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LR Parsing

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LR Parsers The most powerful shift-reduce parsing (yet efficient) is: LR(k) parsing. left to right right-mostk lookhead scanning derivation(k is omitted it is 1) LR parsing is attractive because: –LR parsing is most general non-backtracking shift-reduce parsing, yet it is still efficient. –The class of grammars that can be parsed using LR methods is a proper superset of the class of grammars that can be parsed with predictive parsers. LL(1)-Grammars LR(1)-Grammars –An LR-parser can detect a syntactic error as soon as it is possible to do so a left-to-right scan of the input. –Can recognize virtually all programming language constructs for which CFG can be written

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LR Parsers LR-Parsers –covers wide range of grammars. –SLR – simple LR parser –LR – most general LR parser –LALR – intermediate LR parser (look-ahead LR parser) –SLR, LR and LALR work same (they used the same algorithm), only their parsing tables are different.

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LR Parsing Algorithm SmSm XmXm S m-1 X m-1.. S1S1 X1X1 S0S0 a1a1...aiai anan $ Action Table terminals and $ s t four different a actions t e s Goto Table non-terminal s t each item is a a state number t e s LR Parsing Algorithm stack input output

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A Configuration of LR Parsing Algorithm A configuration of a LR parsing is: ( S o X 1 S 1... X m S m, a i a i+1... a n $ ) StackRest of Input S m and a i decides the parser action by consulting the parsing action table. (Initial Stack contains just S o ) A configuration of a LR parsing represents the right sentential form: X 1... X m a i a i+1... a n $

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Actions of A LR-Parser 1.shift s -- shifts the next input symbol and the state s onto the stack ( S o X 1 S 1... X m S m, a i a i+1... a n $ ) ( S o X 1 S 1... X m S m a i s, a i+1... a n $ ) 2.reduce A (or rn where n is a production number) –pop 2| | (=r) items from the stack; –then push A and s where s=goto[s m-r,A] ( S o X 1 S 1... X m S m, a i a i+1... a n $ ) ( S o X 1 S 1... X m-r S m-r A s, a i... a n $ ) –Output the reducing production reduce A 3.Accept – If action[ S m, a i ] = accept,Parsing successfully completed 4.Error -- Parser detected an error (an empty entry in the action table) and calls an error recovery routine.

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Reduce Action pop 2| | (=r) items from the stack; let us assume that = Y 1 Y 2...Y r then push A and s where s=goto[s m-r,A] ( S o X 1 S 1... X m-r S m-r Y 1 S m-r+1...Y r S m, a i a i+1... a n $ ) ( S o X 1 S 1... X m-r S m-r A s, a i... a n $ ) In fact, Y 1 Y 2...Y r is a handle. X 1... X m-r A a i... a n $ X 1... X m Y 1...Y r a i a i+1... a n $

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LR Parsing Algorithm set ip to point to the first symbol in ω$ initialize stack to S0 repeat forever let ‘s’ be topmost state on stack & ‘a’ be symbol pointed to by ip if action[s,a] = shift s’ –push a then s’ onto stack –advance ip to next input symbol else if action[s,a] = reduce A –pop 2*| | symbols of stack –let s’ be state now on top of stack –push A then goto[s’,A] onto stack –output production A else if action[s,a] == accept –return success else –error()

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SLR Parsing Tables for Expression Grammar stateid+*()$ETF 0s5s4123 1s6acc 2r2s7r2 3r4 4s5s4823 5r6 6s5s493 7s5s410 8s6s11 9r1s7r1 10r3 11r5 Action TableGoto Table 1) E E+T 2) E T 3) T T*F 4) T F 5) F (E) 6) F id

