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中英文翻译系统的分析与设计.doc

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中英文翻译系统的分析与设计.doc

SystemsAnalysisandDesignWorkingundercontrolofastoredprogram,acomputerprocessesdataintoinformation.Thinkaboutthatdefinitionforaminute.Anygivencomputerapplicationinvolvesatleastthreecomponentshardware,software,anddata.Merelywritingaprogramisntenoughbecausetheprogramisbutonecomponentinasystem.Asystemisagroupofcomponentsthatworktogethertoaccomplishanobjective.Forexample,considerapayrollsystem.Itsobjectiveispayingemployees.WhatcomponentsareinvolvedEachday,employeesrecordtheirhoursworkedontimecards.Attheendofeachweek,thetimecardsarecollectedanddeliveredtothecomputercenter,wheretheyarereadintoapayrollprogram.Asitruns,theprogramaccessesdatafiles.Finally,thepaychecksareprintedanddistributed.Forthesystemtowork,people,procedures,inputandoutputmedia,files,hardware,andsoftwaremustbecarefullycoordinated.Notethattheprogramisbutonecomponentinasystem.Computerbasedsystemsaredevelopedbecausepeopleneedinformation.Thosepeople,calledusers,generallyknowwhatisrequired,butmaylacktheexpertisetoobtainit.Technicalprofessionals,suchasprogrammers,havetheexpertise,butmaylacktrainingintheusersfield.Tocomplicatematters,usersandprogrammersoftenseemtospeakdifferentlanguages,leadingtocommunicationproblems.Asystemsanalystisaprofessionalwhotranslatesuserneedsintotechnicalterms,thusservingasabridgebetweenusersandtechnicalprofessionals.Likeanengineeroranarchitect,asystemsanalystsolvesproblemsbycombiningsolidtechnicalskillswithinsight,imagination,andatouchofart.Generally,theanalystfollowsawelldefined,methodicalprocessthatincludesatleastthefollowingsteps1.Problemdefinition2.Analysis3.Design4.Implementation5.MaintenanceAttheendofeachstep,resultsaredocumentedandsharedwithboththeuserandtheprogrammers.Theideaistocatchandcorrecterrorsandmisunderstandingsasearlyaspossible.Perhapsthebestwaytoillustratetheprocessisthroughexample.Pictureasmallclothingstorethatpurchasesmerchandiseatwholesale,displaysthisstock,andsellsittocustomersatretail.Ontheonehand,toomuchstockrepresentsanunnecessaryexpense.Ontheotherhand,apoorselectiondiscouragesshoppers.Ideally,abalancecanbeachievedenough,butnottoomuch.Complicatingmattersisthefactthatinventoryisconstantlychanging,withcustomerpurchasesdepletingstock,andreturnsandreordersaddingtoit.1Theownerwouldliketotrackinventorylevelsandreorderandgivenitemjustbeforethestorerunsout.Forasingleitem,thetaskiseasyjustcountthestockonhand.Unfortunately,thestorehashundredsofdifferentitems,andkeepingtrackofeachoneisimpractical.Perhapsacomputermighthelp.21ProblemDefinitionThefirststepinthesystemsanalysisanddesignprocessisproblemdefinition.Theanalystsobjectiveisdeterminingwhattheuserinthiscase,thestoresownerneeds.Notethat,astheprocessbegins,theuserpossessesthecriticalinformation,andtheanalystmustlistenandlearn.Fewusersaretechnicalexperts.Mostseethecomputerasamagicbox,andarenotconcernedwithhowitworks.Atthisstage,theanalysthasnobusinesseventhinkingaboutprograms,files,andcomputerhardware,butmustcommunicatewiththeuseronhisorherownterm.TheideaistoensurethatboththeuserandtheanalystarethinkingaboutthesamethingThus,aclear,writtenstatementexpressingtheanalystsunderstandingoftheproblemisessential.Theusershouldreviewandcorrectthiswrittenstatement.Thetimetocatchmisunderstandingsandoversightsisnow,beforetime,moneyandeffortarewasted.Often,followingapreliminaryproblemdefinition,theanalystperformsafeasibilitystudy.Thestudyabriefcapsuleversionoftheentiresystemsanalysisanddesignprocess,attemptstoanswerthreequestions1.Cantheproblembesolved2.Canitbesalvedintheusersenvironment3.CanitbesolvedatareasonablecostIftheanswertoanyoneofthesequestionsisno,thesystemshouldnotbedeveloped.Givenagoodproblemdefinitionandapositivefeasibilitystudy,theanalystcanturntoplanninganddevelopingaproblemsolution.22AnalysisAsanalysisbegins,theanalystunderstandstheproblem.Thenextstepisdeterminingwhatmustbedonetosolveit.Theuserknowswhatmustbedone1duringanalysisthisknowledgeisextractedandformallydocumented.Mostusersthinkintermsofthefunctionstobeperformedandthedataelementstobemanipulated.Theobjectiveistoidentifyandlinkthesekeyfunctionsanddataelements,yieldingalogicalsystemdesign.Startwiththesystemsbasicfunctions.Thekeyiskeepingtrackofthestockonhandforeachproductininventory.Inventorychangesbecausecustomerspurchase,exchange,andreturnproducts,sothesystemwillhavetoprocesscustomertransactions.Thestoresownerwantstoselectivelylookattheinventorylevelforanyproductinshortsupplyand,ifappropriate,orderreplacementstock,sothesystemmustbeabletocommunicatewithmanagement.Finally,followingmanagementauthorization,thesystemshouldgenerateareorderreadytosendtoasupplier.Fig1Giventhesystemsbasicfunctions,theanalystsnexttaskisgainingasenseoftheirlogicalrelationship.Agoodwaytostartisbydescribinghowdataflowbetweenthefunctions.Asthenameimplies,dataflowdiagramsareparticularlyusefulforgraphicallydescribingthesedataflows.FoursymbolsareusedFig.1.Datasourcesanddestinationsarerepresentedbysquaresinputdataenterthesystemfromasource,andoutputdataflowtoadestination.Onceinthesystem,thedataaremanipulatedorchangebyprocesses,representedbyroundcornerrectangles.Aprocessmightbeaprogram,aprocedure,oranythingelsethatchangesormovesdata.Datacanbeheldforlaterprocessingindatastores,symbolizedbyopenendedrectangles.Adatastoremightbeadiskfile,atapefile,adatabase,writtennotes,orevenapersonsmemory.Finally,dataflowbetweensources,destinations,processes,enddatastoresoverdataflows,whicharerepresentedbyarrows.Fig2Figure2showsapreliminarydataflowdiagramfortheinventorysystem.StartwithCUSTOMER.Transactionsflowfromacustomerfintothesystem,wheretheyarehandledbyProcesstransaction.Adatastore,STOCK,holdsdataoneachitemininventory.Processtransactionchangesthedatatoreflectthenewtransaction.Meanwhile,MANAGEMENTaccessesthesystemthroughCommunicate,evaluatingthedatainSTOCKand,ifnecessary,requestingareorder.Onceareorderisauthorized.GeneratereordersendsnecessarydatatotheSUPPLIER,whoshipstheitemstothestore.Notethat,becausethereorderrepresentsachangeintheinventorylevelofaparticularproductorproductsitishandledasatransaction.Thedataflowdiagramdescribesthelogicalsystem.Thenextstepistracingthedataflows.StartwiththedestinationSUPPLIER.Reordersflowtosuppliersforexample,thestoremightwant25pairsofjeans.Tofilltheorder,thesupplierneedstheproductdescriptionandthereorderquantity.WheredothesedataelementscomefromSincetheyareoutputbyGeneratereorder,theymusteitherbeInputtoorgeneratedbythisprocess.DataflowintoGeneratereorderforSTOCKthus,productdescriptionsandreorderquantitiesmustbestoredinSTOCK.Otherdataelements,suchastheitempurchasedandthepurchasequantityaregeneratedbyCUSTOMER.Stillothers,forexamplesellingpriceandreorderpoint,aregeneratedbyorneededbyMANAGEMENT.Thecurrentstockonhandforagivenitemisanexampleofadataelementgeneratedbyanalgorithminoneoftheprocedures.Stepbystep,methodically,theanalystidentifiesthedataelementstobeinputto.storedby,manipulatedby,generatedby,oroutputbythesystem.Tokeeptrackofthedataelements,theanalystmightlisteachoneinadatadictionary.Asimpledatadictionarycanbesetuponindexcards,butcomputerizeddatadictionarieshavebecomeincreasinglypopular.Thedatadictionary,acollectionofdatadescribinganddefiningthedata,isusefulthroughoutthesystemsanalysisanddesignprocess,andisoftenusedtobuildadatabaseduringtheimplementationstage.Theideaofanalysisistodefinethesystemsmajorfunctionsanddataelementsmethodically.Rememberthattheobjectiveistranslatinguserneedsintotechnicalterms.Sincethesystemstartswiththeuser,thefirststepisdefiningtheusersneeds.Usersthinkintermsoffunctionsanddata.Theydonotvisualizeprograms,orfiles,orhardware.andduringthisinitial,crucialanalysisstageitisessentialthattheanalystthinklikeauser,notlikeaprogrammer.Dataflowdiagramsanddatadictionariesareusefultools.Theyprovideaformatforrecordingkeyinformationabouttheproposedsystem.Also,theyjogtheanalystsmemoryforexample,iftheanalystdoesnthavesufficientinformationtocompleteadatadictionaryentry,heorshehasprobablymissedsomething.Perhapsmostimportantly,thedataflowdiagramandthedatadictionarydocumenttheanalystsunderstandingofthesystemrequirements.Byreviewingthesedocuments,theusercancorrectmisunderstandingsoroversights.Finally,theyrepresentanexcellentstartingpointthenextstep,design.23DesignAsweenterthedesignstage,weknowwhatthesystemmustdo,andthuscanbeginthinkingabouthowtodoit.Theobjectiveistodevelopastrategyforsolvingtheproblem.Atthisstage,wearenotinterestedinwritingcodeorindefiningprecisedatastructuresinstead,wewanttoidentify,atablackboxlevel,necessaryprograms,files,procedures,andothercomponents.ThedataflowdiagramdefinesthesystemsnecessaryfunctionshowmighttheybeimplementedOnepossibilityiswritingoneprogramforeachprocess.Anotheriscombiningtwoormoreprocessesinasingleprogramtherearedozensofalternativesolutions.Letsfocusononeoptionanddocumentit.Asystemflowchartusessymbolstorepresentprograms,procedures,hardwaredevices,andtheothercomponentsofaphysicalsystemFig.3.Ourflowchart.Fig.4showsthattransactiondataenterthesystemthroughaterminal,areprocessedbyadatacollectionprogram,andthenarestoredonaninventoryfile.Eventually,theinventoryfileisprocessedbyaReportandreorderprogram.Throughit,managementmanipulatesthedataandauthorizesreorders.Fig.4Onasystemflowchart,symbolsrepresentprograms,procedures,hardwaredevices,andtheothercomponentsofaphysicalsystem.Fig3Lookatthesystemflowchart.Itidentifiesseveralhardwarecomponents,includingacomputer,adiskdrive,adataentryterminal,aprinter,andadisplayterminal.TwoprogramsareneededProcesstransactionandReportandreorder.Inadditiontothehardwareandtheprograms,we1lneeddatastructuresfortheinventoryfileandfordataflawsbetweentheI/Odevicesandthesoftware.Notethatthissystemflowchartillustratesonepossiblesolutionagoodanalystwilldevelopseveralfeasiblealternativesbeforechoosingone.Fig4Theflowchartmapsthesystem,highlightingitsmajorphysicalcomponents.Sincethedatalinkthecomponents,thenexttaskisdefiningthedatastructures.Consider,forexample,theinventoryfile.ItcontainsallthedataelementsfromthedatastoreSTOCK.Thedataelementsare

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