计算机专业英语3.docx

计算机专业英语3.docx

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计算机专业英语3

Unit3ComputerLanguageandProgramming

SectionAProgrammingLanguage

I.Introduction

Programminglanguages,incomputerscience,aretheartificiallanguagesusedtowriteasequenceofinstructions（acomputerprogram）thatcanberunbyacomputer.Similartonaturallanguages,suchasEnglish,programminglanguageshaveavocabulary,grammar,andsyntax.However,naturallanguagesarenotsuitedforprogrammingcomputersbecausetheyareambiguous,meaningthattheirvocabularyandgrammaticalstructuremaybeinterpretedinmultipleways.Thelanguagesusedtoprogramcomputersmusthavesimplelogicalstructures,andtherulesfortheirgrammar,spelling,andpunctuationmustbeprecise.

Programminglanguagesvarygreatlyintheirsophisticationandintheirdegreeofversatility.Someprogramminglanguagesarewrittentoaddress（处理）aparticularkindofcomputingproblemorforuseonaparticularmodelofcomputersystem.Forinstance,programminglanguagessuchasFORTRAN1andCOBOL2werewrittentosolvecertaingeneraltypesofprogrammingproblems—FORTRANforscientificapplications,andCOBOLforbusinessapplications.Althoughtheselanguagesweredesigntoaddressspecificcategoriesofcomputerproblems,theyarehighlyportable（可移植）,meaningthattheymaybeusedtoprogrammanytypesofcomputers.Otherlanguages,suchasmachinelanguages,aredesignedtobeusedbyonespecificmodelofcomputersystem,orevenbyonespecificcomputerincertainresearchapplications.Themostcommonlyusedprogramminglanguagesarehighlyportableandcanbeusedtoeffectivelysolvediversetypesofcomputingproblems.LanguageslikeC,PASCALandBASICfallinto（属于）thiscategory.

II.LanguageTypes

Programminglanguagescanbeclassifiedaseitherlow-levellanguagesorhigh-levellanguages.Low-levelprogramminglanguages,ormachinelanguages,arethemostbasictypeofprogramminglanguagesandcanbeunderstooddirectlybyacomputer.Machinelanguagesdifferdependingonthemanufacturerandmodelofcomputer.High-levellanguagesareprogramminglanguagesthatmustfirstbetranslatedintoamachinelanguagebeforetheycanbeunderstoodandprocessedbyacomputer.Examplesofhigh-levellanguagesareC,C++6,PASCAL,andFORTRAN.Assemblylanguagesareintermediatelanguagesthatareveryclosetomachinelanguagesanddonothavetheleveloflinguisticsophisticationexhibitedbyotherhigh-levellanguages,butmuststillbetranslatedintomachinelanguage.

1.MachineLanguages

Inmachinelanguages,instructionsarewrittenassequencesof1sand0s,calledbits,thatacomputercanunderstanddirectly.Aninstructioninmachinelanguagegenerallytellsthecomputerfourthings:

（1）wheretofindoneortwonumbersorsimplepiecesofdatainthemaincomputermemory（RandomAccessMemory,orRAM）,

（2）asimpleoperationtoperform,suchasaddingthetwonumberstogether,（3）whereinthemainmemorytoputtheresultofthissimpleoperation,and（4）wheretofindthenextinstructiontoperform.Whileallexecutableprogramsareeventuallyreadbythecomputerinmachinelanguage,theyarenotallprogrammedinmachinelanguage.Itisextremelydifficulttoprogramdirectlyinmachinelanguagebecausetheinstructionsaresequencesof1sand0s.Atypicalinstructioninamachinelanguagemightread1001011001011andmeanaddthecontentsofstorageregisterAtothecontentsofstorageregisterB.

2.High-LevelLanguages

High-levellanguagesarerelativelysophisticatedsetsofstatementsutilizingwordsandsyntaxfromhumanlanguage.Theyaremoresimilartonormalhumanlanguagesthanassemblyormachinelanguagesandarethereforeeasiertouseforwritingcomplicatedprograms.Theseprogramminglanguagesallowlargerandmorecomplicatedprogramstobedevelopedfaster.However,high-levellanguagesmustbetranslatedintomachinelanguagebyanotherprogramcalledacompilerbeforeacomputercanunderstandthem.Forthisreason,programswritteninahigh-levellanguagemaytakelongertoexecuteanduseup（用完）morememorythanprogramswritteninanassemblylanguage.

3.AssemblyLanguages

Computerprogrammersuseassemblylanguagestomakemachine-languageprogramseasiertowrite.Inanassemblylanguage,eachstatementcorrespondsroughlytoonemachinelanguageinstruction.Anassemblylanguagestatementiscomposedwiththeaidof（借助于，帮助）easytoremembercommands.ThecommandtoaddthecontentsofstorageregisterAtothecontentsofstorageregisterBmightbewrittenADDB,Ainatypicalassemblylanguagestatement.Assemblylanguagessharecertainfeatureswithmachinelanguages.Forinstance,itispossibletomanipulatespecificbitsinbothassemblyandmachinelanguages.Programmersuseassemblylanguageswhenitisimportanttominimizethetimeittakestorunaprogram,becausethetranslationfromassemblylanguagetomachinelanguageisrelativelysimple.Assemblylanguagesarealsousedwhensomepartofthecomputerhastobecontrolleddirectly,suchasindividualdotsonamonitorortheflowofindividualcharacterstoaprinter.

