CS460/IT632 Natural Language Processing/Language Technology for the Web Lecture 6 (14/02/06) Prof. Pushpak Bhattacharyya IIT Bombay Top-Down and Bottom-Up Parsing

14/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 2 Example Grammar S -> NP VP NP -> DT N NP -> N VP -> V ADV VP -> V Note : Order of rules important in some parsing algorithms.

14/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 3 Example Sentence People laugh Lexicon: People - N, V Laugh - N, V These are positions This indicate that both Noun and Verb is possible for the word “People”

14/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 4 Top-Down Parsing State Backup State Action ((S)1) ((NP VP)1) - - 3a. ((DT N VP)1) ((N VP) 1) - 3b. ((N VP)1) ((VP)2) - Consume “People” 5a. ((V ADV)2) ((V)2) - 6. ((ADV)3) ((V)2) Consume “laugh” 5b. ((V)2) ((.)3) - Consume “laugh” Termination Condition : All inputs over. No symbols remaining. Note: Input symbols can be pushed back. Symbol waiting expansion Position of input pointer This will not be possible since DT does not match with the lexical category of “People”

14/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 5 Discussion for Top-Down Parsing This kind of searching is goal driven. Gives importance to textual precedence (rule precedence). No regard for data, a priori (useless expansions made).

14/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 6 Bottom-Up Parsing Some conventions: N 12 S 1? -> NP 12 ° VP 2? Represents positions End position unknown Work on the LHS done, while the work on RHS remaining

14/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 7 Bottom-Up Parsing (pictorial representation) S -> NP 12 VP 23 ° People Laugh N 12 N 23 V 12 V 23 NP 12 -> N 12 ° NP 23 -> N 23 ° VP 12 -> V 12 ° VP 23 -> V 23 ° S 1? -> NP 12 ° VP 2?

14/02/06Prof. Pushpak Bhattacharyya, IIT Bombay 8 Problem with Top-Down Left Recursion Suppose you have A-> AB rule. Then we will have the expansion as follows: ((A)K) -> ((AB)K) -> ((ABB)K) ……..