# Chapter_5 - 1 Chapter 5: Bottom-Up Parsing (Shift-Reduce)...

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Unformatted text preview: 1 Chapter 5: Bottom-Up Parsing (Shift-Reduce) 2- attempts to construct a parse tree for an input string beginning at the leaves (the bottom ) and working towards the root (the top ). i.e., reduce a string w to the start symbol of a grammar. At each reduction step a particular substring matching the right side of a production (grammar rule) is replaced by the left nonterminal symbol. A rightmost derivation is traced out in reverse . Objectives of Bottom-Up Parsing 3 An Example S -> aABe A -> Abc | b B -> d w = abbcde S => aABe => aAde => aAbcde => abbcde LR parsing: abbcde ==> aAbcde ==> aAde ==> aABe ==> S (traced out in reverse) rm rm rm rm S a A B e d 1 2 A b c 3 b 4 LR parsing: a b bcde ==> a A bc de ==> a A de ==> aAB e ==> S 5 Stack Implementation of Bottom-Up Parsing There are four actions a parser can make: (1) shift (2) reduce (3) accept (4) error. There is an important fact that justifies the use of a stack in shift-reduce parsing: the handle will always eventually appear on top of the stack, never inside. Initially, (stack) \$ w\$ (input buffer) Finally, (stack)\$S \$ (input buffer) // S is a start symbol of grammar G 6 7 8 \$ . . . 9 10 \$ \$ \$ \$ \$ \$ \$ \$ \$ \$ \$ \$ \$ \$ 11 Handles A substring that matches the right side of a production, and whose reduction to the nonterminal on the left side of the production represents one step along the reverse of a rightmost derivation. However, in many cases the leftmost substring ' β ' that matches the right side of some production A -> β is not a handle, because a reduction by the production yields a string that cannot be reduced to the start symbol. 12 Handles (Continued) A handle of a right sentential form γ is a production A -> β and a position of γ where the string β may be found and replaced by A to produce the previous right-sentential form in a rightmost derivation of γ . i.e., S => α A w => α β w, then A -> β in the position following α is a handle of αβ w. The string w to the right of the handle contains only terminal symbols . Handle = leftmost complete subtree. * 13 Handle Pruning A rightmost derivation in reverse can be obtained by "handle pruning". Two Problems: 1. To locate the substring to be reduced in right-sentential form. 2. To determine the production with the same substring on the right-hand side to be chosen. 14 Write a LL parser in ? and a LR parser in Yacc separately for the TINY language defined in Fig. 3.6. The parsers will parse any input legal TINY program and generate a parse tree for it. Use the program in Fig. 3.8 to test your parsers and turn in the tested results with your parser codes. Assignment #4 15 Viable Prefixes The set of prefixes of right sentential forms that can appear on the stack of a shift-reduce parser are called viable prefixes....
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## This note was uploaded on 06/28/2011 for the course ENGINEERIN 100 taught by Professor Yangwei during the Spring '10 term at National Cheng Kung University.

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Chapter_5 - 1 Chapter 5: Bottom-Up Parsing (Shift-Reduce)...

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