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Actions of SLR-Parser -- Example stackinputactionoutput 0id*id+id$shift 5 0id5*id+id$reduce by F id F id 0F3*id+id$reduce by T F T F 0T2*id+id$shift 7 0T2*7id+id$shift 5 0T2*7id5+id$reduce by F id F id 0T2*7F10+id$ reduce by T T*F T T*F 0T2+id$reduce by E T E T 0E1+id$shift 6 0E1+6id$shift 5 0E1+6id5$reduce by F id F id 0E1+6F3$reduce by T F T F 0E1+6T9$ reduce by E E+T E E+T 0E1$accept

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LR Grammar A grammar for which we can construct an LR parsing table in which every entry is uniquely defined is an LR grammar. For an LR grammar, the shift reduce parser should be able to recognize handles when they appear on top of the stack. Each state symbol summarizes the information contained in the stack below it. The LR parser can determine from the state on top of the stack everything it needs to know about what is in the stack. The next k input symbols can also help the LR parser to make shift reduce decisions. Grammar that can be parsed by an LR parser by examining upto k input symbols on each move is called an LR(k) grammar.

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Constructing SLR Parsing Tables – LR(0) Item LR parser using SLR parsing table is called an SLR parser. A grammar for which an SLR parser can be constructed is an SLR grammar. An LR(0) item (item) of a grammar G is a production of G with a dot at the some position of the right side. Ex:A aBb Possible LR(0) Items:A .aBb (four different possibility)A a.Bb A aB.b A aBb. Sets of LR(0) items will be the states of action and goto table of the SLR parser. A production rule of the form A yields only one item A . Intuitively, an item shows how much of a production we have seen till the current point in the parsing procedure.

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Constructing SLR Parsing Tables – LR(0) Item A collection of sets of LR(0) items (the canonical LR(0) collection) is the basis for constructing SLR parsers. To construct the of canonical LR(0) collection for a grammar we define an augmented grammar and two functions- closure and goto. Augmented Grammar: G’ is grammar G with a new production rule S’ S where S’ is the new starting symbol. i.e G {S’ S} where S is the start state of G. The start state of G’ = S’. This is done to signal to the parser when the parsing should stop to announce acceptance of input.

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Constructing SLR Parsing Tables – LR(0) Item Complete and Incomplete Items: An LR(0) item is complete if ‘.’ Is the last symbol in RHS else it is incomplete. For every rule A α, α≠ , there is only one complete item A α., but as many incomplete items as there are grammar symbols. Kernel and Non-Kernel items: Kernel items include the set of items that do not have the dot at leftmost end. S’ .S is an exception and is considered to be a kernel item. Non-kernel items are the items which have the dot at leftmost end. Sets of items are formed by taking the closure of a set of kernel items.

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The Closure Operation If I is a set of LR(0) items for a grammar G, then closure(I) is the set of LR(0) items constructed from I by the two rules: 1.Initially, every LR(0) item in I is added to closure(I). 2.If A . B is in closure(I) and B is a production rule of G; then B . will be in the closure(I). We will apply this rule until no more new LR(0) items can be added to closure(I).

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The Closure Operation -- Example E’ E closure({E’ . E}) = E E+T { E’ . Ekernel items E TE . E+T T T*FE . T T FT . T*F F (E)T . F F idF . (E) F . id }

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Computation of Closure function closure ( I ) begin J := I; repeat for each item A .B in J and each production B of G such that B . is not in J do add B . to J until no more items can be added to J return J end

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Goto Operation If I is a set of LR(0) items and X is a grammar symbol (terminal or non- terminal), then goto(I,X) is defined as follows: –If A .X in I then every item in closure({A X. }) will be in goto(I,X). –If I is the set of items that are valid for some viable prefix , then goto(I,X) is the set of items that are valid for the viable prefix X. Example: I ={ E’ E., E E.+T} goto(I,+) = { E E+.T T .T*F T .F F .(E) F .id }

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Example I ={ E’ .E, E .E+T, E .T, T .T*F, T .F, F .(E), F .id } goto (I, E) = { E’ E., E E.+T } goto (I, T) = { E T., T T.*F } goto (I, F) = {T F. } goto(I,id) = { F id. } goto (I, ( ) = { F (.E), E .E+T, E .T, T .T*F, T .F, F .(E), F .id }

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Construction of The Canonical LR(0) Collection To create the SLR parsing tables for a grammar G, we will create the canonical LR(0) collection of the grammar G’. Algorithm: Procedure items( G’ ) begin C := { closure({S’ .S}) } repeat for each set of items I in C and each grammar symbol X if goto(I,X) is not empty and not in C add goto(I,X) to C until no more set of LR(0) items can be added to C. end goto function is a DFA on the sets in C.

