Pushdown Accepters &amp; Context-Free Grammars

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# Pushdown Accepters & Context-Free Grammars - PowerPoint PPT Presentation

Pushdown Accepters &amp; Context-Free Grammars. Sipser, Theorem 2.12 Denning, Chapter 8. Fundamental theorem of CFLs and PDAs. Theorem 2.12 (Sipser) A language L is context-free if and only if (iff) there exists a pushdown accepter M that recognizes L. CFL if PDA; PDA -&gt; CFL.

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### Pushdown Accepters & Context-Free Grammars

Sipser, Theorem 2.12

Denning, Chapter 8

Fundamental theorem of CFLs and PDAs
• Theorem 2.12 (Sipser) A language L is context-free if and only if (iff) there exists a pushdown accepter M that recognizes L
CFL if PDA; PDA -> CFL
• Given a PDA, M, that recognizes language L, then there exists a CFG, G, that generates language L.
• Proof: Denning, Section 8.4, using traverse sets
CFL only if PDA; CFL -> PDA
• Given a CFL, L, generated by grammar G, we can build a PDA, M, which recognizes language L.
• Proof: Denning, Section 8.3
Proof: CFL -> PDA
• Since L is context free, there is a context free grammar, G = (N,T,P,S) that generates L.
• Construct the PDA, M=(Q,T,U,P’,q0,{q3}), as follows:
• T = same input alphabet
• U = N  T  {S} = stack alphabet
• Q = {q0,q1,q2,q3}  {qx | x  U}
Program of M
• q0: , push(\$), goto q1q1: , push(S), goto q2q2: , pop(\$), goto q3
• For each production A->wq2: , pop(A), goto qAqA: , push(wR), goto q2
• For each terminal symbol aq2: a, NOP, goto qaqa: , pop(a), goto q2
Example: L = {0k1k | k  0}
• Grammar G has productions:SA; A01; A0A1

push 1

push A

push 0

pop A

push A

push 0

pop S

push S

push \$

pop \$

pop 0

in 1

pop 1

in 0

Ex 2: L={0j1k2j+k| j,k > 0}
• SA; A0A2; A0B2; B12; B1B2