1. November 2009HLT: Sentence Parsing1 HLT Sentence Parsing Algorithms 2 Problems with Depth First Top Down Parsing

2. November 2009HLT: Sentence Parsing2 Problems with DFTD Parser Left Recursion Handling Ambiguity Inefficiency

3. November 2009HLT: Sentence Parsing3 Left Recursion A grammar is left recursive if it contains at least one non-terminal A for which A  *  A  and   *  (n.b.  * is the transitive closure of  ) Intuitive idea: derivation of that category includes itself along its leftmost branch. NP  NP PP NP  NP and NP NP  DetP Nominal DetP  NP ' s

4. November 2009HLT: Sentence Parsing4 Left Recursion Left recursion can lead to an infinite loop [nltk demo

5. November 2009HLT: Sentence Parsing5 Dealing with Left Recursion Use different parsing strategy Reformulate the grammar to eliminate LR A  A  |  is rewritten as A   A' A'   A' | 

6. November 2009HLT: Sentence Parsing6 Rewriting the Grammar NP → NP ‘and’ NP NP → D N | D N PP

7. November 2009HLT: Sentence Parsing7 Rewriting the Grammar NP → NP ‘and’ NP β NP → D N | D N PP α

8. November 2009HLT: Sentence Parsing8 Rewriting the Grammar NP → NP ‘and’ NP β NP → D N | D N PP α New Grammar NP → α NP1 NP1 → β NP1 | ε

9. November 2009HLT: Sentence Parsing9 Rewriting the Grammar NP → NP ‘and’ NP β NP → D N | D N PP α New Grammar NP → α NP1 NP1 → β NP1 | ε α → D N | D N PP β → ‘and’ NP

10. November 2009HLT: Sentence Parsing10 New Parse Tree D N the cat α NP NP1 ε

11. November 2009HLT: Sentence Parsing11 Rewriting the Grammar Different parse tree Unnatural parse tree?

12. November 2009HLT: Sentence Parsing12 Left Corner Parsing Two kinds of symbol on the stack: “predicted” and “announced”. The basic idea behind LC parsing is is that when the leftmost child of a production is completed, the parent of the category is “announced”, and the remaining right hand categories are “predicted”. When a predicted symbol and an announced symbol of the same category are on top of the stack, we can match them and remove them from the stack

13. November 2009HLT: Sentence Parsing13 Problems with DFTD Parser Left Recursion Handling Ambiguity Inefficiency

14. November 2009HLT: Sentence Parsing14 Handling Ambiguity Coordination Ambiguity: different scope of conjunction: Hot curry and ice taste nice with rice Hot curry and rice taste nice with ice Attachment Ambiguity: a constituent can be added to the parse tree in different places: I shot an elephant in my trousers VP → VP PP NP → NP PP

15. November 2009HLT: Sentence Parsing15 Real sentences are full of attachment ambiguities President Kennedy today pushed aside other White House business to devote all his time and attention to working on the Berlin crisis address he will deliver tomorrow night to the American people over nationwide television and radio

16. November 2009HLT: Sentence Parsing16 Prepositional Phrase Ambiguity No of PPs# parses 22 35 414 5132 6469 71430 84867 he will deliver - to the American people - over nationwide TV - in New York - during September - for very good reasons

17. November 2009HLT: Sentence Parsing17 Growth of Number of Ambiguities The nth Catalan number counts the ways of dissecting a polygon with n+2 sides into triangles by drawing nonintersecting diagonals. No of PPs# parses 22 35 414 5132 6469 71430 84867

18. November 2009HLT: Sentence Parsing18 Handling Ambiguities Statistical disambiguation –which is the most probable interpretation? Semantic knowledge –which is the most sensible interpretation? –Subatomic particles such as positively charged protons and electrons

19. November 2009HLT: Sentence Parsing19 Problems with DFTD Parser Left Recursion Handling Ambiguity Inefficiency

20. November 2009HLT: Sentence Parsing20 Repeated Parsing of Subtrees Local versus global ambiguity. –NP → Det Noun –NP → NP PP Because of the top down depth first, left to right policy, the parser builds trees that fail because they do not cover all of the input. Successive parses cover larger segments of the input, but these include structures that have already been built before.

21. November 2009HLT: Sentence Parsing21 Repeated Parsing of Subtrees a flight4 from Indianapolis3 to Houston2 on TWA1 A flight from Indianapolis3 A flight from Indianapolis to Houston 2 A flight from Indianapolis to Houston on TWA 1 NP Nom Det Noun a flight NP NP PP Nom Det Noun P Noun a flight from Indianapolis

22. November 2009HLT: Sentence Parsing22 Repeated Parsing of Subtrees a flight4 from Indianapolis3 to Houston2 on TWA1 A flight from Indianapolis3 A flight from Indianapolis to Houston 2 A flight from Indianapolis to Houston on TWA 1