reilly media] r cookbook data structures

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1、CHAPTER5DataStructuresIntroductionYoucangetprettyfarinRjustusingvectors.That’swhatChapter2isallabout.Thischaptermovesbeyondvectorstorecipesformatrices,lists,factors,anddataframes.Ifyouhavepreconceptionsaboutdatastructures,Isuggestyouputthemaside.Rdoesdatastructuresdifferently.Ifyouwanttostud

2、ythetechnicalaspectsofR’sdatastructures,IsuggestreadingRinaNutshell(O’Reilly)andtheRLanguageDefinition.Mynotesherearemoreinformal.ThesearethingsIwishI’dknownwhenIstartedusingR.VectorsHerearesomekeypropertiesofvectors:VectorsarehomogeneousAllelementsofavectormusthavethesametypeor,inRterminolo

3、gy,thesamemode.VectorscanbeindexedbypositionSov[2]referstothesecondelementofv.Vectorscanbeindexedbymultiplepositions,returningasubvectorSov[c(2,3)]isasubvectorofvthatconsistsofthesecondandthirdelements.VectorelementscanhavenamesVectorshaveanamesproperty,thesamelengthasthevectoritself,thatgiv

4、esnamestotheelements:>v<-c(10,20,30)>names(v)<-c("Moe","Larry","Curly")>print(v)MoeLarryCurly10203095IfvectorelementshavenamesthenyoucanselectthembynameContinuingthepreviousexample:>v["Larry"]Larry20ListsListsareheterogeneousListscancontainelementsofdifferenttypes;inRterminology,listelements

5、mayhavedifferentmodes.Listscanevencontainotherstructuredobjects,suchaslistsanddataframes;thisallowsyoutocreaterecursivedatastructures.ListscanbeindexedbypositionSolst[[2]]referstothesecondelementoflst.Notethedoublesquarebrackets.ListsletyouextractsublistsSolst[c(2,3)]isasublistoflstthatconsi

6、stsofthesecondandthirdelements.Notethesinglesquarebrackets.ListelementscanhavenamesBothlst[["Moe"]]andlst$Moerefertotheelementnamed“Moe”.Sincelistsareheterogeneousandsincetheirelementscanberetrievedbyname,alistislikeadictionaryorhashorlookuptableinotherprogramminglanguages(Recipe5.9).What’ss

7、urprising(andcool)isthatinR,unlikemostofthoseotherprogramminglanguages,listscanalsobeindexedbyposition.Mode:PhysicalTypeInR,everyobjecthasamode,whichindicateshowitisstoredinmemory:asanumber,asacharacterstring,asalistofpointerstootherobjects,asafunc