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1、数据流图与ER图考题解析2.solutions.Drawingprinciple:Dataflowmustberelatedtoprocessing,Thereisnodataflowbetweenentitiesandentitles,storageandstorage,andnodataflowbetweenentitiesandentitles・Datastreamsthatflowintoprocessingdonothavethesamenameasthedatastreamthatflow
2、sout・(1)databalaneeprinciple・ConcealmentoflocaldatastorageWhenthedatastoredinadataflowgraphisnotthecorrespondingexternalinterfaceprocessedintheparentgraph,butonlythedatainterfacebetweencertainprocessesinthediagram,thesedataarestoredaslocaldatastorage.In
3、ordertoemphasizetheconcealmentoflocaldatastorage,ingeneral,localdatastoragecanonlybedrawnasinputsoroutputsofcertainprocesseddatainterfacesoraparticularprocess・Inaccordaneewiththetop-downmethod,adatastorageforthefirsttimewithonlyoneprocessing,thenthedata
4、storageshouldbeusedasthelocaldatastorageandprocess!ngoftheassociation,inthedatastreaminthesubgraphneednotdraw(thelayer),anddrawintheprocessingofthesubgraph,unlesstheprocessingforatomicprocessingTheprincipleofdatabalanceisdividedintotwoaspects:Accordingt
5、othehierarchicalrelationship,thedataflowgraphisgenerallydividedintothetopleveldataflowgraph,themiddledataflowgraphandthebottomdataflowgraph・Besidesthetoplayerdiagram,theremaininghierarchicaldataflowdiagramsarenumberedfrom0.Inahierarchicaldataflowdiagram
6、,theparentandchildgraphsshouldbebalanced,thatis,theinput/outputstreamofaprocessintheparentgraphmustbethesameastheinputandoutputdatastreamofitssubgraphinquantityandname・Anotheraspectofthedatabalanceissaid:eachprocessingmusthaveinputdatastreamandtheoutput
7、datastream,andaprocessingalloutputdataindatastreamcanbeobtaineddirectlyfromtheinputdataoftheprocessingflow,orthroughtheprocessingofdatagenerated.(1)afewspecificpointstopayspecialattentionto.Inadditiontotheflowofdatastorage(files)ortheflowofdatafromdatas
8、toragewithoutnaming,everyotherdatastreammusthaveanappropriatename・Theflowofdatatoafilethatrepresentsthedatathatiswrittenintoit,andthedatastreamthatflowsoutofthefilerepresentsthereadfile・Inthefulldataflowdiagram,eachfilemusthavebo
