replace a variable in the top of stack by right side of a corresponding rule? (E.g., top of stack is A and we have with a rule A xyz. How to replace A by xyz?) By using dummy states

24 Using dummy states, A z, y, A xyz, x Using two dummy states to replace A by xyz at the top of the stack A shorthand notation

25 PDA for Proof of (1) start shorthand notation used here, S\$, A u for rule A u a, a for terminal a, \$

26 Example of Conversion Convert the following CFG into an equivalent PDA S atb b T Ta

27 Proof of (2) (2) If a language is recognized by a PDA, it can be generated by some CFG. We show how to convert a PDA into an equivalent CFG. Let L be the language, and P be the PDA recognizing L.

28 Proof of (2) We first change P slightly so that: It has a single accept state, q accept It empties the stack before accept Each transition either pushes a symbol on the stack, or pops a symbol off the stack, but not both For the third change, we replace each transition in P that first and second changes are easy (i) pushes and pops at the same time with a two transition sequence that goes through a new state, (ii) neither pushes or pops with a two transition sequence that pushes then pops a dummy stack symbol

29 Proof of (2) Next, for each pair of states p, q in P, we create a CFG variable A pq. Our target is to make A pq generate exactly those strings that can bring P from p with an empty stack to q with an empty stack How to do so?

30 Creating A pq Note that PDA can get from p (with an empty stack) to q (with an empty stack) in two ways: The stack gets empty before reaching q This implies we get from p to some r (with empty stack) and then to q The stack never gets empty before reaching q This implies at p, we push some char t in stack, and then at q, we pop the same char t

31 Creating A pq (cont.) For each p, q, r, add the rule A pq A pr A rq That is, if we can get from p to r, and also from r to q, then we can get from p to q (Here, all starts and ends are with empty stack) For each p, q, r, s with (r, t)2 (p, a, ) and (q, )2 (s, b, t), add the rule A pq aa rs b That is, if we can get from p to r by reading a and pushing t, and we can get from s to q by reading b and popping t, then, if we start with p with an empty stack, we can reach q with an empty stack by reading a, going from r to s, then reading b.

32 Creating A pq (cont.) For each p, we also add the rule A pp That is, if we can get from p to p by reading nothing If A pq really generates exactly those strings that brings P from p to q (with empty stacks), then what is A qstart, q accept? where q start denotes the start state of PDA

33 Proof of (2) So, in our grammar, we will have all the rules A pq and set A qstart, q accept as the start variable What remains is to prove A pq generates exactly those strings that brings P from p to q (with empty stacks) That is, we need to prove If A pq generates x, x brings P from p to q (with empty stacks) If x brings P from p to q (with empty stacks, A pq generates x Exercise: Prove the above two statements (Hint: by induction)

34 Next Time Pumping Lemma for CFL Non-CFL Discuss DPDA, which is DFA + stack Does it have same power as PDA?

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