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INF2080 Context-Free Langugaes Daniel Lupp Universitetet i Oslo 1st February 2018 Department of University of Informatics Oslo INF2080 Lecture :: 1st February 1 / 37 Repetition Weve looked at one of the simpler computational models:

  1. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } Assume s = a k w 1 w 2 · · · w n , for k � = 1 and w ∈ Σ ∗ not starting with a , where n , k are such that | s | ≥ p . if k = 2, choose x = ε, y = aa , z = w 1 w 2 · · · w n . Then | y | > 0 and | xy | ≤ p . INF2080 Lecture :: 1st February 9 / 37

  2. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } Assume s = a k w 1 w 2 · · · w n , for k � = 1 and w ∈ Σ ∗ not starting with a , where n , k are such that | s | ≥ p . if k = 2, choose x = ε, y = aa , z = w 1 w 2 · · · w n . Then | y | > 0 and | xy | ≤ p . The string xz is in Σ ∗ and doesn’t start with a . INF2080 Lecture :: 1st February 9 / 37

  3. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } Assume s = a k w 1 w 2 · · · w n , for k � = 1 and w ∈ Σ ∗ not starting with a , where n , k are such that | s | ≥ p . if k = 2, choose x = ε, y = aa , z = w 1 w 2 · · · w n . Then | y | > 0 and | xy | ≤ p . The string xz is in Σ ∗ and doesn’t start with a . xz ∈ L INF2080 Lecture :: 1st February 9 / 37

  4. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } Assume s = a k w 1 w 2 · · · w n , for k � = 1 and w ∈ Σ ∗ not starting with a , where n , k are such that | s | ≥ p . if k = 2, choose x = ε, y = aa , z = w 1 w 2 · · · w n . Then | y | > 0 and | xy | ≤ p . The string xz is in Σ ∗ and doesn’t start with a . xz ∈ L The string xy i z for i ≥ 1 starts with 2 or more a ’s, followed by a word w ∈ Σ ∗ that does not start with an a . INF2080 Lecture :: 1st February 9 / 37

  5. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } Assume s = a k w 1 w 2 · · · w n , for k � = 1 and w ∈ Σ ∗ not starting with a , where n , k are such that | s | ≥ p . if k = 2, choose x = ε, y = aa , z = w 1 w 2 · · · w n . Then | y | > 0 and | xy | ≤ p . The string xz is in Σ ∗ and doesn’t start with a . xz ∈ L The string xy i z for i ≥ 1 starts with 2 or more a ’s, followed by a word w ∈ Σ ∗ that does not start with an a . ⇒ xy i z ∈ L . INF2080 Lecture :: 1st February 9 / 37

  6. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } Assume s = a k w 1 w 2 · · · w n , for k � = 1 and w ∈ Σ ∗ not starting with a , where n , k are such that | s | ≥ p . if k ≥ 3, choose x = ε, y = a , z = a k − 1 w 2 · · · w n . Then | y | > 0 and | xy | ≤ p . INF2080 Lecture :: 1st February 10 / 37

  7. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } Assume s = a k w 1 w 2 · · · w n , for k � = 1 and w ∈ Σ ∗ not starting with a , where n , k are such that | s | ≥ p . if k ≥ 3, choose x = ε, y = a , z = a k − 1 w 2 · · · w n . Then | y | > 0 and | xy | ≤ p . The string xz is of the form a k − 1 w , where w ∈ Σ ∗ and doesn’t start with a . INF2080 Lecture :: 1st February 10 / 37

  8. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } Assume s = a k w 1 w 2 · · · w n , for k � = 1 and w ∈ Σ ∗ not starting with a , where n , k are such that | s | ≥ p . if k ≥ 3, choose x = ε, y = a , z = a k − 1 w 2 · · · w n . Then | y | > 0 and | xy | ≤ p . The string xz is of the form a k − 1 w , where w ∈ Σ ∗ and doesn’t start with a . xz ∈ L INF2080 Lecture :: 1st February 10 / 37

  9. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } Assume s = a k w 1 w 2 · · · w n , for k � = 1 and w ∈ Σ ∗ not starting with a , where n , k are such that | s | ≥ p . if k ≥ 3, choose x = ε, y = a , z = a k − 1 w 2 · · · w n . Then | y | > 0 and | xy | ≤ p . The string xz is of the form a k − 1 w , where w ∈ Σ ∗ and doesn’t start with a . xz ∈ L The string xy i z for i ≥ 1 is of the form a k + i − 1 w where w ∈ Σ ∗ that does not start with an a . INF2080 Lecture :: 1st February 10 / 37

