第8章计算机学科基础

Chapter8,High-LevelProgrammingLanguages高级程序设计语言,8-2,ChapterGoals本章目标,Describethetranslationprocessanddistinguishbetweenassembly,compilation,interpretation,andexecution描述翻译过程，区别汇编、编译、解释和执行。Namefourdistinctprogrammingparadigmsandnamealanguagecharacteristicofeach列出四种不同的程序设计范型，并说明每种的语言特征。Describethefollowingconstructs:streaminputandoutput,selection,looping,andsubprograms描述下列结构：流输入和输出、选择、循环和子程序。,8-3,ChapterGoals,ConstructBooleanexpressionsanddescribehowtheyareusedtoaltertheflowofcontrolofanalgorithm构造Boolean表达式，并说明如何用它们改变算法的控制流Definetheconceptsofadatatypeandstrongtyping定义数据类型和强类型化Explaintheconceptofaparameteranddistinguishbetweenvalueandreferenceparameters解释参数的概念，区别值参和引用参数Describetwocompositedata-structuringmechanisms描述两种复合数据结构机制Name,describe,andgiveexamplesofthethreeessentialingredientsofanobject-orientedlanguage列出并描述面向对象语言的三要素，给出每种要素的实例,8-4,Compilers编译器,CompilerAprogramthattranslatesahigh-levellanguageprogramintomachinecode把用高级语言编写的程序翻译成机器码的程序。High-levellanguagesprovidearichersetofinstructionsthatmakestheprogrammerslifeeveneasier高级程序设计语言提供的指令集要丰富得多，大大简化了程序员的工作。,8-5,Compilers编译器,Figure8.1Compilationprocess,8-6,Interpreters解释器,InterpreterAtranslatingprogramthattranslatesandexecutesthestatementsinsequence解释器是一种翻译程序，用于解释和执行语句序列。Unlikeanassemblerorcompilerwhichproducemachinecodeasoutput,whichisthenexecutedinaseparatestep不像汇编器和编译器只输出需在其他步骤中运行机器码Aninterpretertranslatesastatementandthenimmediatelyexecutesthestatement解释器在翻译过语句之后会立即执行这个语句。Interpreterscanbeviewedassimulators解释器可以看成是模拟器。,8-7,Java,Introducedin1996andsweptthecomputingcommunitybystormPortabilitywasofprimaryimportanceJavaiscompiledintoastandardmachinelanguagecalledBytecode字节码AsoftwareinterpretercalledtheJVM(JavaVirtualMachineJava虚拟机)takestheBytecodeprogramandexecutesit,8-8,ProgrammingLanguageParadigms程序设计语言的范型,Whatisaparadigm?Asetofassumptions,concepts,values,andpracticesthatconstituteawayofviewingreality一组假设、概念、数值和规则，构成了观察现实的方式。,8-9,ProgrammingLanguageParadigms,Figure8.2PortabilityprovidedbystandardizedlanguagesversusinterpretationbyBytecode,8-10,ProgrammingLanguageParadigms,Figure8.2PortabilityprovidedbystandardizedlanguagesversusinterpretationbyBytecode,8-11,ProgrammingLanguageParadigms,Imperativeorproceduralmodel命令模型或过程模型FORTRAN,COBOL,BASIC,C,Pascal,Ada,andC+Functionalmodel函数模型LISP,Scheme(aderivativeofLISP),andML,命令式编程：命令“机器”如何去做事情(how)，这样不管你想要的是什么(what)，它都会按照你的命令实现。声明式编程：告诉“机器”你想要的是什么(what)，让机器想出如何去做(how)。,函数式（functional）语言采用一种基于函数的递归定义的计算模型。他们的灵感来自于lambda演算。本质上，程序被看作是一种从输入到输出的函数，基于一些更简单的函数，通过一种逐步精化的过程定义。,8-12,ProgrammingLanguageParadigms,Logicprogramming逻辑程序设计PROLOGObject-orientedparadigm面向对象范型SIMULAandSmalltalkC+isasanimperativelanguagewithsomeobject-orientedfeaturesJavaisanobject-orientedlanguagewithsomeimperativefeatures,逻辑式（Logic）或基于约束的（constrained-based）语言有命题逻辑得到灵感，他们把计算看作是一种目标制导的搜索过程，设法根据一集逻辑规则找出满足某些特定关系的值。,8-13,FunctionalityofImperativeLanguages命令式语言的功能性,Sequence顺序ExecutingstatementsinsequenceuntilaninstructionisencounteredthatchangesthissequencingSelection选择DecidingwhichactiontotakeIteration迭代(looping循环)RepeatinganactionBothselectionanditerationrequiretheuseofaBooleanexpression,8-14,BooleanExpressionsBoolean表达式,BooleanexpressionAsequenceofidentifiers,separatedbycompatibleoperators,thatevaluatestotrueorfalse一个标识符序列，标示符之间由相容的运算符分隔，求得的值是true或falseBooleanexpressioncanbeABooleanvariableAnarithmeticexpressionfollowedbyarelationaloperatorfollowedbyanarithmeticexpressionABooleanexpressionfollowedbyaBooleanoperatorfollowedbyaBooleanexpression,8-15,BooleanExpressionsBoolean表达式,VariableAlocationinmemorythatisreferencedbyanidentifierthatcontainsadatavalue由存放数值的标识符引用的内存单元Thus,aBooleanvariableisalocationinmemorythatcancontaineithertrueorfalse,8-16,BooleanExpressions,ArelationaloperatorbetweentwoarithmeticexpressionsisaskingiftherelationshipexistsbetweenthetwoexpressionsForexample,xValueyValue,Page233,8-17,StrongTyping强类型,StrongtypingTherequirementthatonlyavalueofthepropertypecanbestoredintoavariable只有类型相符的值才能存入变量的要求成为强类型。DatatypeAdescriptionofthesetofvaluesandthebasicsetofoperationsthatcanbeappliedtovaluesofthetype数据类型：一组值以及能够应用于这种类型的值的基本操作集合的说明。,8-18,DataTypes数据类型,Integernumbers整形数Realnumbers实型数Characters字符Booleanvalues布尔值Strings字符串,8-19,Integers,TherangevariesdependinguponhowmanybytesareassignedtorepresentanintegervalueSomehigh-levellanguagesprovideseveralintegertypesofdifferentsizesOperationsthatcanbeappliedtointegersarethestandardarithmeticandrelationaloperations,8-20,Reals,Liketheintegerdatatype,therangevariesdependingonthenumberofbytesassignedtorepresentarealnumberManyhigh-levellanguageshavetwosizesofrealnumbersTheoperationsthatcanbeappliedtorealnumbersarethesameasthosethatcanbeappliedtointegernumbers,8-21,Characters,IttakesonebytetorepresentcharactersintheASCIIcharactersetTwobytestorepresentcharactersintheUnicodecharactersetOurEnglishalphabetisrepresentedinASCII,whichisasubsetofUnicode,8-22,Characters,ApplyingarithmeticoperationstocharactersdoesntmakemuchsenseComparingcharactersdoesmakesense,sotherelationaloperatorscanbeappliedtocharactersThemeaningof“lessthan”and“greaterthan”whenappliedtocharactersis“comesbefore”and“comesafter”inthecharacterset,8-23,Boolean,TheBooleandatatypeconsistsoftwovalues:trueandfalseNotallhigh-levellanguagessupporttheBooleandatatypeIfalanguagedoesnot,thenyoucansimulateBooleanvaluesbysayingthattheBooleanvaluetrueisrepresentedby1andfalseisrepresentedby0,8-24,Strings,AstringisasequenceofcharactersconsideredasonedatavalueForexample:“Thisisastring.”Containing17characters:oneuppercaseletter,12lowercaseletters,threeblanks,andaperiodTheoperationsdefinedonstringsvaryfromlanguagetolanguageTheyincludeconcatenationofstringsandcomparisonofstringsintermsoflexicographicorder,8-25,Declarations声明,DeclarationAstatementthatassociatesanidentifierwithavariable,anaction,orsomeotherentitywithinthelanguagethatcanbegivenanamesothattheprogrammercanrefertothatitembyname把变量、动作或语言中的其他实体与标识符关联起来的语句，使程序员可以通过名字引用这些项目。,Declarations,8-26,8-27,Declarations,Reservedword保留字AwordinalanguagethathasspecialmeaningCase-sensitive大小写敏感Uppercaseandlowercaselettersareconsideredthedifferent,8-28,Assignmentstatement赋值语句,AssignmentstatementAnactionstatement(notadeclaration)thatsaystoevaluatetheexpressionontheright-handsideofthesymbolandstorethatvalueintotheplacenamedontheleft-handsideNamedconstantAlocationinmemory,referencedbyanidentifier,thatcontainsadatavaluethatcannotbechanged,AssignmentStatement,Page238,8-29,8-30,Input/OutputStructures输入/输出结构,InourpseudocodealgorithmswehaveusedtheexpressionsReadandWriteHigh-levellanguagesviewinputdataasastreamofcharactersdividedintolines,8-31,Input/OutputStructures输入/输出结构,Thekeytotheprocessingisinthedatatypethatdetermineshowcharactersaretobeconvertedtoabitpattern(input)andhowabitpatternistobeconvertedtocharacters(output)Wedonotgiveexamplesofinput/outputstatementsbecausethesyntaxisoftenquitecomplexanddifferssowidelyamonghigh-levellanguages,8-32,ControlStructures控制结构,ControlstructureAninstructionthatdeterminestheorderinwhichotherinstructionsinaprogramareexecuted确定程序中的其他指令的执行顺序的指令。