PrinciplesofCompiler6PrinciplesofCompiler6AnalysisLecturer:HaiNotethatmostoftheseslideswerecreatedProf.Wen-junLI(SchoolofDr.Zhong-meiSHU(DepartmentofComputerScience)Dr.HanLIN(DepartmentofComputerScience)CourseWebsite:問問Recursive-Recursive-ChooseanA-productionA X1X2Xkfor(i=1tok)if(Xiisanonterminal&&!Xi())returnfalse;elseif isaif(Xiequalsthecurrentinputtokent)getthenexttokent;elsereturn}}return}EliminationofLeftEliminationofLeftEliminationofLeftAEliminationofLeftAgrammarisleftrecursiveifithasanonterminalsuchthatthereisastringforGenerally,Generally,However,ThisHowever,Thisprocedureeliminatesimmediaterecursion,butdoesnoteliminateleftrecursioninvolvingderivationsoftwoormoresteps.TheAlgorithmTheAlgorithmforEliminationofDragon,Dragon,??Whyis??WhyisthealgorithmWhatwillhappeniftheinputgrammarhasacycle?Whatwillhappeniftheinputgrammarhasan?-productions???Giveanalgorithmtoconvertagrammarintoan?Giveanalgorithmtoconvertagrammarintoanequivalentgrammarthathasno?-GiveanalgorithmtoconvertagrammarintoanequivalentgrammarthathasnoSeeExercise4.4.6and4.4.7(DragonBook?Recursive-Recursive-對于字符串對于字符串cabdcad回MoreEfficientMoreEfficient??BacktrackingisWeneedmoreefficientRecursive-Recursive-SASAcAd|ab|LeftLeftleftleftIsleftIsleftfactoringenoughforpredictiveSAbc|BdaIfweonlylookaheadonlyonecharacter,wedonotknowwhichproductionshouldbechosen.FIRSTConsiderFIRSTConsidertwoA-productionsAα|β,whereFIRST(α)andaredisjoint.WecanthenchoosebetweentheseA-productionsbylookingatthenextinputsymbola,sinceacanbeinatmostoneofFIRST(α)andFIRST(β),notboth.ComputingComputingComputetheComputetheFIRST(X)foreachnonterminalSaA|?,SaA|?,ifthenextinputsymbolisnot‘a’,thenwillwillchooseS?;otherwisebothproductionscanbeifAcanbetherightmostsymbolinsomesententialform,then$isinFOLLOW(A).ComputingComputingComputetheComputetheFOLLOW(X)foreachnonterminalConstructionofConstructionofaParsingPredictiveParsingPredictiveParsingLL(1)LL(1)SinceM[S’,SinceM[S’,e]containsmorethanoneproduction,thegrammarisnotLL(1).作作ImplementingtheTwo?ImplementingtheTwo??Non-table-CodingforRecursivePredictiveCodingforRecursivePredictive?CodingforRecursiveCodingforRecursivePredictive?CodingforRecursiveCodingforRecursivePredictive?CodingforRecursiveCodingforRecursivePredictiveModelofModelofaTable-PredictiveTabl