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Top down parsing corresponds to which kind of derivation?

  • A.no derivation at all
  • B.a leftmost derivation of the input string
  • C.a rightmost derivation of the input string in reverse order
  • D.a derivation in which nonterminals are expanded at random

Answer: B

Correct answer: a leftmost derivation of the input string

Since the parser always expands the leftmost unexpanded nonterminal, the sequence of productions it applies is exactly a leftmost derivation, produced in order. Bottom up parsing produces a rightmost derivation, and produces it backwards - which is the essential contrast between the two families.

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