Context-Free Grammar Study Note, Part Two

Challenges of Parsing CFG Correctly

Syntactic Ambiguity

  1. Part-of-speech Ambiguity
    : The grammar can assign multiple part-of-speech tags (word categories) to a lexicon
  2. Structural Ambiguity
    : The grammar can assign more than one possible parse to a sentence

    • Attachment Ambiguity
      : A particular constituent can be attached to the parse tree at more than one place
    • Coordination Ambiguity
      : The reference of a lexicon has multiple choices
    • Local Ambiguity
      : Some part of the sentence has more that one parse while the whole sentence is not ambiguous
    • etc
      : ...

Phrasal Nodes

  1. a phrasal node is a non-terminal node

CFG Tree v.s. Dependency Grammar Tree

Strucutrally cfg tree has little difference with dp tree.
1. In cfg tree, the internal nodes are phrasal nodes (non-lexicalized). In dp tree, the internal nodes are by definition lexicalized.
2. Both cfg and dp tree has pos tags as the next-to-leaf node. For dp tree this pos tag can be replaced by synthetic parts-of-speech (subject, object, predicate, etc).