LALR Parser Presentation ppt

LALR Parser Presentation ppt

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  Presentationon LALR PARSER ( LookAhead Parser ) Submitted To Dharemendra Sir Submitted By Vivek Kr Poddar
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  Table Of Content oIntroduction To LALR Parser o LALR Table Construction Method o Examples – Related To Grammer, First, CLR, etc. o Computed LR( 0 ) Items o GOTO Graph o Canonical Parsing Table o LALR Parsing Table 2
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  What is LALRParser o LALR stands for (look ahead LR) parser. o LALR parser starts with the idea of building an LR parsing table o Tables generated by LALR parser are smaller in size as compared to that of Canonical LR ( CLR ) and Simple LR ( SLR ) techniques. o LALR parsers are slightly less powerful than LR parsers, but still more powerful than SLR parsers. o LALR is used by YACC and other parser generators because of its effectiveness. 3
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  LALR TableConstruction Idea oConstruct the set of LR (1) items. o Merge the sets with common core together in CLR table. o If any problem arises then grammar is not LALR. 4
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  Example Grammar: 1. S’ ->S 2. S -> CC 3. C -> cC 4. C -> d 5 First : Where F Means First F( S’ ) -> F( S ) -> F( C ) -> { c,d } F ( C ) -> { c, d } -> c/d Look Ahead Symbol Note :: CC are two different items. Note :: Non-terminals denoted by upper-case letters, terminals denoted by lower-case letters S’ -> .S,$ :: This (a rule with a dot in it) is called an item, it indicates what is in the stack [ left side of . ] and what is to be expected on input [ right side of . ] Anything after ( , ) comma is termed as look ahead.
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  Computed LR( 0)& LR( 1 )Items6 I5: S -> CC., $ I6: C -> c.C, $ C -> .cC, $ C -> .d, $ I7: C -> d., $ I8: C -> cC., c /d I9: C -> cC., $ I0 : S’ -> .S, $ S -> .CC, $ C -> .c C, c /d C -> .d, c /d I1: S’ -> S., $ I2: S -> C.C, $ C -> .c C, $ C -> .d, $ I3: C -> c. C, c /d C -> .Cc, c /d C -> .d, c /d 14: C -> d., c / d Grammar: 1. S’ -> S 2. S -> CC 3. C -> cC 4. C -> d Grammar: 1. S’ -> S 2. S -> CC 3. C -> cC/d First ( c/d ) Note : I0 consist of LR ( 0 ). Items while rest are LR ( 1 ) items. Its always suggested not to work beyond State 2 by hand as they are compicated and should be calculated using standard tools.
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  8 Canonical ParsingTable States cd $ S C 0 S3 S4 1 2 1 acc 2 S6 S7 5 3 S3 S4 8 4 R4 R4 5 R2 6 S6 S7 9 7 R4 8 R3 R3 9 R3 Actions GOTO
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  9 LALR Parser ActionsGOTO Merge the Cores: What is core ? A core is a set of LR (0) (SLR) items for the grammar. o 3 & 6 o 4 & 7 o 8 & 9
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  10 LALR ParsingTable Actions Start c d $ S C 0 S36 S47 1 2 1 acc 2 S36 S47 5 36 S36 S47 89 47 R47 R47 R47 5 R2 89 R36 R36 R36 GOTOAction
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  Conflicts In LALRParser o LALR Parser cannot introduce shift/reduce conflicts. o Such conflicts arises when the look ahead is same as the token on which we can shift. o They depend on the core of the item but we merge only those rows which have common cores. o The only way by which this conflict can arise in LALR is when the conflict is already their in the LR(1). 11
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  11 Refrence Actions GOTOActionUsed togenerate First of Given Grammer http://hackingoff.com/compilers/predict-first-follow-set IITKGP notes for making presentation http://www.facweb.iitkgp.ernet.in/~niloy/COURSE/ Compilers Principles Techniques and Tools (2nd Edition) - BOOK
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  Thank You For Listening 12