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LALR PARSER

LALR PARSER. Prepared by VISHWA DEEPAK 04CS1029. There are three issues which arise here— Whether LALR is equivalent to SLR. Shift/Reduce conflict in the cases of LR(1) and LALR. Reduce/Reduce conflict in the cases of LR(1) and LALR. Taking an example Grammar. S’ -> S S -> Bbb

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LALR PARSER

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  1. LALR PARSER Prepared by VISHWA DEEPAK 04CS1029 Vishwa Deepak 04CS1029

  2. There are three issues which arise here— • Whether LALR is equivalent to SLR. • Shift/Reduce conflict in the cases of LR(1) and LALR. • Reduce/Reduce conflict in the cases of LR(1) and LALR. Vishwa Deepak 04CS1029

  3. Taking an example Grammar • S’ -> S • S -> Bbb • S -> aab • S -> bBa • B -> a Vishwa Deepak 04CS1029

  4. Items using SLR PARSER • I0 : S’ -> .S • S -> .Bbb • S -> .aab • S -> .bBa • B -> .a • I1: S’ -> S. • I2: S -> B.bb • I3: S -> a.ab • B -> a. Vishwa Deepak 04CS1029

  5. I4: S -> b.Ba • B -> .a • I5: S-> Bb.b • I6: S -> aa.b • I7: S -> bB.a • I8: B -> a. • I9: S-> Bbb. • I10: S -> aab. • I11: S -> bBa. Underlined item shows shift/reduce conflict. Vishwa Deepak 04CS1029

  6. Items using LR(1) PARSER • I0 : S’ -> .S , $ • S -> .Bbb ,$ • S -> .aab , $ • S -> .bBa , $ • B -> .a ,a|b • I1: S’ -> S. ,$ • I2: S -> B.bb , $ • I3: S -> a.ab ,$ • B -> a. , b • I4: S -> b.Ba , $ • B -> .a ,a Vishwa Deepak 04CS1029

  7. I5: S-> Bb.b ,$ • I6: S -> aa.b ,$ • I7: S -> bB.a ,$ • I8: B -> a. ,a • I9: S-> Bbb. ,$ • I10: S -> aab. ,$ • I11: S -> bBa. ,$ In the underlined item , we find that there is no shift / reduce conflict. Here LALR = LR(1) & Number of states in SLR parser = Number of states in LR(1) parser. Vishwa Deepak 04CS1029

  8. NEXT GRAMMAR (EXAMPLE)#To discuss reduce/reduce conflict • S’-> S • S-> aAd • S-> bBd • S-> aBe • S-> bAe • A-> c • B-> c Vishwa Deepak 04CS1029

  9. Items using LR(1) PARSER • I0 : S’-> .S , $ • S-> . aAd, $ • S-> . bBd, $ • S-> . aBe, $ • S-> . bAe, $ • I1: S’-> S ., $ • I2: S-> a . Ad, $ • S-> a . Be, $ • A-> .c, d • B->.c, e • I3: S-> b . Bd, $ • S-> b . Ae, $ • A->.c, e • B->.c,d Vishwa Deepak 04CS1029

  10. I4: S->aA.d, $ • I5: S-> aB.e,$ • I6: A->c. , d • B->c. , e • I7: S->bB.d, $ • I8: S->bA.e, $ • I9: B->c. , d • A->c. , e • I10: S->aAd. , $ • I11: S->aBe., $ • I12: S->bBd., $ • I13: S->aBe., $ The underlined items are of our interest. There is no reduce/reduce conflict in the underlined items, due to different follows. Vishwa Deepak 04CS1029

  11. LR parser Item d e I1 .. .. I2 .. .. .. .. .. I6 r6 r7 .. .. .. I9 r7 r6 .. .. .. LALR parser Item d e I1 .. .. I2 .. .. .. .. .. I69 r6/r7 r6/r7 .. .. .. I10 .. .. .. .. .. LR and LALR parsing tables Vishwa Deepak 04CS1029

  12. In the last slide we found that even there was no R/R conflict in LR(1) parser, it appeared in LALR parser. • This shows that LALR is less potent than LR(1) parser ,since it gains R/R conflict. • LALR is free of S/R conflict , if LR(1) is. But SLR may be having this conflict. So, SLR is least potent among all the three. Vishwa Deepak 04CS1029

  13. CONCLUSION SHIFT-REDUCE CONFLICT present in SLR  Some of them are solved in…. LR (1)  All those solved are preserved in… LALR Vishwa Deepak 04CS1029

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