Structuredprogramming结构化程序设计Aprogrammingmethodologyinwhicheachlogicalunitofaprogramshouldhavejustoneentryandoneexit程序中的每个逻辑部件都只有一个入口和一个出口。Sequence,selectionstatements,loopingstatements,andsubprogramstatementsarecontrolstructures,8-33,SelectionStatements,TheifstatementallowstheprogramtotestthestateoftheprogramvariablesusingaBooleanexpression,Page243,8-34,SelectionStatements,Figure8.3Flowofcontrolofifstatement,8-35,SelectionStatements,8-36,SelectionStatements,Page245,8-37,caseStatement,Forconvenience,manyhigh-levellanguagesincludeacase(orswitch)statementAllowsustomakemultiple-choicedecisionseasier,providedthechoicesarediscrete,Page246,8-38,LoopingStatements,ThewhilestatementisusedtorepeatacourseofactionLetslookattwodistincttypesofrepetitions,8-39,LoopingStatements,Count-controlledloopsRepeataspecifiednumberoftimesUseofaspecialvariablecalledaloopcontrolvariable,Figure8.4Flowofcontrolofwhilestatement,8-40,LoopingStatements,Count-controlledloops,8-41,LoopingStatements,Event-controlledloopsThenumberofrepetitionsiscontrolledbyaneventthatoccurswithinthebodyoftheloopitself,Page249,8-42,LoopingStatements,Event-controlledloops,Page249,8-43,SubprogramStatements,WecangiveasectionofcodeanameandusethatnameasastatementinanotherpartoftheprogramWhenthenameisencountered,theprocessingintheotherpartoftheprogramhaltswhilethenamedcodeisexecuted,8-44,SubprogramStatements,TherearetimeswhenthecallingunitneedstogiveinformationtothesubprogramtouseinitsprocessingAparameterlistisalistoftheidentifierswithwhichthesubprogramistowork,alongwiththetypesofeachidentifierplacedinparenthesesbesidethesubprogramname,8-45,SubprogramStatements,Figure8.5Subprogramflowofcontrol,8-46,SubprogramStatements,Figure8.5Subprogramflowofcontrol,8-47,SubprogramStatements,ParametersIdentifierslistedinparenthesesbesidethesubprogramdeclaration;sometimestheyarecalledformalparametersArgumentsIdentifierslistedinparenthesesonthesubprogramcall;sometimestheyarecalledactualparameters,8-48,SubprogramStatements,ValueparameterAparameterthatexpectsacopyofitsargumenttobepassedbythecallingunit(putonthemessageboard)ReferenceparameterAparameterthatexpectstheaddressofitsargumenttobepassedbythecallingunit(putonthemessageboard),8-49,SubprogramStatements,Page253,8-50,Recursion,RecursionTheabilityofasubprogramtocallitselfEachrecursivesolutionhasatleasttwocasesBasecaseThecasetowhichwehaveananswerGeneralcaseThecasethatexpressesthesolutionintermsofacalltoitselfwithasmallerversionoftheproblemForexample,thefactorialofanumberisdefinedasthenumbertimestheproductofallthenumbersbetweenitselfand0:N!=N*(N1)!,8-51,AsynchronousProcessing异步处理,AsynchronousprocessingTheconceptthatinputandoutputcanbeaccomplishedthroughwindowsonthescreenClickinghasbecomeamajorformofinputtothecomputerMouseclickingisnotwithinthesequenceoftheprogramAusercanclickamouseatanytimeduringtheexecutionofaprogramThistypeofprocessingiscalledasynchronous,8-52,CompositeDataTypes复合数据类型,Records记录Arecordisanamedheterogeneous异构collectionofitemsinwhichindividualitemsareaccessedbynameTheelementsinthecollectioncanbeofvarioustypes,8-53,CompositeDataTypes复合数据类型,Page258,8-54,CompositeDataTypes复合数据类型,Page259,8-55,Arrays数组,AnarrayisanamedcollectionofhomogeneousitemsinwhichindividualitemsareaccessedbytheirplacewithinthecollectionTheplacewithinthecollectioni