COSC 3340: Introduction to Theory of Computation

COSC 3340: Introduction to Theory of Computation University of Houston Dr. Verma Lecture 14 UofH - COSC 3340 - Dr. Verma

PDA’s and CFG’s • For every CFG G there is a PDA M such that L(G) = L(M) • For every PDA M there is a CFG G such that L(M) = L(G) UofH - COSC 3340 - Dr. Verma

CFG  PDA • Given CFG G = (V, , R, S) • Let PDA M = (Q, ,   V  {$}, , qstart, {qaccept}) • Q = {qstart, qloop,qaccept} •  contains transitions for the form • ((qstart,, ), (qloop, S$))   • For each rule A  w  R(G) there is a transition ((qloop,, A),(qloop, w))   *** • For each symbol    ((qloop, , ), (qloop, ))   • ((qloop,, $), (qaccept, ))   , S$ qstart ,   , A w qloop , $  qaccept UofH - COSC 3340 - Dr. Verma

CFG  PDA • The PDA simulates a leftmost derivation of the string. • Place the marker symbol $ and the start variable on the stack. • Repeat the following steps forever (a) If the top of stack is a variable symbol A, nondeterministically select on of the rules for A and substitute A by the string on the right-hand side of the rule. (b) If the top of stack is terminal symbol , read the next symbol from the input and compare it to . If they match, repeat. If they do not match, reject on this branch of the nondeterminism. (c) If the top of stack is the symbol $, enter the accept state. Doing so accepts the input if it has all been read. UofH - COSC 3340 - Dr. Verma

Example: S  aSb |  Z is used instead of $  Instead of  q0 = qstart q1 = qloop q2 = qaccept UofH - COSC 3340 - Dr. Verma

JFLAP SIMULATION UofH - COSC 3340 - Dr. Verma

Idea of PDA  CFG • First, we simplify our task by modifying P slightly to give it the following three features • It has a single accept state, qaccept. • It empties its stack before accepting. • Each transition either pushes a symbol onto the stack (a push move) or pops one off the stack (a pop move), but does not do both at the same time. UofH - COSC 3340 - Dr. Verma

Idea of PDA  CFG • Apq generates all strings that take PDA, P, from p to q, starting and ending with empty stack. • For any string x, P’s first move on x must be a push (why?) and last move must be a pop (why?). • Two possibilities occur during P’s computation on x: either the symbol popped at the end is the symbol pushed at beginning, or not. • The first type of rule simulates the first possibility • The second type of rule simulates the second. UofH - COSC 3340 - Dr. Verma

PDA  CFG • Say that P = {Q, , , , q0, {qaccept}} and construct G. The variable of G are {Apq| p, q Q}. The start variable is Aq0qaccept. Now we describe G’s rules. • For each p, q, r, s Q, t  , and a, b  , if (p, a, ) contains (r, t) and (s, b, t) contains (q, ) put the rule Apq aArsb in G. • For each p, q, r  Q put the rule Apq AprArq in G. • Finally, for each p  Q put the rule App  in G. UofH - COSC 3340 - Dr. Verma

Example • Let M be the PDA for {anbn | n > 0} • Note that n cannot be 0, which makes the example a little simpler. M = {{p, q}, {a, b}, {a}, , p, {q}}, where  = {((p, a, ),(p, a)),((p, b, a), (q, )),((q, b, a),(q, ))} p q UofH - COSC 3340 - Dr. Verma

Example: contd. • CFG, G = (V, {a, b}, Apq, R) corresponding to M has V = {App, Apq, Aqp, Aqq}. R contains the following rules: • Type I: • Apq aAppb • Apq aApqb • Type II: • App App App| Apq Aqp • Apq App Apq| Apq Aqq • Aqp Aqq Aqp| Aqp App • Aqq Aqq Aqq| Aqp Apq • Type III: • App  • Aqq  We can discard all rules containing the variables Aqq and Aqp. And we can also simplify the rules containing App and get the grammar with just two rules Apqab and Apq  aApqb. UofH - COSC 3340 - Dr. Verma

JFLAP SIMULATION UofH - COSC 3340 - Dr. Verma

COSC 3340: Introduction to Theory of Computation