Natural Language Processing Ch3. Parsing with PSG

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Fig. 5 left-corner table Using this left-corner table, the process of sentence “book that flight”will become simple and quick. The process is as follows: First ply: S S S VP NP VP Aux NP VP Verb NP Det Nominal Book Book Book [Fail !] [Fail !] Fig. 5 Verb is the left-corner of S. “Det”and “Aux”can not match <strong>with</strong> “Book”. Second ply: S VP Verb NP Book Det Nominal that Fig. 6 Det is the left-corner of NP. Third ply: S VP Verb NP Book Det Nominal That Noun Fig. 7 flight
Noun is the left-corner of Nominal. The Top-Down parsing process using left-corner filter is as follows: Searching goal Rule The rest part of input string S Book that flight + VP 3 Book that flight Verb 9 Book that flight Verb NP 10 Book that flight NP that flight Det Nominal 4 that flight Nominal flight Noun 5 flight 3.3 Problems <strong>with</strong> Top-Down Parser 3.3.1 Left-recursion: In top-down, depth-first, left-to-right parser, It may dive down an infinitely deeper path never return to visit space if it use the left-recursive grammar. Formally, a grammar is left-recursive if it contains at least one non-terminal A, such that , A à A and A more obvious and common case of left-recursion in natural language grammar involves immediately left-recursive rules. The left-recursive rules are rules of the form A à A, where the first constituent of the RHS is identical to the LHS. E.g. NP à NP PP VP à VP PP S à S and S A left-recursive non-terminal can lead a top-down, depth-first, left-to-right parser to recursively expand the same non-terminal over again in exactly the same way, leading to an infinite expansion of the trees. E.g. if we have the left recursive rule NP à NP PP as first rule in our small grammar, we may get the infinite search as following: S è S è S è S NP VP NP VP NP VP
Page 1 and 2: Natural Language Processing Prof. F
Page 3 and 4: Fifth ply: VP NP NP NP Nominal Nomi
Page 5 and 6: S VP Verb NP Book Fifth ply: S S VP
Page 7: [Success ! ] 3.2.2 Comparing Top-Do
Page 11 and 12: à à 1 1 n +
Page 13 and 14: NP NP PP Nom NP Det Noun Prep Prop-
Page 15 and 16: 3.4.1.2 Three operators in Early al
Page 17 and 18: The Earley algorithm just described
Page 19 and 20: Chart [0] à .S [0,0] Dummy start s
Page 21 and 22: Example-2: using Earley algorithm t
Page 23 and 24: PrN à ASIANA. [9,10] Scanner NPàP
Page 25 and 26: 3.4.2. CYK approach: CYK approach i
Page 27 and 28: à à à
Page 29 and 30: ij (NP1): i=1. j=1+1=2 b ij (NP2):

Natural Language Processing Ch3. Parsing with PSG

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