# CS453 : Shift Reduce Parsing Unambiguous Grammars LR(0) and SLR Parse Tables by Wim Bohm and Michelle Strout. CS453 Shift-reduce Parsing 1

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1 CS453 : Shift Reduce Parsing Unambiguous Grammars LR(0) and SLR Parse Tables by Wim Bohm and Michelle Strout CS453 Shift-reduce Parsing 1

2 Plan for Today Finish PA1 this week Friday recitation: help with PA1 Also on Thursday and Friday, form groups for PA2 and on Shift-reduce parsing, LR(1) The problem with predictive top down parsing LR parsing: bottom up Performs a right-most derivation in reverse Parsing unambiguous grammars LR parsing table and parsing algorithm CS453 Shift-reduce Parsing 2

3 LL vs LR LL(k) must predict which production looking ahead k: S à SS (S) ε is it S à SS or (S) when I see ((((.? produces the parse tree TOP DOWN, with a left to right derivation LR(k) postpones the decision until all tokens of the rhs plus k more tokens have been seen. It therefore is more powerful. It does this by parsing BOTTOM UP CS453 Shift-reduce Parsing 3

4 Shift-reduce parsing in an LR parser LR(k) parser Left-to-right parse Right-most derivation K-token look ahead LR parsing algorithm Performs a shift-reduce parse with an explicit stack stack contains grammar symbols (T or V) mixed with states Look at state at top of stack and input symbol to find action in table shift(n): advance input, push state n on stack reduce(k): pop rhs of grammar rule k, look up state on top of stack and lhs for goto n, push lhs(k) and n onto stack accept: stop and success error: stop and fail CS453 Shift-reduce Parsing 4

5 Example LR parse S à AB Aà Aa a B à Bb b S à S \$ aaabb\$ß Aaabb\$ß Aabb\$ß Abb\$ß ABb\$ ß AB\$ß S\$ß S Notice that this is the rightmost derivation S à S\$à AB\$à ABb\$à Abb\$à Aabb\$à Aaabb\$à aaabb\$ in reverse! It does not start with the start symbol, it ends with it LR(k) parsing scans the input Left to right and produces the Rightmost derivation (looking k tokens ahead) in reverse. CS453 Shift-reduce Parsing 5

6 LR parsing engine The LR parsing engine uses an Explicit Stack (as opposed to the implicit stack of LL) and the input token string and performs two kinds of actions Shift -- push input token onto the stack Reduce -- pop a rhs off the stack and push the corresponding lhs on the stack It accepts the string of tokens when the input is empty or \$/(EOF) and the stack contains the start symbol. CS453 Shift-reduce Parsing 6

7 Simplified example LR parsing engine ac4ons S à AB Aà Aa a B à Bb b S à S \$ Stack input action aaabb\$ shift a aabb\$ reduce : Aà a A aabb\$ shift Aa abb\$ reduce: Aà Aa A abb\$ shift Aa bb\$ reduce: Aà Aa A bb\$ shift Ab b\$ reduce: Bà b AB b\$ shift ABb \$ reduce: Bà Bb AB \$ reduce: Sà AB S \$ accept CS453 Shift-reduce Parsing 7

8 Shift reduce parsing applied to unambiguous grammars [0] S à ( S ) [1] S à S \$ [2] S à ID Single parentheses nest Start symbol is S Stack input action ((ID))\$ shift ( (ID))\$ shift (( ID))\$ shift ((ID ))\$ reduce: Sà Id ((S ))\$ shift ((S) )\$ reduce: Sà (S) (S )\$ shift (S) \$ reduce: Sà (S) S \$ accept CS453 Shift-reduce Parsing 8

9 Table driven LR parse algorithm The stack contains grammar symbols and states: after a symbol is pushed, the next state is pushed The actions can now be formulated as: Sn: push input token; push n (ie goto state n) Rk: rule k: lhs à rhs pop rhs of rule k off the stack, this exposes a state s look up in row s and column lhs the next state gn to goto push lhs; push n (ie goto state n) a: stop parsing, report success _: stop parsing, report error CS453 Shift-reduce Parsing 9

10 LR(0) SLR LALR LR(k) Given the shift reduce machinery described, there are various algorithms for creating the parse table. These algorithms get more and more sophisticated, starting from simple LR(0) Parser generators such as java-cup use the more sophisticated LALR parse table creation algorithm Today the goal is to understand how to do a shift and reduce parse given a table. Later we will discuss how to get the table. CS453 Shift-reduce Parsing 10

11 Table Driven LR Parsing The parsing engine is a DFA plus a stack: a determinis7c pushdown automaton. The DFA is described by a transi7on table: Rows are states Columns are Grammar symbols (terminals, non- terminals) terminals: shid or reduce and go to next state non- terminals: decide which state to go to ader a reduc7on So there are four kinds of ac7ons sn: shid input and go to state n rk: reduce according to rule k: lhs à rhs, this exposes a state s look up in row s and column lhs the next state gn to goto push lhs; push n (ie goto state n) a: accept _: error (indicated by blank state) CS453 Shift-reduce Parsing 11

12 Example LR(0) Parse Table, Single Parentheses Nest [0] S -> ( S ) [1] S -> S \$ [2] S -> ID Action Goto State ( ) \$ ID S 0 s3 s1 2 1 r2 r2 r2 r2 2 a 3 s3 s1 4 4 s5 5 r0 r0 r0 r0 In LR(0) parse tables, reduce actions happen for all terminals, as the parser does not look ahead. CS453 Shift-reduce Parsing 12

13 Parse table for List grammar 0: S à S\$ 1: Sà ( L ) 2: Sà x 3: Là S 4: Là L, S ( ) x, \$ S L 0 s2 s1 g3 1 r2 r2 r2 r2 r2 2 s2 s1 g6 g4 3 a 4 s5 s7 5 r1 r1 r1 r1 r1 6 r3 r3 r3 r3 r3 7 s2 s1 g8 8 r4 r4 r4 r4 r4 parse (x,(x))\$ stack input action 0 (x,(x))\$ s2 0(2 x,(x))\$ s1 0(2x1,(x))\$ r2: Sà x 0(2S6,(x))\$ r3: Là S 0(2L4,(x))\$ s7 0(2L4,7 (x))\$ s2 0(2L4,7(2 x))\$ s1 0(2L4,7(2x1 ))\$ r2: Sà x 0(2L4,7(2S6 ))\$ r3: Là S 0(2L4,7(2L4 ))\$ s5 0(2L4,7(2L4)5 )\$ r1: S à (L) 0(2L4,7S8 0(2L4 )\$ s5 0(2L4)5 03S )\$ r4: Là L,S \$ r1:sà (L) \$ a CS453 Shift-reduce Parsing 13

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