Plan for Today and Thursday Bottom-Up Parsing with LR parse tables - - PDF document

CS453 Lecture Building LR Parse Tables 1

Plan for Today and Thursday

Bottom-Up Parsing with LR parse tables

– JavaCUP debug information – using an LR parser

Building the LR Parsing Table for LR(0)

– items, closure, and goto, oh my! – why does it need to know input token if LR(0)? Building the LR Parsing Table for LR(1) – First and Follow sets – building the table

Debugging shift/reduce and reduce/reduce errors

LR parse tables and pushdown automata

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Example LR Parse Table

[0] S -> ( S ) [1] S’ -> S EOF [2] S -> ID

Action Goto State ( ) $/EOF ID S s3 s1 2 1 r2 r2 r2 r2 2 accept 3 s3 s1 4 4 s5 5 r0 r0 r0 r0

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Building the LR Parse Table for LR(0)

[0] S -> ( S ) [1] S’ -> S EOF [2] S -> ID

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LR(0) states for nested parens example

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Building the Table from the State Diagram

Action Goto State ( ) $/EOF ID S s3 s1 2 1 r2 r2 r2 r2 2 accept 3 s3 s1 4 4 s5 5 r0 r0 r0 r0

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Building the LR Parse Table for LR(0), another example

(0) S’ -> E $ (1) E -> E || B (2) E -> B (3) B -> t (4) B -> f

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FIRST and FOLLOW sets

FIRST( gamma )

– gamma is a string of terminals and nonterminals – FIRST(gamma) is any terminals that can start a string derived from gamma

nullable(X)

– X is a nonterminal – nullable(X) is true if X can derive the empty string

FOLLOW( X )

– X is a nonterminal – FOLLOW(X) is all terminals that can immediately follow X in strings generated from the grammar

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(0) S’ -> E $ (1) E -> T + E (2) E -> T (3) T -> x

Building the LR Parse Table for LR(1)

Symbol FIRST x x + + T x E x S x Action Goto State x + $ T E s3 g2 g1 1 accept 2 s4 r2 3 r3 r3 4 s3 g2 g5 5 r1

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Example LR Parse Table

[0] S -> ( S ) [1] S’ -> S EOF [2] S -> ID

Action Goto State ( ) $/EOF ID S s3 s1 2 1 r2 r2 r2 r2 2 accept 3 s3 s1 4 4 s5 5 r0 r0 r0 r0

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Pushdown Automata for Grammar