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The Canonical LR(0) Collection -- Example I 0 : E’ .E. I 4 : goto(I 0, ( ) E .E+T F (.E) I 7 : goto(I 2, *) E .T E .E+T T T*.F T .T*F E .T F .(E) T .F T .T*F F .id F .(E) T .F F .id F .(E) I 8 : goto( I 4, E) F .id F (E.) I 1 : goto(I 0, E) E E.+T E’ E. I 5 : goto(I 0, id) I 9 :goto(I 6, T) E E.+T F id. E E+T. I 6 : goto(I 1, +) T T.*F I 2 : goto(I 0, T) E E+.T I 10 : goto(I 7, F) E T. T .T*F T T*F. T T.*F T .F I 11 : goto(I 8, ) ) I 3 : goto(I 0, F) F .(E)F (E). T F. F .id

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Transition Diagram (DFA) of Goto Function I0I0 I1I2I3I4I5I1I2I3I4I5 I 6 I 7 I 8 to I 2 to I 3 to I 4 I 9 to I 3 to I 4 to I 5 I 10 to I 4 to I 5 I 11 to I 6 to I 7 id ( F * E E + T T T ) F F F ( ( * ( +

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Constructing SLR Parsing Table (of an augumented grammar G’) 1.Construct the canonical collection of sets of LR(0) items for G’. C {I 0,...,I n } 2.State i is constructed from I i. The parsing actions for state I are determined as follows: If a is a terminal, A .a in I i and goto(I i,a)=I j then action[i,a] is shift j. If A . is in I i, then action[i,a] is reduce A for all a in FOLLOW(A) where A S’. If S’ S. is in I i, then action[i,$] is accept. If any conflicting actions generated by these rules, the grammar is not SLR(1). 3.Create the parsing goto table for all non-terminals A, if goto(I i,A)=I j then goto[i,A]=j 4.All entries not defined by (2) and (3) are errors. 5.Initial state of the parser is the one construcetd from the sets of items containing [S’ .S]

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Parsing Tables of Expression Grammar stateid+*()$ETF 0s5s4123 1s6acc 2r2s7r2 3r4 4s5s4823 5r6 6s5s493 7s5s410 8s6s11 9r1s7r1 10r3 11r5 Action TableGoto Table

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SLR(1) Grammar An LR parser using SLR(1) parsing tables for a grammar G is called as the SLR(1) parser for G. If a grammar G has an SLR(1) parsing table, it is called SLR(1) grammar (or SLR grammar in short). Every SLR grammar is unambiguous, but every unambiguous grammar is not a SLR grammar.

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shift/reduce and reduce/reduce conflicts If a state does not know whether it will make a shift operation or reduction for a terminal, we say that there is a shift/reduce conflict. If a state does not know whether it will make a reduction operation using the production rule i or j for a terminal, we say that there is a reduce/reduce conflict. If the SLR parsing table of a grammar G has a conflict, we say that that grammar is not SLR grammar.

