编译原理教学课件：Chapter 5 - Bottom-Up Parsing.pptVIP

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编译原理教学课件：Chapter 5 - Bottom-Up Parsing.ppt

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* * If there is a legal action on the current input token from one of the goto states, push that state onto the stack and restart the parse. If there are several such states, prefer a shift to a reduce. Among the reduce actions, prefer one whose associated nonterminal is least general If there is no legal action on the current input token from one of the goto states, advance the input until there is a legal action or the end of the input reached Example:LR parsing of string “(2+*)” ACTION GOTO num ( + - * ) $ E T F 4 r6 r6 r6 r6 r6 r6 r6 7 S5 S6 ? 11 4 9 S5 S6 13 11 r2 r2 r2 r2 S9 r2 r2 13 r5 r5 r5 r5 r5 r5 r5 reduce T→T*F )$ $0(6E10+7T11*9F13 error;push F,goto 13 )$ $0(6E10+7T11*9 S9 *)$ $0(6E10+7T11 error;push T,goto 11 *)$ $0(6E10+7 Action Input Parsing stack * * Assignment Consider the following grammar: E-(L) | a L-L,E | E Construct the DFA of LR(0) items for this grammar Construct the SLR(1) parsing table Show the parsing stack and the actions of an SLR(1) parser for the input string ((a),a,(a,a)) Is this grammar an LR(0) grammar? If not, describe the LR(0) conflict. If so, construct the LR(0) parsing table, and describe how a parse might differ from an SLR(1) parse. * * * Example Grammar of real variable declarationreal-declaration→ real identifier-listidentifier-list→identifier-list, identifier identifier-list →identifier Rewrite the G‘： (0) S’ ? S (1) S ? r D (2) D? D, i (3) D ? i * * Example G’: (0) S’→S (1) S→rD(2) D→D,i (3) D→i In state I3, S→rD? is a complete item, D→D?,i is a shift item, there is shift-reduce conflict, so G’ is not LR(0) grammar I0:S‘ → ? SS‘ → ?rD ? I1: S‘ → S ? S I2: S → r?DD → ?D,iD → ? i r I4: D → i ? i I3: S → rD ? D →D ?,i D I5: D → D,?i , I6: D →D,i ? i * * 2. Eliminating Conflicts in SLR(1) The Main Idea of SLR(1)(Simple LR(1)) SLR(1) parsing is a simple, effective extension of LR(0) It uses the DFA of sets of LR(0) items. Increases the power of LR(0) parsing by using the next token in the input s