1. Normal Forms for Context-free Grammars

2. Chomsky Normal Form All productions have form: and variable variable
terminal

3. Examples:
Chomsky
Normal Form
Not Chomsky
Normal Form

4. Convertion to Chomsky Normal Form
Example:
Not Chomsky
Normal Form

5. Introduce variables for terminals:

6. Introduce intermediate variable:

7. Introduce intermediate variable:

8. Final grammar in Chomsky Normal Form:
Initial grammar

9. In general:
From any context-free grammar
not in Chomsky Normal Form
we can obtain:
An equivalent grammar
in Chomsky Normal Form

10. The Procedure
First remove:
Nullable variables
Unit productions

11. For every symbol :
Add production
In productions: replace with
New variable:

12. Replace any production
with
New intermediate variables:

13. Theorem: For any context-free grammar there is an equivalent grammar
in Chomsky Normal Form

14. Observations Chomsky normal forms are good
for parsing and proving theorems
It is very easy to find the Chomsky normal
form of any context-free grammar

15. Greinbach Normal Form
All productions have form:
symbol
variables

16. Examples:
Greinbach
Normal Form
Not Greinbach
Normal Form

17. Conversion to Greinbach Normal Form:

18. Theorem: For any context-free grammar there is an equivalent grammar
in Greinbach Normal Form

19. Observations Greinbach normal forms are very good for parsing
It is hard to find the Greinbach normal
form of any context-free grammar

20. An Application of Chomsky Normal Forms

21. The CYK Membership Algorithm
Input:
Grammar in Chomsky Normal Form
String
Output:
find if

22. The Algorithm
Input example:
Grammar :
String :

27. Therefore:
Time Complexity:
Observation:
The CYK algorithm can be
easily converted to a parser

28. Pushdown Automata PDAs

29. Pushdown Automaton -- PDA
Input String
Stack
States

30. Initial Stack Symbol
Stack
Stack
bottom
special symbol

31. The States
Pop
symbol
Input
symbol
Push
symbol

32. input
stack
top
Replace

33. input
stack
top
Push

34. input
stack
top
Pop

35. input
stack
top
No Change

36. Non-Determinism

37. NPDA: Non-Deterministic PDA
Example:

38. Execution Example:
Time 0
Input
Stack
current
state

39. Time 1
Input
Stack

40. Time 2
Input
Stack

41. Time 3
Input
Stack

42. Time 4
Input
Stack

43. Time 5
Input
Stack

44. Time 6
Input
Stack

45. Time 7
Input
Stack

46. Time 8
Input
Stack
accept

47. A string is accepted if:
All the input is consumed
The last state is a final state
We do not care about the stack contents

48. The input string
is accepted by the NPDA:

49. In general,
is the language accepted by the NPDA:

50. Another NPDA example
NPDA

51. Execution Example:
Time 0
Input
Stack

52. Time 1
Input
Stack

53. Time 2
Input
Stack

54. Time 3
Input
Stack

55. Time 4
Input
Stack

56. Time 5
Input
Stack

57. Time 6
Input
Stack
accept