chart-parsing.4 - Natural Language Processing Natural Language Processing Bottom-up Chart Parsing Chart Parsing 1 If the agenda is empty look up the

chart-parsing.4 - Natural Language Processing Natural...

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Natural Language Processing 1 Chart Parsing General search methods are not best for syntactic parsing because the same syntactic constituent may be rederived many times as a part of larger constituents. For example, an NP could be part of different VPs or PPs. Chart parsing uses a chart to keep track of partial derivations so nothing has to be rederived. Chart parsers also use an agenda to prioritize the constituents to be processed. The agenda can be implemented as a stack or a queue to simulate depth-first or breadth-first search, or as a priority queue for best-first search. Natural Language Processing 2 Bottom-up Chart Parsing 1. If the agenda is empty, look up the next word (scanning the sentence left to right) in the lexicon and add its possible parts-of-speech to the agenda. Include its position in the input sentence. For example, if the first word is an article then it would be added as ART ( p 0 - p 1 ). 2. Select a constituent C from the agenda. 3. Add C to the chart using the arc extension algorithm. Natural Language Processing 3 The Arc Extension Algorithm To add a constituent C that spans positions p i - p j 1. INSERT C into the chart in positions p i - p j . 2. SEARCH for grammar rules that begin with the constituent C . For each rule, add an active arc to the chart of the form: X C * X 1 X 2 ... X n , spanning positions p i - p j . 3. EXTEND all active arcs of the form X X 1 ... * C ... X n spanning positions p m - p i by adding a COPY of the arc to the chart that includes the new constituent: X X 1 ... C * ... X n , spanning positions p m - p j .. Do not delete the original active arc, always create a copy!! 4. For each NEWLY COMPLETED ARC of the form X X 1 ... X n C *, spanning positions p m - p j , add a new constituent of type X onto the agenda for positions p m - p j . Natural Language Processing 4 An Example: the grammar and lexicon Sentence: The smelly dog drank the water.
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