notes20 Context-free Grammars and Ambiguity

# notes20 Context-free Grammars and Ambiguity - CS 373 Theory...

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CS 373: Theory of Computation Gul Agha Mahesh Viswanathan Fall 2010 1

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1 Context Free Grammars 1.1 Definitions Context-Free Grammars Definition 1. A context-free grammar (CFG) is G = ( V, Σ , R, S ) where V is a finite set of variables/non-terminals. Σ is a finite set of terminals. Σ is disjoint from V . R is a finite set of rules or productions of the form A α where A V and α ( V Σ) * S V is the start symbol Example: Palindromes Example 2 . A string w is a palindrome if w = w R . G pal = ( { S } , { 0 , 1 } , R, S ) defines palindromes over { 0 , 1 } , where R is S S 0 S 1 S 0 S 0 S 1 S 1 Or more briefly, R = { S | 0 | 1 | 0 S 0 | 1 S 1 } Language of a CFG Derivations Expand the start symbol using one of its rules. Further expand the resulting string by expanding one of the variables in the string, by the RHS of one of its rules. Repeat until you get a string of terminals. For the grammar G pal = ( { S } , { 0 , 1 } , { S | 0 | 1 | 0 S 0 | 1 S 1 } , S ) we have S 0 S 0 00 S 00 001 S 100 0010100 Derivations Formal Definition Definition 3. Let G = ( V, Σ , R, S ) be a CFG. We say αAβ G αγβ , where α, β, γ ( V Σ) * and A V if A γ is a rule of G . We say α * G β if either α = β or there are α 0 , α 1 , . . . α n such that α = α 0 G α 1 G α 2 G · · · ⇒ G α n = β 2
Notation When G is clear from the context, we will write and * instead of G and * G . Context-Free Language Definition 4. The language of CFG G = ( V, Σ , R, S ), denoted L ( G ) is the collection of strings over the terminals derivable from S using the rules in R . In other words, L ( G ) = { w Σ * | S * w } Definition 5. A language L is said to be context-free if there is a CFG G such that L = L ( G ). 1.2 Proving Properties Palindromes Revisited Recall, L pal = { w ∈ { 0 , 1 } * | w = w R } is the language of palindromes. Consider G pal = ( { S } , { 0 , 1 } , R, S ) defines palindromes over { 0 , 1 } , where R = { S | 0 | 1 | 0 S 0 | 1 S 1 } Proposition 6. L ( G pal ) = L pal Proving Correctness of CFG L pal L ( G pal ) Proof. Let w L pal . We prove that S * w by induction on | w | . Base Cases: If | w | = 0 or | w | = 1 then w = or 0 or 1. And S | 0 | 1.

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