嵌入式拼图游戏-外文翻译

0外文原文CMainarticle:Cvariabletypesanddeclarations，ChasastaticweaktypingtypesystemthatsharessomesimilaritieswiththatofotherALGOLdescendantssuchasPascal.Therearebuilt-intypesforintegersofvarioussizes,bothsignedandunsigned,floating-pointnumbers,characters,andenumeratedtypes(enum).C99addedabooleandatatype.Therearealsoderivedtypesincludingarrays,pointers,records(struct),anduntaggedunions(union).Cisoftenusedinlow-levelsystemsprogrammingwhereescapesfromthetypesystemmaybenecessary.Thecompilerattemptstoensuretypecorrectnessofmostexpressions,buttheprogrammercanoverridethechecksinvariousways,eitherbyusingatypecasttoexplicitlyconvertavaluefromonetypetoanother,orbyusingpointersorunionstoreinterprettheunderlyingbitsofadataobjectinsomeotherway.SomefindCsdeclarationsyntaxunintuitive,particularlyforfunctionpointers.(Ritchiesideawastodeclareidentifiersincontextsresemblingtheiruse:declarationreflectsuse.)Csusualarithmeticconversionsallowforefficientcodetobegenerated,butcansometimesproduceunexpectedresults.Forexample,acomparisonofsignedandunsignedintegersofequalwidthrequiresaconversionofthesignedvaluetounsigned.Thiscangenerateunexpectedresultsifthesignedvalueisnegative.Csupportstheuseofpointers,atypeofreferencethatrecordstheaddressorlocationofanobjectorfunctioninmemory.Pointerscanbedereferencedtoaccessdatastoredattheaddresspointedto,ortoinvokeapointed-tofunction.Pointerscanbemanipulatedusingassignmentorpointerarithmetic.Therun-timerepresentationofapointervalueistypicallyarawmemoryaddress(perhapsaugmentedbyanoffset-within-wordfield),butsinceapointerstypeincludesthetypeofthething，pointedto,expressionsincludingpointerscanbetype-checkedatcompiletime.Pointerarithmeticisautomaticallyscaledbythesizeofthepointed-todatatype.PointersareusedformanydifferentpurposesinC.Textstringsarecommonlymanipulatedusingpointersintoarraysofcharacters.Dynamicmemoryallocation1isperformedusingpointers.Manydatatypes,suchastrees,arecommonlyimplementedasdynamicallyallocatedstructobjectslinkedtogetherusingpointers.Pointerstofunctionsareusefulforpassingfunctionsasargumentstohigher-orderfunctions(suchasqsortorbsearch)orascallbackstobeinvokedbyeventhandlers.Anullpointervalueexplicitlypointstonovalidlocation.Dereferencinganullpointervalueisundefined,oftenresultinginasegmentationfault.Nullpointervaluesareusefulforindicatingspecialcasessuchasnonextpointerinthefinalnodeofalinkedlist,orasanerrorindicationfromfunctionsreturningpointers.Inappropriatecontextsinsourcecode,suchasforassigningtoapointervariable,anullpointerconstantcanbewrittenas0,withorwithoutexplicitcastingtoapointertype,orastheNULLmacrodefinedbyseveralstandardheaders.Inconditionalcontexts,nullpointervaluesevaluatetofalse,whileallotherpointervaluesevaluatetotrue.Voidpointers(void*)pointtoobjectsofunspecifiedtype,andcanthereforebeusedasgenericdatapointers.Sincethesizeandtypeofthepointed-toobjectisnotknown,voidpointerscannotbedereferenced,norispointerarithmeticonthemallowed,althoughtheycaneasilybe(andinmanycontextsimplicitlyare)convertedtoandfromanyotherobjectpointertype.Carelessuseofpointersispotentiallydangerous.Becausetheyaretypicallyunchecked,apointervariablecanbemadetopointtoanyarbitrarylocation,whichcancauseundesirableeffects.Althoughproperlyusedpointerspointtosafeplaces,theycanbemadetopointtounsafeplacesbyusinginvalidpointerarithmetic;theobjectstheypointtomaybedeallocatedandreused(danglingpointers);theymaybeusedwithouthavingbeeninitialized(wildpointers);ortheymaybedirectlyassignedanunsafevalueusingacast,union,orthroughanothercorruptpointer.Ingeneral,Cispermissiveinallowingmanipulationofandconversionbetweenpointertypes,althoughcompilerstypicallyprovideoptionsforvariouslevelsofchecking.Someotherprogramminglanguagesaddresstheseproblemsbyusingmorerestrictivereferencetypes.2ArraytypesinCaretraditionallyofafixed,staticsizespecifiedatcompiletime.