  10. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } Assume s = a k w 1 w 2 · · · w n , for k � = 1 and w ∈ Σ ∗ not starting with a , where n , k are such that | s | ≥ p . if k ≥ 3, choose x = ε, y = a , z = a k − 1 w 2 · · · w n . Then | y | > 0 and | xy | ≤ p . The string xz is of the form a k − 1 w , where w ∈ Σ ∗ and doesn’t start with a . xz ∈ L The string xy i z for i ≥ 1 is of the form a k + i − 1 w where w ∈ Σ ∗ that does not start with an a . ⇒ xy i z ∈ L . INF2080 Lecture :: 1st February 10 / 37

  11. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } can be pumped!! Does that mean L is regular? INF2080 Lecture :: 1st February 11 / 37

  12. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } can be pumped!! Does that mean L is regular? If L is regular, then so is L ∩ ab Σ ∗ (recall: regular languages are closed under intersection). INF2080 Lecture :: 1st February 11 / 37

  13. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } can be pumped!! Does that mean L is regular? If L is regular, then so is L ∩ ab Σ ∗ (recall: regular languages are closed under intersection). L ∩ ab Σ ∗ = { ab n c n | n ≥ 1 } INF2080 Lecture :: 1st February 11 / 37

  14. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } can be pumped!! Does that mean L is regular? If L is regular, then so is L ∩ ab Σ ∗ (recall: regular languages are closed under intersection). L ∩ ab Σ ∗ = { ab n c n | n ≥ 1 } Exercise: show that this language is nonregular! (analogous to proof for a n b n ) So L is nonregular...is this a counter-example to the pumping lemma? INF2080 Lecture :: 1st February 11 / 37

  15. Pumping Lemma revisited L = { ab n c n | n ≥ 0 } ∪ { a k w | k � = 1 , and w ∈ Σ ∗ doesn’t start with a } can be pumped!! Does that mean L is regular? If L is regular, then so is L ∩ ab Σ ∗ (recall: regular languages are closed under intersection). L ∩ ab Σ ∗ = { ab n c n | n ≥ 1 } Exercise: show that this language is nonregular! (analogous to proof for a n b n ) So L is nonregular...is this a counter-example to the pumping lemma? No, pumping lemma is not an if and only if statement! INF2080 Lecture :: 1st February 11 / 37

  16. Context-Free Grammars Today: Context-free grammars and languages INF2080 Lecture :: 1st February 12 / 37

  17. Context-Free Grammars Today: Context-free grammars and languages grammars describe the syntax of a language; they try to describe the relationship of all the parts to one another, such as placement of nouns/verbs in sentences INF2080 Lecture :: 1st February 12 / 37

  18. Context-Free Grammars Today: Context-free grammars and languages grammars describe the syntax of a language; they try to describe the relationship of all the parts to one another, such as placement of nouns/verbs in sentences useful for programming languages, specifically compilers and parsers: if the grammar of a programming language is available, parsing is very straightforward. INF2080 Lecture :: 1st February 12 / 37

  19. Context-Free Grammars First example: S → aSb S → ε INF2080 Lecture :: 1st February 13 / 37

  20. Context-Free Grammars First example: S → aSb S → ε Every grammar consists of rules , which are a pair consisting of one variable (to the left of → ) and a string of variables and symbols (to the right of → ) INF2080 Lecture :: 1st February 13 / 37

  21. Context-Free Grammars First example: S → aSb S → ε Every grammar consists of rules , which are a pair consisting of one variable (to the left of → ) and a string of variables and symbols (to the right of → ) Every grammar contains a start variable (above: variable S ). Common convention: the first listed variable is the start variable (if you choose a different start variable, you must specify!). INF2080 Lecture :: 1st February 13 / 37

  22. Context-Free Grammars First example: S → aSb S → ε Every grammar consists of rules , which are a pair consisting of one variable (to the left of → ) and a string of variables and symbols (to the right of → ) Every grammar contains a start variable (above: variable S ). Common convention: the first listed variable is the start variable (if you choose a different start variable, you must specify!). Words are generated by starting with the start variable and recursively replacing variables with the righthand side of a rule. S � aSb � aaSbb � aa ε bb � aabb INF2080 Lecture :: 1st February 13 / 37

