Earley’s Algorithm J. Earley, "An efficient context-free parsing algorithm", Communications of the Association for Computing Machinery, 13:2:94-102, 1970."An.

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Presentation transcript:

Earley’s Algorithm J. Earley, "An efficient context-free parsing algorithm", Communications of the Association for Computing Machinery, 13:2:94-102, 1970."An efficient context-free parsing algorithm"

Z ::= X YZ parses w pq CYK: if d pr parses X and d (r+1)q parses Y, then in d pq stores symbol Z Earley’s parser: in set S q stores item (Z ::= XY., p) Move forward, similar to top-down parsers Use dotted rules to avoid binary rules CYK vs Earley’s Parser Comparison (()()())

Example: expressions D ::= e EOF e ::= ID | e – e | e == e Rules with a dot inside D ::=. e EOF | e. EOF | e EOF. e ::=. ID | ID. |. e – e | e. – e | e –. e | e – e. |. e == e | e. == e | e ==. e | e == e.

ID- ==IDEOF IDID-ID-IDID-ID==ID-ID==ID ID --ID-ID==-ID==ID - IDID==ID==ID ID ====ID == ID EOF e ::=. ID | ID. |. e – e | e. – e | e –. e | e – e. |. e == e | e. == e | e ==. e | e == e.

Remark: Grammars and Languages Language S is a set of words For each language S, there can be multiple possible grammars G such that S=L(G) Language S is –Non-ambiguous if there exists a non-ambiguous grammar for it –LL(1) if there is an LL(1) grammar for it Even if a language has ambiguous grammar, it can still be non-ambiguous if it also has a non-ambiguous grammar

regular Languages semi-decidable decidable context-sensitive context-free unambiguous deterministic = LR(1) LL(1) LALR(1) SLR LR(0)