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Conflict Example S L=R I 0 : S’ .S I 1 :S’ S. I 6 :S L=.R I 9 : S L=R. S R S .L=RR .L L *R S .R I 2 :S L.=RL .*R L id L .*RR L.L .id R L L .id R .L I 3 :S R. I 4 :L *.R I 7 :L *R. ProblemR .L FOLLOW(R)={=,$}L .*R I 8 :R L. = shift 6L .id reduce by R L shift/reduce conflict I 5 :L id. Action[2,=] = shift 6 Action[2,=] = reduce by R L [ S L=R *R=R] so follow(R) contains, =

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Conflict Example2 S AaAb I 0 :S’ .S S BbBaS .AaAb A S .BbBa B A . B . Problem FOLLOW(A)={a,b} FOLLOW(B)={a,b} areduce by A breduce by A reduce by B reduce/reduce conflict

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Constructing Canonical LR(1) Parsing Tables In SLR method, the state i makes a reduction by A when the current token is a: –if the A . in the I i and a is FOLLOW(A) In some situations, A cannot be followed by the terminal a in a right-sentential form when and the state i are on the top stack. This means that making reduction in this case is not correct. Back to Slide no 22.

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LR(1) Item To avoid some of invalid reductions, the states need to carry more information. Extra information is put into a state by including a terminal symbol as a second component in an item. A LR(1) item is: A . ,awhere a is the look-head of the LR(1) item (a is a terminal or end-marker.) Such an object is called LR(1) item. –1 refers to the length of the second component –The lookahead has no effect in an item of the form [A . ,a], where is not . –But an item of the form [A .,a] calls for a reduction by A only if the next input symbol is a. –The set of such a’s will be a subset of FOLLOW(A), but it could be a proper subset.

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LR(1) Item (cont.) When ( in the LR(1) item A . ,a ) is not empty, the look-head does not have any affect. When is empty (A .,a ), we do the reduction by A only if the next input symbol is a ( not for any terminal in FOLLOW(A) ). A state will contain A .,a 1 where {a 1,...,a n } FOLLOW(A)... A .,a n

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Canonical Collection of Sets of LR(1) Items The construction of the canonical collection of the sets of LR(1) items are similar to the construction of the canonical collection of the sets of LR(0) items, except that closure and goto operations work a little bit different. closure(I) is: ( where I is a set of LR(1) items) –every LR(1) item in I is in closure(I) –if A . B ,a in closure(I) and B is a production rule of G; then B . ,b will be in the closure(I) for each terminal b in FIRST( a).

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goto operation If I is a set of LR(1) items and X is a grammar symbol (terminal or non-terminal), then goto(I,X) is defined as follows: –If A .X ,a in I then every item in closure({A X. ,a}) will be in goto(I,X).

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Construction of The Canonical LR(1) Collection Algorithm: C is { closure({S’ .S,$}) } repeat the followings until no more set of LR(1) items can be added to C. for each I in C and each grammar symbol X if goto(I,X) is not empty and not in C add goto(I,X) to C goto function is a DFA on the sets in C.

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A Short Notation for The Sets of LR(1) Items A set of LR(1) items containing the following items A . ,a 1... A . ,a n can be written as A . ,a 1 /a 2 /.../a n

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Canonical LR(1) Collection -- Example S AaAb I 0 :S’ .S,$ I 1 : S’ S.,$ S BbBaS .AaAb,$ A S .BbBa,$ I 2 : S A.aAb,$ B A .,a B .,b I 3 : S B.bBa,$ I 4 : S Aa.Ab,$ I 6 : S AaA.b,$ I 8 : S AaAb.,$ A .,b I 5 : S Bb.Ba,$ I 7 : S BbB.a,$ I 9 : S BbBa.,$ B .,a S A B a b A B a b to I 4 to I 5

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An Example I 0 : closure({(S’ S, $)}) = (S’ S, $) (S C C, $) (C c C, c/d) (C d, c/d) I 1 : goto(I 1, S) = (S’ S , $) I 2 : goto(I 1, C) = (S C C, $) (C c C, $) (C d, $) I 3 : goto(I 1, c) = (C c C, c/d) (C c C, c/d) (C d, c/d) I 4 : goto(I 1, d) = (C d , c/d) I 5 : goto(I 3, C) = (S C C , $) 1. S’ S 2. S C C 3. C c C 4. C d