  23. Parse Trees Derivations of the form S � aSb � aaSbb � aa ε bb � aabb can also be encoded as a parse tree: S a S b b S b ε INF2080 Lecture :: 1st February 14 / 37

  24. Context-Free Grammars Second example: S → aSa S → bSb S → cSc S → ε INF2080 Lecture :: 1st February 15 / 37

  25. Context-Free Grammars Second example: S → aSa S → bSb S → cSc S → ε To simplify notation, you can summarize multiple rules into one line: S → aSa | bSb | cSc | ε. INF2080 Lecture :: 1st February 15 / 37

  26. Context-Free Grammars Second example: S → aSa S → bSb S → cSc S → ε To simplify notation, you can summarize multiple rules into one line: S → aSa | bSb | cSc | ε. The symbol | takes on the meaning of “or.” INF2080 Lecture :: 1st February 15 / 37

  27. Context-Free Grammars Second example: S → aSa S → bSb S → cSc S → ε To simplify notation, you can summarize multiple rules into one line: S → aSa | bSb | cSc | ε. The symbol | takes on the meaning of “or.” → palindromes of even length over { a , b , c } . INF2080 Lecture :: 1st February 15 / 37

  28. Context-Free Grammar Definition (Context-Free Grammar) A context-free grammar is a 4-tuple ( V , Σ , R , S ) where 1 V is a finite set of variables 2 Σ is a finite set disjoint from V of terminals 3 R is a finite set of rules , each consisting of a variable and of a string of variables and terminals 4 and S is the start variable INF2080 Lecture :: 1st February 16 / 37

  29. Context-Free Grammar Definition (Context-Free Grammar) A context-free grammar is a 4-tuple ( V , Σ , R , S ) where 1 V is a finite set of variables 2 Σ is a finite set disjoint from V of terminals 3 R is a finite set of rules , each consisting of a variable and of a string of variables and terminals 4 and S is the start variable We call L ( G ) the language generated by a context-free grammar. A language is called a context-free language if it is generated by a context-free grammar. INF2080 Lecture :: 1st February 16 / 37

  30. Context-Free Grammar So what can context-free grammars (CFGs) express? INF2080 Lecture :: 1st February 17 / 37

  31. Context-Free Grammar So what can context-free grammars (CFGs) express? Regular languages? INF2080 Lecture :: 1st February 17 / 37

  32. Context-Free Grammar So what can context-free grammars (CFGs) express? Regular languages? Is the class of context-free languages closed under union/intersection/concatanation/complement/Kleene star? INF2080 Lecture :: 1st February 17 / 37

  33. Context-Free Grammar So what can context-free grammars (CFGs) express? Regular languages? Is the class of context-free languages closed under union/intersection/concatanation/complement/Kleene star? Regular languages could be modelled by an automaton with finite memory...what about context-free languages? INF2080 Lecture :: 1st February 17 / 37

  34. Context-Free Grammar So what can context-free grammars (CFGs) express? Regular languages? Is the class of context-free languages closed under union/intersection/concatanation/complement/Kleene star? Regular languages could be modelled by an automaton with finite memory...what about context-free languages? Answers to these over the course of this and next lecture (and group sessions) INF2080 Lecture :: 1st February 17 / 37

  35. RLs and CFLs Can regular languages be described using context-free grammars? INF2080 Lecture :: 1st February 18 / 37

  36. RLs and CFLs Can regular languages be described using context-free grammars? Given a RL L , there exists some DFA ( Q , Σ , δ, q 0 , F ) that accepts L INF2080 Lecture :: 1st February 18 / 37

  37. RLs and CFLs Can regular languages be described using context-free grammars? Given a RL L , there exists some DFA ( Q , Σ , δ, q 0 , F ) that accepts L What if we encode traversing the DFA into grammar rules, i.e., for each transition δ ( q 1 , a ) = q 2 we create a rule Q 1 → aQ 2 INF2080 Lecture :: 1st February 18 / 37

  38. RLs and CFLs Can regular languages be described using context-free grammars? Given a RL L , there exists some DFA ( Q , Σ , δ, q 0 , F ) that accepts L What if we encode traversing the DFA into grammar rules, i.e., for each transition δ ( q 1 , a ) = q 2 we create a rule Q 1 → aQ 2 the variables of our grammar correspond to the states in Q , with Q 0 as the start variable. INF2080 Lecture :: 1st February 18 / 37

