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top-down parsing

Delve into the concepts of top-down parsing strategies in COP4620 - Programming Language Translators under the guidance of Dr. Manuel E. Bermudez. Explore the game-like approach of syntactic dominoes, parsing techniques, and the use of Omniscient Parsing Functions (OPF) to decode complex grammars efficiently.

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top-down parsing

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  1. top-down parsing COP4620 – Programming Language Translators Dr. Manuel E. Bermudez

  2. topics • The Game of Syntactic Dominoes • Strategies for parsing • Top-Down Parsing • Define OPF

  3. The grammar: E → E+T T → P*T P → (E) → T → P →i The playing pieces: An arbitrary supply of each piece (one per grammar rule). The game board: Start domino at the top. Bottom dominoes are the "input." The Game of Syntactic Dominoes

  4. SyntacticdominoesTheGameBoard

  5. Game rules: Add game pieces to the board. Match the flat parts and the symbols. Lines are infinitely elastic (and cannot cross). Object of the game: Connect start domino with the input dominoes. Leave no unmatched flat parts. The Game of Syntactic Dominoes

  6. Same as for the game of syntactic dominoes. “Top-down” parsing: start at the start symbol, work toward the input string. “Bottom-up” parsing: start at the input string, work towards the goal symbol. In either strategy, can process the input left-to-right  or right-to-left  Parsing Strategies

  7. Attempt a left-most derivation, by predicting the re-write that will match the remaining input. Use a string (a stack, really) from which the input can be derived. Start with S on the stack. At every step, two alternatives:  (the stack) begins with a terminal t: match t against input.  begins with a nonterminal A: Consult an OPF (Omniscient Parsing Function) to determine which production for A to use. Top-Down Parsing

  8. Sample top-downparse • E → E+T • → T • T → P*T • → P • P → (E) • → i

  9. Push (Stack, S); while not Empty (Stack) do if Top(Stack) ∊  then if Top(Stack) = Head(input) then input := tail(input) Pop(Stack) else error (Stack, input) else P:= OPF (Stack, input) Push (Pop(Stack), RHS(P)) od Classic Top-Down Parsing Algorithm

  10. top-down parsing: general scheme

  11. Most parsing methods (for PL’s) impose bounds: input lookahead: 1. We define OPF (A,t), where A: top element of the stack, and t: first symbol on the input. Storage requirements: O(n2), where n is the size of the grammar vocabulary (a few hundred). Top-Down Parsing

  12. S → A A → bAd → Sample opf Stack Input OPF S bdd S bbdd

  13. summary • The Game of Syntactic Dominoes • Strategies for parsing • Top-Down Parsing • Define OPF

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