编译原理2E全套配套课件龙书 Chapter_three(1).ppt

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1、COMPILERCONSTRUCTIONPrinciplesandPracticeKennethC.Louden3.Context-FreeGrammarsandParsingPARTONEContentsPARTONE3.1TheParsingProcess[More]3.2Context-FreeGrammars[More]3.3ParseTreesandAbstract[More]3.4Ambiguity[More]PARTTWO3.5ExtendedNotations:EBNFandSyntaxDiagrams3.6

2、FormalPropertiesofContext-FreeLanguages3.7SyntaxoftheTINYLanguageIntroductionParsingisthetaskofSyntaxAnalysisDeterminingthesyntax,orstructure,ofaprogram.ThesyntaxisdefinedbythegrammarrulesofaContext-FreeGrammarTherulesofacontext-freegrammararerecursiveThebasicdatas

3、tructureofSyntaxAnalysisisparsetreeorsyntaxtreeThesyntacticstructureofalanguagemustalsoberecursive3.1TheParsingProcessFunctionofaParserTakesthesequenceoftokensproducedbythescannerasitsinputandproducesthesyntaxtreeasitsoutput.ParserSequenceoftokensSyntax-TreeIssueso

4、ftheParsingThesequenceoftokensisnotanexplicitinputparameterTheparsercallsascannerproceduregetTokentofetchthenexttokenfromtheinputasitisneededduringtheparsingprocess.Theparsingstepofthecompilerreducestoacalltotheparserasfollows:SyntaxTree=parse()IssuesoftheParsingTh

5、eparserincorporatealltheotherphasesofacompilerinasingle-passcompilerNoexplicitsyntaxtreeneedstobeconstructedTheparserstepsthemselveswillrepresentthesyntaxtreeimplicitlybyacallParse()IssuesoftheParsingInMulti-Pass,thefurtherpasseswillusethesyntaxtreeastheirinputThes

6、tructureofthesyntaxtreeisheavilydependentontheparticularsyntacticstructureofthelanguageThistreeisusuallydefinedasadynamicdatastructureEachnodeconsistsofarecordwhosefieldsincludetheattributesneededfortheremainderofthecompilationprocess(i.e.,notjustthosecomputedbythe

7、parser).IssuesoftheParsingWhatismoredifficultfortheparserthanthescanneristhetreatmentoferrors.ErrorinthescannerGenerateanerrortokenandconsumetheoffendingcharacter.IssuesoftheParsingErrorintheparserTheparsermustnotonlyreportanerrormessagebutitmustrecoverfromtheerror

8、andcontinueparsing(tofindasmanyerrorsaspossible)AparsermayperformerrorrepairErrorrecoveryisthereportingofmeaningfulerrormessagesandtheresumptiono