  39. RLs and CFLs Can regular languages be described using context-free grammars? Given a RL L , there exists some DFA ( Q , Σ , δ, q 0 , F ) that accepts L What if we encode traversing the DFA into grammar rules, i.e., for each transition δ ( q 1 , a ) = q 2 we create a rule Q 1 → aQ 2 the variables of our grammar correspond to the states in Q , with Q 0 as the start variable. How do we deal with accept states? INF2080 Lecture :: 1st February 18 / 37

  40. RLs and CFLs Can regular languages be described using context-free grammars? Given a RL L , there exists some DFA ( Q , Σ , δ, q 0 , F ) that accepts L What if we encode traversing the DFA into grammar rules, i.e., for each transition δ ( q 1 , a ) = q 2 we create a rule Q 1 → aQ 2 the variables of our grammar correspond to the states in Q , with Q 0 as the start variable. How do we deal with accept states? � for each q i ∈ F , add rule Q i → ε INF2080 Lecture :: 1st February 18 / 37

  41. RLs and CFLs Can regular languages be described using context-free grammars? Given a RL L , there exists some DFA ( Q , Σ , δ, q 0 , F ) that accepts L What if we encode traversing the DFA into grammar rules, i.e., for each transition δ ( q 1 , a ) = q 2 we create a rule Q 1 → aQ 2 the variables of our grammar correspond to the states in Q , with Q 0 as the start variable. How do we deal with accept states? � for each q i ∈ F , add rule Q i → ε Theorem Every regular language is context-free. INF2080 Lecture :: 1st February 18 / 37

  42. Properties of CFLs Closure under union/concatanation/Kleene star? INF2080 Lecture :: 1st February 19 / 37

  43. Properties of CFLs Closure under union/concatanation/Kleene star? Let G 1 = ( V 1 , Σ 1 , R 1 , S 1 ) and G 2 = ( V 2 , Σ 2 , R 2 , S 2 ) be two grammars that generate L 1 , L 2 respectively. INF2080 Lecture :: 1st February 19 / 37

  44. Properties of CFLs Closure under union/concatanation/Kleene star? Let G 1 = ( V 1 , Σ 1 , R 1 , S 1 ) and G 2 = ( V 2 , Σ 2 , R 2 , S 2 ) be two grammars that generate L 1 , L 2 respectively. Union: create grammar G L 1 ∪ L 2 that generates all words w ∈ L 1 ∪ L 2 . INF2080 Lecture :: 1st February 19 / 37

  45. Properties of CFLs Closure under union/concatanation/Kleene star? Let G 1 = ( V 1 , Σ 1 , R 1 , S 1 ) and G 2 = ( V 2 , Σ 2 , R 2 , S 2 ) be two grammars that generate L 1 , L 2 respectively. Union: create grammar G L 1 ∪ L 2 that generates all words w ∈ L 1 ∪ L 2 . Create new start variable S . G L 1 ∪ L 2 = ( V , Σ , R , S ) where V = V 1 ∪ V 2 ∪ { S } , Σ = Σ 1 ∪ Σ 2 , and R = R 1 ∪ R 2 ∪ { S → S 1 | S 2 } . INF2080 Lecture :: 1st February 19 / 37

  46. CFL Union: Example S 1 → aS 1 b | ε ∪ S 2 → aS 2 a | bS 2 b | cS 2 c | ε INF2080 Lecture :: 1st February 20 / 37

  47. CFL Union: Example S 1 → aS 1 b | ε ∪ S 2 → aS 2 a | bS 2 b | cS 2 c | ε S → S 1 | S 2 S 1 → aS 1 b | ε S 2 → aS 2 a | bS 2 b | cS 2 c | ε INF2080 Lecture :: 1st February 20 / 37

  48. Properties of CFLs: Concatanation Let G 1 = ( V 1 , Σ 1 , R 1 , S 1 ) and G 2 = ( V 2 , Σ 2 , R 2 , S 2 ) be two grammars that generate L 1 , L 2 respectively. Concatanation: create grammar G L 1 L 2 = ( V , Σ , R , S ) that accepts all words w = w 1 w 2 , where w 1 ∈ L 1 and w 2 ∈ L 2 . INF2080 Lecture :: 1st February 21 / 37

  49. Properties of CFLs: Concatanation Let G 1 = ( V 1 , Σ 1 , R 1 , S 1 ) and G 2 = ( V 2 , Σ 2 , R 2 , S 2 ) be two grammars that generate L 1 , L 2 respectively. Concatanation: create grammar G L 1 L 2 = ( V , Σ , R , S ) that accepts all words w = w 1 w 2 , where w 1 ∈ L 1 and w 2 ∈ L 2 . new start variable S V = V 1 ∪ V 2 ∪ { S } , Σ = Σ 1 ∪ Σ 2 , and R = R 1 ∪ R 2 ∪ { S → S 1 S 2 } . INF2080 Lecture :: 1st February 21 / 37

