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# Topdown parsing

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Top-Down Parsing

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### Topdown parsing

1. 1. A . Antony Alex MCA Dr G R D College of Science – CBE Tamil Nadu - India
2. 2. Recursive Descent Parsing • Attempt to find a leftmost derivation for the input streaminput stream • Attempt to construct a parse tree for the input starting from the root and creating the nodes of the parse tree in preorder • A general form of top down parsing called• A general form of top down parsing called Recursive Descent Parsing that involve Backtracking
3. 3. Predictive Parsers • Eliminating left recursion and left factoring the resulting grammar, a new grammar can beresulting grammar, a new grammar can be obtained that can be parsed by a recursive- descent parser without backtracking stmt if expr then stmt else stmtstmt if expr then stmt else stmt while expr do stmt begin stmt_list end
4. 4. Transition Diagrams for predictive parsers • Create an initial and final (return) state• Create an initial and final (return) state • For each production A → X1 X2 ….. X n create a path from the initial to the final state with edges labeled X1 , X2 , …… X n
5. 5. Transition Diagrams for Predictive Parsers Example
6. 6. Grammer • E → TE’ • E’ → +TE’ | ε• E’ → +TE’ | ε • T → FT’ • T’ → *FT’ | ε • F → (E) | id
7. 7. 0 1 E → TE’ 2E: T E’ E’ → +TE’ | ε 4 5 6E’: T E’ 3 +E’: ε
8. 8. 7 8 T → FT’ 9T: F T’ T’ → *FT’ | ε 11 12 13T’: F T’ 10 *T’: ε
9. 9. F → (E) | id 15 16 17F: E ) 14 ( 15 16F: 14 id
10. 10. Thank You