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C d , c/d C (S’ S , $ S C C, $ C c C, $ C d, $ C c C, c/d C c C, c/d C d, c/d S C C , $ C c C, $ C c C, $ C d, $ C d , $ C c C , c/d S’ S, $ S C C, $ C c C, c/d C d, c/d C cC , $ SCcdSCcd CcdCcd c c C I0I0 I2I2 I3I3 I4I4 I5I5 I1I1 I6I6 I7I7 I8I8 I9I9 d d

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An Example I 6 : goto(I 3, c) = (C c C, $) (C c C, $) (C d, $) I 7 : goto(I 3, d) = (C d , $) I 8 : goto(I 4, C) = (C c C , c/d) : goto(I 4, c) = I 4 : goto(I 4, d) = I 5 I 9 : goto(I 7, c) = (C c C , $) : goto(I 7, c) = I 7 : goto(I 7, d) = I 8

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An Example I0I1I2I5I6I9I7I3I8I4I0I1I2I5I6I9I7I3I8I4 S C C C C c c c d d d d

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c d $ S C 0 s3 s4 g1 g2 1 a 2 s6 s7 g5 3 s3 s4 g8 4 r3 r3 5 r1 6 s6 s7 g9 7 r3 8 r2 r2 9 r2

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The Core of LR(1) Items The core of a set of LR(1) Items is the set of their first components (i.e., LR(0) items) The core of the set of LR(1) items { (C c C, c/d), (C c C, c/d), (C d, c/d) } is { C c C, C c C, C d }

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Construction of LR(1) Parsing Tables 1.Construct the canonical collection of sets of LR(1) items for G’. C {I 0,...,I n } 2.Create the parsing action table as follows If a is a terminal, A . a ,b in I i and goto(I i,a)=I j then action[i,a] is shift j. If A .,a is in I i, then action[i,a] is reduce A where A S’. If S’ S.,$ is in I i, then action[i,$] is accept. If any conflicting actions generated by these rules, the grammar is not LR(1). 3.Create the parsing goto table for all non-terminals A, if goto(I i,A)=I j then goto[i,A]=j 4.All entries not defined by (2) and (3) are errors. 5.Initial state of the parser contains S’ .S,$

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LALR Parsing Tables 1.LALR stands for Lookahead LR. 2.LALR parsers are often used in practice because LALR parsing tables are smaller than LR(1) parsing tables. 3.The number of states in SLR and LALR parsing tables for a grammar G are equal. 4.But LALR parsers recognize more grammars than SLR parsers. 5.yacc creates a LALR parser for the given grammar. 6.A state of LALR parser will be again a set of LR(1) items.

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Creating LALR Parsing Tables Canonical LR(1) Parser LALR Parser shrink # of states This shrink process may introduce a reduce/reduce conflict in the resulting LALR parser (so the grammar is NOT LALR) But, this shrik process does not produce a shift/reduce conflict.

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The Core of A Set of LR(1) Items The core of a set of LR(1) items is the set of its first component. Ex:S L. =R,$ S L. =RCore R L.,$ R L. We will find the states (sets of LR(1) items) in a canonical LR(1) parser with same cores. Then we will merge them as a single state. I 1 :L id.,= A new state: I 12 : L id.,= L id.,$ I 2 :L id.,$ have same core, merge them We will do this for all states of a canonical LR(1) parser to get the states of the LALR parser. In fact, the number of the states of the LALR parser for a grammar will be equal to the number of states of the SLR parser for that grammar.