  50. CFL Concatanation: Example S 1 → aS 1 b | ε S 2 → aS 2 a | bS 2 b | cS 2 c | ε INF2080 Lecture :: 1st February 22 / 37

  51. CFL Concatanation: Example S 1 → aS 1 b | ε S 2 → aS 2 a | bS 2 b | cS 2 c | ε S → S 1 S 2 S 1 → aS 1 b | ε S 2 → aS 2 a | bS 2 b | cS 2 c | ε INF2080 Lecture :: 1st February 22 / 37

  52. Properties of CFLs: Kleene star Let G 1 = ( V 1 , Σ 1 , R 1 , S 1 ) generate language L 1 . Kleene star: create grammar G = ( V , Σ , R , S ) that generates all words in L ∗ 1 . INF2080 Lecture :: 1st February 23 / 37

  53. Properties of CFLs: Kleene star Let G 1 = ( V 1 , Σ 1 , R 1 , S 1 ) generate language L 1 . Kleene star: create grammar G = ( V , Σ , R , S ) that generates all words in L ∗ 1 . V = V 1 , Σ = Σ 1 , R = R 1 ∪ { S 1 → ε, S 1 → S 1 S 1 } , S = S 1 . INF2080 Lecture :: 1st February 23 / 37

  54. Properties of CFLs: Kleene star Let G 1 = ( V 1 , Σ 1 , R 1 , S 1 ) generate language L 1 . Kleene star: create grammar G = ( V , Σ , R , S ) that generates all words in L ∗ 1 . V = V 1 , Σ = Σ 1 , R = R 1 ∪ { S 1 → ε, S 1 → S 1 S 1 } , S = S 1 . Example: S 1 → ε | S 1 S 1 S 1 → aS 1 b | ε S 1 → aS 1 b | ε INF2080 Lecture :: 1st February 23 / 37

  55. Properties of CFLs Closure under complement/intersection? INF2080 Lecture :: 1st February 24 / 37

  56. Properties of CFLs Closure under complement/intersection? � No, but we need to know more before we can determine if a language is not context-free. (next week) INF2080 Lecture :: 1st February 24 / 37

  57. Ambiguity Consider the grammar E → E + E | E × E | ( E ) | a INF2080 Lecture :: 1st February 25 / 37

  58. Ambiguity Consider the grammar E → E + E | E × E | ( E ) | a Here: the alphabet is { a , + , × , ( , ) } . INF2080 Lecture :: 1st February 25 / 37

  59. Ambiguity Consider the grammar E → E + E | E × E | ( E ) | a Here: the alphabet is { a , + , × , ( , ) } . → arithmetic expressions over a INF2080 Lecture :: 1st February 25 / 37

  60. Ambiguity Consider the grammar E → E + E | E × E | ( E ) | a Here: the alphabet is { a , + , × , ( , ) } . → arithmetic expressions over a What does the parse tree for the string a + a × a look like? INF2080 Lecture :: 1st February 25 / 37

  61. Ambiguity E + E E a × E E a a INF2080 Lecture :: 1st February 26 / 37

  62. Ambiguity E + E E a × E E a a Intuitively corresponds to a + ( a × a ) INF2080 Lecture :: 1st February 26 / 37

  63. Ambiguity E E + × E E E E a × + a E E E E a a a a Intuitively corresponds to a + ( a × a ) INF2080 Lecture :: 1st February 26 / 37

  64. Ambiguity E E + × E E E E a × + a E E E E a a a a Intuitively corresponds to a + ( a × a ) Intuitively corresponds to ( a + a ) × a INF2080 Lecture :: 1st February 26 / 37

  65. Ambiguity E E + × E E E E a × + a E E E E a a a a Intuitively corresponds to a + ( a × a ) Intuitively corresponds to ( a + a ) × a This is called ambiguity INF2080 Lecture :: 1st February 26 / 37

  66. Ambiguity But just having multiple possible derivations does not mean that a grammar is ambiguous. INF2080 Lecture :: 1st February 27 / 37

  67. Ambiguity But just having multiple possible derivations does not mean that a grammar is ambiguous. Two derivations could look different, yet “structurally” the same: apply the same rules to the same variables, yet in a different order. INF2080 Lecture :: 1st February 27 / 37