(ThemorerecentC99standardalsoallowsaformofvariable-lengtharrays.)However,itisalsopossibletoallocateablockofmemory(ofarbitrarysize)atrun-time,usingthestandardlibrarysmallocfunction,andtreatitasanarray.Csunificationofarraysandpointersmeansthatdeclaredarraysandthesedynamicallyallocatedsimulatedarraysarevirtuallyinterchangeable.Sincearraysarealwaysaccessed(ineffect)viapointers,arrayaccessesaretypicallynotcheckedagainsttheunderlyingarraysize,althoughsomecompilersmayprovideboundscheckingasanoption.31Arrayboundsviolationsarethereforepossibleandrathercommonincarelesslywrittencode,andcanleadtovariousrepercussions,includingillegalmemoryaccesses,corruptionofdata,bufferoverruns,andrun-timeexceptions.Ifboundscheckingisdesired,itmustbedonemanually.Cdoesnothaveaspecialprovisionfordeclaringmultidimensionalarrays,butratherreliesonrecursionwithinthetypesystemtodeclarearraysofarrays,whicheffectivelyaccomplishesthesamething.Theindexvaluesoftheresultingmultidimensionalarraycanbethoughtofasincreasinginrow-majororder.Multidimensionalarraysarecommonlyusedinnumericalalgorithms(mainlyfromappliedlinearalgebra)tostorematrices.ThestructureoftheCarrayiswellsuitedtothisparticulartask.However,sincearraysarepassedmerelyaspointers,theboundsofthearraymustbeknownfixedvaluesorelseexplicitlypassedtoanysubroutinethatrequiresthem,anddynamicallysizedarraysofarrayscannotbeaccessedusingdoubleindexing.(Aworkaroundforthisistoallocatethearraywithanadditionalrowvectorofpointerstothecolumns.)C99introducedvariable-lengtharrayswhichaddresssome,butnotall,oftheissueswithordinaryArray-pointerinterchangeability。Thesubscriptnotationxi(wherexdesignatesapointer)isasyntacticsugarfor*(x+i).32Takingadvantageofthecompilersknowledgeofthepointertype,theaddressthatx+ipointstoisnotthebaseaddress(pointed3tobyx)incrementedbyibytes,butratherisdefinedtobethebaseaddressincrementedbyimultipliedbythesizeofanelementthatxpointsto.Thus,xidesignatesthei+1thelementofthearray.Furthermore,inmostexpressioncontexts(anotableexceptionisasoperandofsizeof),thenameofanarrayisautomaticallyconvertedtoapointertothearraysfirstelement.Thisimpliesthatanarrayisnevercopiedasawholewhennamedasanargumenttoafunction,butratheronlytheaddressofitsfirstelementispassed.Therefore,althoughfunctioncallsinCusepass-by-valuesemantics,arraysareineffectpassedbyreference.Thesizeofanelementcanbedeterminedbyapplyingtheoperatorsizeoftoanydereferencedelementofx,asinn=sizeof*xorn=sizeofx0,andthenumberofelementsinadeclaredarrayAcanbedeterminedassizeofA/sizeofA0.Thelatteronlyappliestoarraynames:variablesdeclaredwithsubscripts(intA20).DuetothesemanticsofC,itisnotpossibletodeterminetheentiresizeofarraysthroughpointerstoarraysorthosecreatedbydynamicallocation(malloc);codesuchassizeofarr/sizeofarr0(wherearr=Adesignatesapointer)willnotworksincethecompilerassumesthesizeofthepointeritselfisbeingrequested.3334Sincearraynameargumentstosizeofarenotconvertedtopointers,theydonotexhibitsuchambiguity.However,arrayscreatedbydynamicallocationareinitializedtopointersratherthantruearrayvariables,sotheysufferfromthesamesizeofissuesasarraypointers.Thus,despitethisapparentequivalencebetweenarrayandpointervariables,thereisstilladistinctiontobemadebetweenthem.Eventhoughthenameofanarrayis,inmostexpressioncontexts,convertedintoapointer(toitsfirstelement),thispointerdoesnotitselfoccupyanystorage;thearraynameisnotanl-value