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Creation of LALR Parsing Tables 1.Create the canonical LR(1) collection of the sets of LR(1) items for the given grammar. 2.For each core present; find all sets having that same core; replace those sets having same cores with a single set which is their union. C={I 0,...,I n } C’={J 1,...,J m }where m n 3.Create the parsing tables (action and goto tables) same as the construction of the parsing tables of LR(1) parser. 1.Note that: If J=I 1 ... I k since I 1,...,I k have same cores cores of goto(I 1,X),...,goto(I 2,X) must be same. 1.So, goto(J,X)=K where K is the union of all sets of items having same cores as goto(I 1,X). 4.If no conflict is introduced, the grammar is LALR(1) grammar. (We may only introduce reduce/reduce conflicts; we cannot introduce a shift/reduce conflict)

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C d , c/d C (S’ S , $ S C C, $ C c C, $ C d, $ C c C, c/d C c C, c/d C d, c/d S C C , $ C c C, $ C c C, $ C d, $ C d , $ C c C , c/d S’ S, $ S C C, $ C c C, c/d C d, c/d C cC , $ SCcdSCcd CcdCcd c c C I0I0 I2I2 I3I3 I4I4 I5I5 I1I1 I6I6 I7I7 I8I8 I9I9 d d

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C d , c/d C (S’ S , $ S C C, $ C c C, $ C d, $ C c C, c/d C c C, c/d C d, c/d S C C , $ C c C, $ C c C, $ C d, $ C d , $ C c C , c/d/$ S’ S, $ S C C, $ C c C, c/d C d, c/d SCcdSCcd CcdCcd c c C I0I0 I2I2 I3I3 I4I4 I5I5 I1I1 I6I6 I7I7 I 89 d d

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C (S’ S , $ S C C, $ C c C, $ C d, $ C c C, c/d C c C, c/d C d, c/d S C C , $ C c C, $ C c C, $ C d, $ C d , c/d/$ C c C , c/d/$ S’ S, $ S C C, $ C c C, c/d C d, c/d SCcSCc CcdCcd c c C I0I0 I2I2 I3I3 I5I5 I1I1 I6I6 I 47 I 89 d d d

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C (S’ S , $ S C C, $ C c C, $ C d, $ S C C , $ C c C, c/d/$ C c C,c/d/$ C d,c/d/$ C d , c/d/$ C c C , c/d/$ S’ S, $ S C C, $ C c C, c/d C d, c/d SCdSCd CcdCcd c I0I0 I2I2 I5I5 I1I1 I 36 I 47 I 89 d c

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LALR Parse Table cd $S C 0 s36s acc 2 s36 s s36s r3 r3r3 5 r1 89 r2r2r2

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Shift/Reduce Conflict We say that we cannot introduce a shift/reduce conflict during the shrink process for the creation of the states of a LALR parser. Assume that we can introduce a shift/reduce conflict. In this case, a state of LALR parser must have: A .,aandB . a ,b This means that a state of the canonical LR(1) parser must have: A .,aandB . a ,c But, this state has also a shift/reduce conflict. i.e. The original canonical LR(1) parser has a conflict. (Reason for this, the shift operation does not depend on lookaheads)

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Reduce/Reduce Conflict But, we may introduce a reduce/reduce conflict during the shrink process for the creation of the states of a LALR parser. I 1 : A .,a I 2 : A .,b B .,b B .,c I 12 : A .,a/b reduce/reduce conflict B .,b/c

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Canonical LALR(1) Collection – Example2 S’ S 1) S L=R 2) S R 3) L *R 4) L id 5) R L I 0 :S’ . S,$ S . L=R,$ S . R,$ L . *R,$/= L . id,$/= R . L,$ I 1 :S’ S.,$ I 2 :S L. =R,$ R L.,$ I 3 :S R.,$ I 411 :L *. R,$/= R . L,$/= L . *R,$/= L . id,$/= I 512 :L id.,$/= I 6 :S L=. R,$ R . L,$ L . *R,$ L . id,$ I 713 :L *R.,$/= I 810 : R L.,$/= I 9 :S L=R.,$ to I 6 to I 713 to I 810 to I 411 to I 512 to I 810 to I 411 to I 512 to I 9 S L L L R R id R * * * Same Cores I 4 and I 11 I 5 and I 12 I 7 and I 13 I 8 and I 10