  68. Ambiguity But just having multiple possible derivations does not mean that a grammar is ambiguous. Two derivations could look different, yet “structurally” the same: apply the same rules to the same variables, yet in a different order. E � E + E � E + E × E � a + E × E � a + a × E � a + a × a E � E + E � a + E � a + E × E � a + a × E � a + a × a INF2080 Lecture :: 1st February 27 / 37

  69. Ambiguity But just having multiple possible derivations does not mean that a grammar is ambiguous. Two derivations could look different, yet “structurally” the same: apply the same rules to the same variables, yet in a different order. E � E + E � E + E × E � a + E × E � a + a × E � a + a × a E � E + E � a + E � a + E × E � a + a × E � a + a × a E + E E Both have the same parse tree! a × E E a a INF2080 Lecture :: 1st February 27 / 37

  70. Ambiguity We are interested in structurally different derivations, i.e., two derivations of the same word that, given a predefined order of derivation, are different INF2080 Lecture :: 1st February 28 / 37

  71. Ambiguity We are interested in structurally different derivations, i.e., two derivations of the same word that, given a predefined order of derivation, are different Definition A leftmost derivation of a string replaces, in each derivation step, the leftmost variable. Then a string is derived ambiguously over a grammar G if it has two or more leftmost derivations over G . INF2080 Lecture :: 1st February 28 / 37

  72. Ambiguity We are interested in structurally different derivations, i.e., two derivations of the same word that, given a predefined order of derivation, are different Definition A leftmost derivation of a string replaces, in each derivation step, the leftmost variable. Then a string is derived ambiguously over a grammar G if it has two or more leftmost derivations over G . If L ( G ) contains a string that is derived ambiguously, we say that G is ambiguous. INF2080 Lecture :: 1st February 28 / 37

  73. Chomsy Normal Form Context-free languages have a nice property: Every CFL can be described by a CFG in Chomsky Normal Form : Definition A grammar is in Chomsky Normal Form if every rule is of the form: A → BC A → a where a is any terminal, A is any variable, B , C are any variables that are not the start variable. In addition the rule S → ε is permitted. INF2080 Lecture :: 1st February 29 / 37

  74. Definition A grammar is in Chomsky Normal Form if every rule is of the form: A → BC A → a where a is any terminal, A is any variable, B , C are any variables that are not the start variable. In addition the rule S → ε is permitted. Proof sketch: Given an arbitrary grammar G . First, add new start variable S 0 and new rule S 0 → S to G . INF2080 Lecture :: 1st February 30 / 37

  75. Definition A grammar is in Chomsky Normal Form if every rule is of the form: A → BC A → a where a is any terminal, A is any variable, B , C are any variables that are not the start variable. In addition the rule S → ε is permitted. Proof sketch: Given an arbitrary grammar G . First, add new start variable S 0 and new rule S 0 → S to G .Then, remove all rules A → ε , followed by all “unit” rules A → B . INF2080 Lecture :: 1st February 30 / 37

  76. Definition A grammar is in Chomsky Normal Form if every rule is of the form: A → BC A → a where a is any terminal, A is any variable, B , C are any variables that are not the start variable. In addition the rule S → ε is permitted. Proof sketch: Given an arbitrary grammar G . First, add new start variable S 0 and new rule S 0 → S to G .Then, remove all rules A → ε , followed by all “unit” rules A → B . For each such occurence of A in the righthand side of a rule, add a new rule with ε (resp. B ) substituted for A (see examples on next slide). INF2080 Lecture :: 1st February 30 / 37

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INF2080 Church Turing Thesis and Decidability Daniel Lupp Universitetet i Oslo 28th February

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INF2080 Context-Sensitive Langugaes Daniel Lupp Universitetet i Oslo 24th February 2016

INF2080 Context-Sensitive Langugaes Daniel Lupp Universitetet i Oslo 24th February 2016

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INF2080 Context-Sensitive Langugaes Daniel Lupp Universitetet i Oslo 15th February 2017

INF2080 Context-Sensitive Langugaes Daniel Lupp Universitetet i Oslo 15th February 2017

INF2080 Context-Sensitive Langugaes Daniel Lupp Universitetet i Oslo 15th February 2017 Department of University of Informatics Oslo INF2080 Lecture :: 15th February 1 / 20 Context-Free Grammar Definition (Context-Free Grammar) A

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