,anditsaddressisaconstant,unlikeapointervariable.Consequently,whatanarraypointstocannotbechanged,anditisimpossibletoassignanewaddresstoanarrayname.Arraycontentsmaybecopied,however,byusingthememcpyfunction,orbyaccessingtheindividualelements.Puzzles4Manypuzzlesaretermedfullyinterlocking.Thismeansthatadjacentpiecesareconnectingsuchthatifyoumoveonepiecehorizontallyyoumoveall,preservingtheconnection.Sometimestheconnectionistightenoughtopickupasolvedpartholdingonepiece.Somefullyinterlockingpuzzleshavepiecesallofasimilarshape,withroundedtabsoutonoppositeends,withcorrespondingblankscutintotheinterveningsidestoreceivethetabsofadjacentpieces.Otherfullyinterlockingpuzzlesmayhavetabsandblanksvariouslyarrangedoneachpiece,buttheyusuallyhavefoursides,andthenumbersoftabsandblanksthusadduptofour.Theuniform-shapedfullyinterlockingpuzzles,sometimescalledJapaneseStyle,arethemostdifficult,becausethedifferencesinshapesbetweenpiecescanbeverysubtle.citationneededSomepuzzlesalsohavepieceswithnon-interlockingsidesthatareusuallyslightlycurvedincomplexcurves.Theseareactuallytheeasiestpuzzlestosolve,sincefewerotherpiecesarepotentialcandidatesformating.Mostjigsawpuzzlesaresquare,rectangular,orround,withedgepiecesthathaveonesidethatiseitherstraightorsmoothlycurvedtocreatethisshape,plusfourcornerpiecesifthepuzzleissquareorrectangular.Somejigsawpuzzleshaveedgepiecesthatarecutjustlikealltherestoftheinterlockingpieces,withnosmoothedge,tomakethemmorechallenging.Otherpuzzlesaredesignedsotheshapeofthewholepuzzleformsafigure,suchasananimal.Theedgepiecesmayvarymoreinthesecases.RequirementsPhaseThechancesofaproductbeingdevelopedontimeandwithinbudgetaresomewhatslimunlessthemembersofthesoftwaredevelopmentteamagreeonwhatthesoftwareproductwilldo.Thefirststepinachievingthisunanimityistoanalyzetheclientscurrentsituationaspreciselyaspossible.Forexample,itisinadequatetosay,“Theyneedacomputer-aideddesignsystembecausetheyclaimtheirmanualdesignsystem,thereislousy.“Unlessthedevelopmentteamknowsexactlywhatiswrongwiththecurrentmanualsystem,thereisahighprobabilitythataspectsofthenewcomputerizedsystemwill5beequally“lousy.“Similarly,ifapersonalcomputermanufactureriscontemplatingdevelopmentofanewoperatingsystem,thefirststepistoevaluatethefirmscurrentoperatingsystemandanalyzecarefullyexactlywhyitisunsatisfactory.Totakeanextremeexample,itisvitaltoknowwhethertheproblemexistsonlyinthemindofthesalesmanager,whoblamestheoperatingsystemforpoorsales,orwhetherusersoftheoperatingsystemarethoroughlydisenchantedwithitsfunctionalityandreliability.Onlyafteraclearpictureofthepresentsituationhasbeengainedcantheteamattempttoanswerthecriticalquestion,Whatmustthenewproductbeabletodo?Theprocessofansweringthisquestioniscarriedoutduringtherequirementsphase.Acommonlyheldmisconceptionisthat,duringtherequirementsphase,thedevelopersmustdeterminewhatsoftwaretheclientwants.Onthecontrary,therealobjectiveoftherequirementsphaseistodeterminewhatsoftwaretheclientneeds.Theproblemisthatmanyclientsdonotknowwhattheyneed.Furthermore,evenaclientwhohasagoodideaofwhatisneededmayhavedifficultyinaccuratelyconveyingtheseideastothedevelopers,becausemostclientsarelesscomputerliteratethanthemembersofthedevelopmentteam.Toelicittheclientsneeds,themembersoftherequirementsteammustbefamiliarwiththeapplicationdomain,thatis,thegeneralareainwhichtheproposedsoftwareproductistobeused.Forexample,itisnoteasytoaskmeaningfulquestionsofabankeroranursewithoutfirstacquiringsomefamiliaritywithbankingornursing.Therefore,oneoftheinitialtasksofeachmemberoftherequirementsanalysisteamistoacquirefamiliaritywiththeapplicationdomainunlessheorshealreadyhasexperienceinthatgeneralarea.Itisparticularlyimportanttousecorrectterminologywhencommunicatingwiththeclientandpotentialusersofthetargetsoftware.Afterall,itishardtobetakenseriouslybyapersonworkinginaspecificdomainunlesstheinterviewerusesthenomenclatureappropriateforthatdomain.Moreimportant,useofaninappropriatewordmayleadtoamisunderstanding,eventuallyresultinginafaultyproductbeingdelivered.Thesameproblemcanariseifthe6membersoftherequirementsteamdonotunderstandthesubtletiesoftheterminologyofthedomain.Onewaytosolvetheproblemwithterminologyistobuildaglossary.Theinitialentriesareinsertedwhiletheteamlearnstheapplicationdomain.Thentheglossaryisupdatedwheneverthemembersoftherequirementsteamencounternewterminology.Notonlydoessuchaglossaryreduceconfusionbetweenclientanddevelopers,italsoisusefulinlesseningmisunderstandingsbetweenmembersofthedevelopmentteam.Oncetherequirementsteamhaveacquiredfamiliaritywiththedomain,thenextstepisforthemtostarttodeterminetheclientsneeds,thatis,requirementselicitation.Elicitationtechniqueasfollows:Interviews.Themembersoftherequirementsteammeetwithmembersoftheclientorganizationuntiltheyareconvincedthattheyhaveelicitedallrelevantinformationfromtheclientandfutureusersoftheproduct.Therearetwobasictypesofinterview,structuredandunstructured.Inastructuredinterview,specific,preplanned,close-endedquestionsareposed.Inanunstructuredinterview,open-endedquestionsareasked,toencouragethepersonbeinginterviewedtospeakout.Someofthesefactsmightnothavecometolighthadtheinterviewbeenmorestructured.Atthesametime,itisnotagoodideaiftheinterviewistoounstructured.Therefore,questionsshouldbeposedinsuchawayastoencouragethepersonbeinginterviewedtogivewide-ranginganswersbutwithinthecontextoftheinformationneededbytheinterviewer.Conductingagoodinterviewisnotalwayseasy.First,theinterviewermustbefamiliarwiththeapplicationdomain.Second,thereisnopointininterviewingamemberoftheclientorganizationiftheintervieweralreadyhasmadeuphisorhermindregardingtheclientsneeds.Nomatterwhatheorshepreviouslyhasbeentoldorlearnedbyothermeans,theinterviewermustapproacheveryinterviewwiththeintentionoflisteningcarefullytowhatthepersonbeinginterviewedhastosaywhilefirmlysuppressinganypreconceivednotionsregardingtheclientcompanyortheneedsoftheclientsandpotentialusesofthesoftwareproducttobebuilt.7Scenarios.Ascenarioisawayausermightutilizethetargetproducttoaccomplishsomeobjective.Ascenariocanbedepictedinanumberofways.Onetechniqueissimplytolisttheactionscomprisingthescenario.Anothertechniqueistosetupastoryboard,aseriesofdiagramsdepictingthesequenceofevents.Theycandemonstratethebehavioroftheproductinawaythatiscomprehensibletotheuser.Thiscanresultinadditionalrequirementscomingtolight,asintheweight-lossplannerexample.Becausescenarioscanbeunderstoodbyusers,theutilizationofscenarioscanensurethattheclientandusersplayanactiverolethroughouttherequirementsanalysisprocess.Afterall,theaimoftherequirementsanalysisphaseistoelicittherealneedsoftheclient,andtheonlysourceofthisinformationistheclientandtheusers.Scenarios(ormoreprecisely,usecases)playanimportantroleinobject-orientedanalysis.Tosendaquestionnairetotherelevantmembersoftheclientorganization.Thistechniqueisusefulwhentheopinionsof,say,hundredsofindividualsneedtobedetermined.Furthermore,acarefullythought-outwrittenanswermaybemoreaccuratethananimmediateverbalresponsetoaquestionposedbyaninterviewer.However,anunstructuredinterviewconductedbyamethodicalinterviewerwholistenscarefullyandposesquestionsthatexpandoninitialresponsesusuallyyieldsfarbetterinformationthanathoughtfullywordedquestionnaire.Becausequestionnairesarepreplanned,thereisnowaythataquestioncanbeposedinresponsetoananswer.Toexaminethevariousformsusedbytheclient.Forexample,aforminaprintshopmightreflectpressnumber,paperrollsize,humidity,inktemperature,papertension,andsoon.Thevariousfieldsinthisformshedlightontheflowofprintjobsandtherelativeimportanceofthestepsintheprintingprocess.Otherdocuments,suchasoperatingproceduresandjobdescriptions,alsocanbepowerfultoolsfor3findingoutexactlywhatisdoneandhow.Suchcomprehensiveinformationregardinghowtheclientcurrentlydoesbusinesscanbeextraordinarilyhelpfulindeterminingtheclients8needs.Therefore,carefulperusalofclientdocumentationshouldneverbeoverlookedasasourceofinformationthatcanleadtoanaccurateassessmentoftheclientsneeds.Tosetupvideotapecameraswithintheworkplacetorecord(withthepriorwrittenpermissionofthosebeingobserved)exactlywhatisbeingdone.Onedifficultyofthistechniqueisthatitcantakealongtimetoanalyzethetapes.Ingeneral,oneormoremembersoftherequirementsanalysisteamhastospendanhourplayingbackthetapeforeveryhourthatthecamerasrecord.Thistimeisinadditiontowhatisneededtoassesswhatwasobserved.Moreseriously,thistechniquehasbeenknowntobackfirebadlybecauseemployeesmayviewthecamerasasanunwarrantedinvasionofprivacy.Itisimportantthattherequirementsanalysisteamhavethefullcooperationofallemployees;itcanbeextremelydifficulttoobtainthenecessaryinformationifpeoplefeelthreatenedorharassed.Thepossiblerisksshouldbeconsideredcarefullybeforeintroducingcamerasor,forthatmatter,takinganyotheractionthathasthepotentialtoangeremployees.Aninitialsetofrequirementshasbeenelicited,thenextstepistorefinethem,aprocesscalledrequirementsanalysis.Themembersoftherequirementsteamdiscussthelistofrequirementswiththevariousindividualsinterviewedtodetermineifanythinghasbeenomitted.Then,becausethemostaccurateandpowerfulrequirementsanalysistechniqueisrapidprototyping,arapidprototypeisbuilt.Arapidprototypeishastilybuiltsoftwarethatexhibitsthekeyfunctionalityofthetargetproduct.Theclientandintendedusersoftheproductnowexperimentwiththerapidprototype,whilemembersofthedevelopmentteamwatchandtakenotes.Basedontheirhands-onexperience,userstellthedevelopershowtherapidprototypesatisfiestheirneedsand,moreimportant,identifytheareasthatneedimprovement.Thedeveloperschangetherapidprototypeuntilbothsidesareconvincedthattheneedsoftheclientareaccuratelyencapsulatedintherapidprototype.Therapidprototypethenisusedasthebasisfordrawingupthespecifications.Animportantaspectoftherapidprototypingmodelisembodiedinthewordrapid.The9wholeideaistobuildtheprototypeasquicklyaspossible.Afterall,thepurposeoftherapidprototypeistoprovidetheclientwithanunderstandingoftheproduct,andthesooner,thebetter.Itdoesnotmatteriftherapidprototypehardlyworks,ifitcrasheseveryfewminutes,orifthescreenlayoutsarelessthanperfect.Thepurposeoftherapidprototypeistoenabletheclientandthedeveloperstoagreeasquicklyaspossibleonwhattheproductistodo.Therefore,anyimperfectionsintherapidprototypemaybeignored,providedtheydonotseriouslyimpairthefunctionalityoftherapidprototypeandtherebygiveamisleadingimpressionofhowtheproductwillbehave.Onedifficultywithrapidprototypingisthattheeasewithwhichchangesgenerallycanbemadetoarapidprototypemayencouragetheclienttorequestallsortsofmajorchangestothedeliveredoperational-qualityversionoftheproduct.Furthermore,theclientmayexpectthechangestobeimplementedasrapidlyaschangestotherapidprototype.Arelatedchallengeishavingtoexplaintotheclientthattherapidprototypeisnotofoperationalqualityandtheclientwillhavetowaitfortheoperational-qualityversion,eventhoughtherapidprototypeappearstodoeverythingneeded.Beforerapidprototypingis