III.ClassificationofHigh-LevelLanguages

High-levellanguagesarecommonlyclassifiedasprocedure-oriented,functional,object-oriented,orlogiclanguages.Themostcommonhigh-levellanguagestodayareprocedure-orientedlanguages.Inlanguages,oneormorerelatedblocksofstatementsthatperformsomecompletefunctionaregroupedtogetherintoaprogrammodule,orprocedure,andgivenanamesuchas"procedureA."Ifthesamesequenceofoperationsisneededelsewhereintheprogram,asimplestatementcanbeusedtoreferbacktotheprocedure.Inessence,aprocedureisjustamini-program.Alargeprogramcanbeconstructedbygroupingtogetherproceduresthatperformdifferenttasks.Procedurallanguagesallowprogramstobeshorterandeasierforthecomputertoread,buttheyrequiretheprogrammertodesigneachproceduretobegeneralenoughtobeuseindifferentsituations.

Functionallanguagestreatprocedureslikemathematicalfunctionandallowthemtobeprocessedlikeanyotherdatainaprogram.Thisallowsamuchhigherandmorerigorouslevelofprogramconstruction,Functionallanguagesalsoallowvariables—symbolsfordatathatcanbespecifiedandchangedbytheuserastheprogramisrunning—tobegivenvaluesonlyonce.Thissimplifiesprogrammingbyreducingtheneedtobeconcernedwiththeexactorderofstatementexecution,sinceavariabledoesnothavetoberedeclared,orrestated,eachtimeitisusedinaprogramstatement.Manyoftheideasfromfunctionallanguageshavebecomekey|partsofmanymodemprocedurallanguages.

Object-orientedlanguagesareoutgrowthsoffunctionallanguages.Inobject-orientedlanguages,thecodeusedtowritetheprogramandthedataprocessedbytheprogramaregroupedtogetherintounitscalledobjects,Objectsarefurthergroupedintoclasses,whichdefinetheattributesobjectsmusthave.AsimpleexampleofaclassistheclassBook.ObjectswithinthisclassmightbeNovelandShortStory.Objectsalsohavecertainfunctionsassociatedwiththem,calledmethods.Thecomputeraccessesanobjectthroughtheuseofoneoftheobject'smethods.Themethodperformssomeactiontothedataintheobjectandreturnsthisvaluetothecomputer.Classesofobjectscanalsobefurthergroupedintohierarchies,inwhichobjectsofoneclasscaninheritmethodsfromanotherclass.Thestructureprovidedinobject-orientedlanguagesmakesthemveryusefulforcomplicatedprogrammingtasks.

Logiclanguagesuselogicastheirmathematicalbase.Alogicprogramconsistsofsetsoffactsandif-thenrules,whichspecifyhowonesetoffactsmaybededucedfrom（推断）others,forexample:

IfthestatementXistrue,thenthestatementYisfalse.

Intheexecutionofsuchaprogram,aninputstatementcanbelogicallydeducedfromotherstatementsintheprogram.Manyartificialintelligenceprogramsarewritteninsuchlanguages.

IV.LanguageStructureandComponents

Programminglanguagesusespecifictypesofstatements,orinstructions,toprovidefunctionalstructuretotheprogram.Astatementinaprogramisabasicsentencethatexpressesasimpleidea—itspurposeistogivethecomputerabasicinstruction.Statementsdefinethetypesofdataallowed,howdataaretobemanipulated,andthewaysthatproceduresandfunctionswork.Programmersusestatementstomanipulatecommoncomponentsofprogramminglanguages,suchasvariablesandmacros（mini-programswithinaprogram）.

Statementsknownasdatadeclarationsgivenamesandpropertiestoelementsofaprogramcalledvariables.Variablescanbeassigneddifferentvalueswithintheprogram.Thepropertiesvariablescanhavearecalledtypes,andtheyincludesuchthingsaswhatpossiblevaluesmightbesavedinthevariables,howmuchnumericalaccuracyistobeusedinthevalues,andhowonevariablemayrepresentacollectionofsimplervaluesinanorganizedfashion,suchasatableorarray.Inmanyprogramminglanguages,akeydatatypeisapointer.Variablesthatarepointersdonotthemselveshavevalues;instead,theyhaveinformationthatthecomputercanusetolocatesomeothervariable—thatis,theypointtoanothervariable.

Anexpressionisapieceofastatementthatdescribesaseriesofcomputationstobeperformedonsomeoftheprogram'svariables,suchasX+Y/Z,inwhichthevariablesareX,Y,andZandthecomputationsareadditionanddivision.Anassignmentstatementassignsavariableavaluederivedfrom（得自）someexpression,whileconditionalstatementsspecifyexpressionstobetestedandthenusedtoselectwhichotherstatementsshouldbeexecutednext.

Procedureandfunctionstatementsdefinecertainblocksofcodeasproceduresorfunct