Dependency-based methods for syntactic parsing have become increasingly popular in natural language processing in recent years. This book gives a thorough introduction to the methods that are most ...
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BOTTOM UP U P PARSING PARSING A bottom-up parse corresponds to the construction of a parse tree for an input string beginning at the leaves lea ves (the bottom) and working work ing up towards the root (the top) to p) . It is convenient to describe parsing as the process of building parse trees, although a front end may in fact carry out a translation directly without building an explicit tree. e can think of bottom-up parsing as the process of !reducing! a string w to the start symbol of the grammar. At each reduction step, a specific substring matching the body of a production is replaced repl aced by the non terminal at the head of that production. produ ction. "he key decisions during bottom-up parsing are about when to reduce and about what production to apply, as the parse proceeds.
LR PARSERS: "he #most prevalent type of bottom-up parser today is based on a concept called $%(k) parsing& the !$! is for left-to-right left- to-right scanning of o f the input, the !%! for constructing construc ting a rightmost derivation in reverse, and the k for the number of input symbols of lookahead that are used in making parsing decisions. $% parsing is attractive because of variety of reasons' $% parsers can be constructed to recognie virtually all programming language constructs for which context-free grammars can be written. on $% context-free grammars exist, but these can generally be avoided for typical programminglanguage constructs. "he $%-parsing method is the most general non back tracking shift-reduce parsing method known, yet it can be implemented as efficiently as other, more primitive shiftreduce methods . An $% parser can detect a syntactic error as soon as it is possible to do so on a left-toright scan of the input. $% grammars can describe more languages than $$ grammars. •
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ITEMS AND THE LR(0) AUTOMATION: AUTOMATION: An $% parser makes shift-reduce decisions by maintaining states to keep track of where we are in a parse. *tates represent sets of !items.! An $%(+) item (item for short) of a grammar is a production of with a dot at some position of the body. "hus, production A - /0 yields the four items A - 1/0 A - 1/0 A - /1 0 A - /01 "he production production A -ϵ generates generates only one one item, item, A - . . Intuitively, an item indicates how much of a production we have seen at a given point in the parsing process. 2or example, exa mple, the item A - 1/ 0 indicates that we hope to see a string derivable from / 0 next on the input. Item A - 1 / 0 indicates that we have 3ust seen on the input a string derivable from and that we hope next to see a string derivable from / 0. Item A -/ 01 indicates that we have seen the body / 0 and that it may be time to reduce /0 to A.
CLOSURE OF ITEM SETS:
If I is a set of items for a grammar , then 4$+*5%6(I) is the set of items constructed from I by the two rules' 7. Initially, add every item in I to 4$+*5%6(I). 8. If A -91:; is in 4$+*5%6(I) and : - ᵞ is a production, then add the item : - ᵞ to 4$+*5%6(I), if it is not already there. Apply this rule until no more new items can be added to 4$+*5%6 (I). THE FUNCTION GOTO:
