1. Prepared by VISHWA DEEPAK 04CS1029
LALR PARSER
Compiler
Prepared by
VISHWA DEEPAK
04CS1029
24-Feb-19
Vishwa Deepak CS1029

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.
24-Feb-19
Vishwa Deepak CS1029

3. Taking an example Grammar
S’ -> S
S -> Bbb
S -> aab
S -> bBa
B -> a
24-Feb-19
Vishwa Deepak CS1029

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.
24-Feb-19
Vishwa Deepak CS1029

5. Underlined item shows shift/reduce conflict.
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.
24-Feb-19
Vishwa Deepak CS1029

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
24-Feb-19
Vishwa Deepak CS1029

7. & Number of states in SLR parser = Number of states in LR(1) parser.
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.
24-Feb-19
Vishwa Deepak CS1029

8. NEXT GRAMMAR (EXAMPLE) #To discuss reduce/reduce conflict
S-> aAd
S-> bBd
S-> aBe
S-> bAe
A-> c
B-> c
24-Feb-19
Vishwa Deepak CS1029

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
24-Feb-19
Vishwa Deepak CS1029

10. The underlined items are of our interest.
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.
24-Feb-19
Vishwa Deepak CS1029

11. LR and LALR parsing tables
LR parser
Item d e I I
I r r7
I r r6
LALR parser
Item d e I I
I r6/r7 r6/r7 I
24-Feb-19
Vishwa Deepak CS1029

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.
24-Feb-19
Vishwa Deepak CS1029

13. CONCLUSION SHIFT-REDUCE CONFLICT present in SLR 
Some of them are solved in….
LR (1)
All those solved are preserved in…
LALR
24-Feb-19
Vishwa Deepak CS1029