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LALR(1) Parsing Tables – (for Example2) id*=$SLR 0s5s4123 1acc 2s6r5 3r2 4s5s487 5r4 6s12s r3 8r5 9r1 no shift/reduce or no reduce/reduce conflict so, it is a LALR(1) grammar

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Using Ambiguous Grammars All grammars used in the construction of LR-parsing tables must be un-ambiguous. Can we create LR-parsing tables for ambiguous grammars ? –Yes, but they will have conflicts. –We can resolve these conflicts in favor of one of them to disambiguate the grammar. –At the end, we will have again an unambiguous grammar. Why we want to use an ambiguous grammar? –Some of the ambiguous grammars are much natural, and a corresponding unambiguous grammar can be very complex. –Usage of an ambiguous grammar may eliminate unnecessary reductions. Ex. E E+T | T E E+E | E*E | (E) | id T T*F | F F (E) | id

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Sets of LR(0) Items for Ambiguous Grammar I 0 : E’ . E E . E+E E . E*E E . (E) E . id I 1 : E’ E. E E. +E E E. *E I 2 : E (. E) E . E+E E . E*E E . (E) E . id I 3 : E id. I 4 : E E +. E E . E+E E . E*E E . (E) E . id I 5 : E E *. E E . E+E E . E*E E . (E) E . id I 6 : E (E. ) E E. +E E E. *E I 7 : E E+E. E E. +E E E. *E I 8 : E E*E. E E. +E E E. *E I 9 : E (E). I5I5 ) E E E E * * * * ( ( ( ( id I4I4 I2I2 I2I2 I3I3 I3I3 I4I4 I4I4 I5I5 I5I5

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SLR-Parsing Tables for Ambiguous Grammar FOLLOW(E) = { $,+,*,) } State I 7 has shift/reduce conflicts for symbols + and *. I0I0 I1I1 I7I7 I4I4 E+E when current token is + shift + is right-associative reduce + is left-associative when current token is * shift * has higher precedence than + reduce + has higher precedence than *

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SLR-Parsing Tables for Ambiguous Grammar FOLLOW(E) = { $,+,*,) } State I 8 has shift/reduce conflicts for symbols + and *. I0I0 I1I1 I8I8 I5I5 E*E when current token is * shift * is right-associative reduce * is left-associative when current token is + shift + has higher precedence than * reduce * has higher precedence than +

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SLR-Parsing Tables for Ambiguous Grammar id+*()$E 0s3s21 1s4s5acc 2s3s26 3r4 4s3s27 5s3s28 6s4s5s9 7r1s5r1 8r2 9r3 ActionGoto

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Error Recovery in LR Parsing An LR parser will detect an error when it consults the parsing action table and finds an error entry. All empty entries in the action table are error entries. Errors are never detected by consulting the goto table. An LR parser will announce error as soon as there is no valid continuation for the scanned portion of the input. A canonical LR parser (LR(1) parser) will never make even a single reduction before announcing an error. The SLR and LALR parsers may make several reductions before announcing an error. But, all LR parsers (LR(1), LALR and SLR parsers) will never shift an erroneous input symbol onto the stack.

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Panic Mode Error Recovery in LR Parsing Scan down the stack until a state s with a goto on a particular nonterminal A is found. (Get rid of everything from the stack before this state s). Discard zero or more input symbols until a symbol a is found that can legitimately follow A. –The symbol a is simply in FOLLOW(A), but this may not work for all situations. The parser stacks the nonterminal A and the state goto[s,A], and it resumes the normal parsing. This nonterminal A is normally is a basic programming block (there can be more than one choice for A). –stmt, expr, block,...

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Phrase-Level Error Recovery in LR Parsing Each empty entry in the action table is marked with a specific error routine. An error routine reflects the error that the user most likely will make in that case. An error routine inserts the symbols into the stack or the input (or it deletes the symbols from the stack and the input, or it can do both insertion and deletion). –missing operand –unbalanced right parenthesis

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