Types of Parsing in NLP

Parsing is the process of analyzing the grammatical structure of a sentence. It helps identify the syntactic relationships between words and phrases.

1. Top-Down Parsing

• Starts from the highest-level rule (the start symbol, usually S for sentence) and tries to rewrite it to match the input sentence.

• Works by expanding the grammar rules to generate the sentence.

• Example techniques: Recursive descent parsing.

• Pros: Intuitive and easy to implement.

• Cons: Can be inefficient due to backtracking, and may not handle left-recursive grammars well.

2. Bottom-Up Parsing

• Starts from the input words and tries to combine them into higher-level constructs until it reaches the start symbol.

• Works by reducing the sentence to the start symbol using grammar rules.

• Example techniques: Shift-reduce parsing, LR parsing.

• Pros: Can handle a larger class of grammars efficiently.

• Cons: More complex to implement than top-down parsing.

3. Chart Parsing

• Uses dynamic programming to store intermediate parsing results to avoid redundant computations.

• Combines features of both top-down and bottom-up parsing.

• Useful for ambiguous or complex grammars.

4. Dependency Parsing

• Focuses on relationships between “head” words and their dependents, rather than phrase structure.

• Useful for understanding syntactic relations like subject, object, modifiers.

5. Constituency Parsing (Phrase Structure Parsing)

• Breaks sentence into sub-phrases or constituents (like noun phrases, verb phrases).

• Often represented as a parse tree.

• The type of parsing needed for the parse tree example below.

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Parse Tree for the Sentence:

“The cat ate the fish.”

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Step 1: Identify parts of speech (POS) for each word:

• The — Determiner (Det)

• cat — Noun (N)

• ate — Verb (V)

• the — Determiner (Det)

• fish — Noun (N)

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Step 2: Phrase structure (simple example):

• Sentence (S)

  • Noun Phrase (NP)

    • Det: The

    • Noun: cat

  • Verb Phrase (VP)

    • Verb: ate

    • Noun Phrase (NP)

      • Det: the

      • Noun: fish

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Step 3: Visual parse tree (constituency):

            S
           / \
         NP   VP
        / \   /  \
      Det  N  V   NP
      |    |  |   /  \
     The  cat ate Det  N
                      |   |
                     the fish

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Explanation:

• The sentence (S) splits into a subject noun phrase (NP) and a predicate verb phrase (VP).

• The subject NP is “The cat”.

• The VP contains the verb “ate” and the object NP “the fish”.

• Each NP breaks down into a determiner (Det) and a noun (N).