CS460/IT632 Natural Language Processing/Language Technology for the Web Lecture 13 (17/02/06) Prof. Pushpak Bhattacharyya IIT Bombay Top-Down Bottom-Up Chart Parsing

17/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 2 Top-Down Bottom-Up Chart Parsing Combines advantages of top-down & bottom-up parsing. Does not work in case of left recursion. –e.g. – “People laugh” People – noun, verb Laugh – noun, verb –Grammar – S  NP VP NP  DT N | N VP  V ADV | V

17/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 3 Transitive Closure People laugh 123 S  NP VPNP  N  VP  V  NP  DT NS  NP  VPS  NP VP  NP  NVP  V ADVsuccess VP  V

17/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 4 Arcs in Parsing Each arc represents a chart which records –Completed work (left of  ) –Expected work (right of  )

17/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 5 Example People laugh loudly 1234 S  NP VPNP  N  VP  V  VP  V ADV  NP  DT NS  NP  VPVP  V  ADVS  NP VP  NP  NVP   V ADVS  NP VP  VP   V

17/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 6 Dealing With Structural Ambiguity Multiple parses for a sentence –The man saw the boy with a telescope. –The man saw the mountain with a telescope. – The man saw the boy with the ponytail. At the level of syntax, all these sentences are ambiguous. But semantics can disambiguate 2 nd & 3 rd sentence.

17/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 7 Prepositional Phrase (PP) Attachment Problem V – NP 1 – P – NP 2 (Here P means preposition) NP 2 attaches to NP 1 ? or NP 2 attaches to V ?

17/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 8 Parse Trees for a Structurally Ambiguous Sentence Let the grammar be – S  NP VP NP  DT N | DT N PP PP  P NP VP  V NP PP | V NP For the sentence, “I saw a boy with a telescope”

17/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 9 Parse Tree - 1 S NPVP NVNP DetNPP PNP DetN I saw a boy with atelescope

17/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 10 Parse Tree -2 S NPVP NVNP DetN PP PNP DetN I saw a boy with atelescope

17/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 11 Exercise For the sentence, “The man saw the boy with a telescope” & the grammar given previously, compare the performance of top-down, bottom-up & top-down chart parsing.