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WelcometoSwift

AboutSwift

SwiftisanewprogramminglanguageforiOS,OSX,watchOS,andtvOSappsthatbuildsonthebestofCandObjective-C,withouttheconstraintsofCcompatibility.Swiftadoptssafeprogrammingpatternsandaddsmodernfeaturestomakeprogrammingeasier,moreflexible,andmorefun.Swift’scleanslate,backedbythematureandmuch-lovedCocoaandCocoaTouchframeworks,isanopportunitytoreimaginehowsoftwaredevelopmentworks.

Swifthasbeenyearsinthemaking.ApplelaidthefoundationforSwiftbyadvancingourexistingcompiler,debugger,andframeworkinfrastructure.WesimplifiedmemorymanagementwithAutomaticReferenceCounting(ARC).Ourframeworkstack,builtonthesolidbaseofFoundationandCocoa,hasbeenmodernizedandstandardizedthroughout.Objective-Citselfhasevolvedtosupportblocks,collectionliterals,andmodules,enablingframeworkadoptionofmodernlanguagetechnologieswithoutdisruption.Thankstothisgroundwork,wecannowintroduceanewlanguageforthefutureofApplesoftwaredevelopment.

SwiftfeelsfamiliartoObjective-Cdevelopers.ItadoptsthereadabilityofObjective-C’snamedparametersandthepowerofObjective-C’sdynamicobjectmodel.ItprovidesseamlessaccesstoexistingCocoaframeworksandmix-and-matchinteroperabilitywithObjective-Ccode.Buildingfromthiscommonground,Swiftintroducesmanynewfeaturesandunifiestheproceduralandobject-orientedportionsofthelanguage.

Swiftisfriendlytonewprogrammers.Itisthefirstindustrial-qualitysystemsprogramminglanguagethatisasexpressiveandenjoyableasascriptinglanguage.Itsupportsplaygrounds,aninnovativefeaturethatallowsprogrammerstoexperimentwithSwiftcodeandseetheresultsimmediately,withouttheoverheadofbuildingandrunninganapp.

SwiftcombinesthebestinmodernlanguagethinkingwithwisdomfromthewiderAppleengineeringculture.Thecompilerisoptimizedforperformance,andthelanguageisoptimizedfordevelopment,withoutcompromisingoneither.It’sdesignedtoscalefrom“hello,world”toanentireoperatingsystem.AllthismakesSwiftasoundfutureinvestmentfordevelopersandforApple.

SwiftisafantasticwaytowriteiOS,OSX,watchOS,andtvOSapps,andwillcontinuetoevolvewithnewfeaturesandcapabilities.OurgoalsforSwiftareambitious.Wecan’twaittoseewhatyoucreatewithit.

InstruktorXWyróżnienie









InstruktorXPodkreślenie

ASwiftTour

Traditionsuggeststhatthefirstprograminanewlanguageshouldprintthewords“Hello,world!”onthescreen.InSwift,thiscanbedoneinasingleline:

print("Hello,world!")

IfyouhavewrittencodeinCorObjective-C,thissyntaxlooksfamiliartoyou—inSwift,thislineofcodeisacompleteprogram.Youdon’tneedtoimportaseparatelibraryforfunctionalitylikeinput/outputorstringhandling.Codewrittenatglobalscopeisusedastheentrypointfortheprogram,soyoudon’tneedamain()function.Youalsodon’tneedtowritesemicolonsattheendofeverystatement.

ThistourgivesyouenoughinformationtostartwritingcodeinSwiftbyshowingyouhowtoaccomplishavarietyofprogrammingtasks.Don’tworryifyoudon’tunderstandsomething—everythingintroducedinthistourisexplainedindetailintherestofthisbook.

NOTE

OnaMac,downloadthePlaygroundanddouble-clickthefiletoopenitinXcode:https://developer.apple.com/go/?id=swift-tour

SimpleValues

Uselettomakeaconstantandvartomakeavariable.Thevalueofaconstantdoesn’tneedtobeknownatcompiletime,butyoumustassignitavalueexactlyonce.Thismeansyoucanuseconstantstonameavaluethatyoudetermineoncebutuseinmanyplaces.

1 varmyVariable=42

2 myVariable=50

3 letmyConstant=42

Aconstantorvariablemusthavethesametypeasthevalueyouwanttoassigntoit.However,youdon’talwayshavetowritethetypeexplicitly.Providingavaluewhenyoucreateaconstantorvariableletsthecompilerinferitstype.Intheexampleabove,thecompilerinfersthatmyVariableisanintegerbecauseitsinitialvalueisaninteger.

https://developer.apple.com/go/?id=swift-tourInstruktorXWyróżnienie


















InstruktorXProstokąt


Iftheinitialvaluedoesn’tprovideenoughinformation(orifthereisnoinitialvalue),specifythetypebywritingitafterthevariable,separatedbyacolon.

1 letimplicitInteger=70

2 letimplicitDouble=70.0

3 letexplicitDouble:Double=70

EXPERIMEN T

CreateaconstantwithanexplicittypeofFloatandavalueof4.

Valuesareneverimplicitlyconvertedtoanothertype.Ifyouneedtoconvertavaluetoadifferenttype,explicitlymakeaninstanceofthedesiredtype.

1 letlabel="Thewidthis"

2 letwidth=94

3 letwidthLabel=label+String(width)

EXPERIMEN T

TryremovingtheconversiontoStringfromthelastline.Whaterrordoyouget?

There’sanevensimplerwaytoincludevaluesinstrings:Writethevalueinparentheses,andwriteabackslash(\)beforetheparentheses.Forexample:

1 letapples=3

2 letoranges=5

3 letappleSummary="Ihave\(apples)apples."

4 letfruitSummary="Ihave\(apples+oranges)piecesoffruit."

EXPERIMEN T

Use\()toincludeafloating-pointcalculationinastringandtoincludesomeone’snameinagreeting.














Createarraysanddictionariesusingbrackets([]),andaccesstheirelementsbywritingtheindexorkeyinbrackets.Acommaisallowedafterthelastelement.

1 varshoppingList=["catfish","water","tulips","bluepaint"]

2 shoppingList[1]="bottleofwater"

3

4 varoccupations=[

5 "Malcolm":"Captain",

6 "Kaylee":"Mechanic",

7 ]

8 occupations["Jayne"]="PublicRelations"

Tocreateanemptyarrayordictionary,usetheinitializersyntax.

1 letemptyArray=[String]()

2 letemptyDictionary=[String:Float]()

Iftypeinformationcanbeinferred,youcanwriteanemptyarrayas[]andanemptydictionaryas[:]—forexample,whenyousetanewvalueforavariableorpassanargumenttoafunction.

1 shoppingList=[]

2 occupations=[:]

ControlFlow

Useifandswitchtomakeconditionals,andusefor-in,for,while,andrepeat-whiletomakeloops.Parenthesesaroundtheconditionorloopvariableareoptional.Bracesaroundthebodyarerequired.
























1 letindividualScores=[75,43,103,87,12]

2 varteamScore=0

3 forscoreinindividualScores{

4 ifscore>50{

5 teamScore+=3

6 }else{

7 teamScore+=1

8 }

9 }

print(teamScore)

Inanifstatement,theconditionalmustbeaBooleanexpression—thismeansthatcodesuchasifscore{...}isanerror,notanimplicitcomparisontozero.

Youcanuseifandlettogethertoworkwithvaluesthatmightbemissing.Thesevaluesarerepresentedasoptionals.Anoptionalvalueeithercontainsavalueorcontainsniltoindicatethatavalueismissing.Writeaquestionmark(?)afterthetypeofavaluetomarkthevalueasoptional.

1 varoptionalString:String?="Hello"

2 print(optionalString==nil)

3

4 varoptionalName:String?="JohnAppleseed"

5 vargreeting="Hello!"

6 ifletname=optionalName{

7 greeting="Hello,\(name)"

8 }

EXPERIMEN T

ChangeoptionalNametonil.Whatgreetingdoyouget?AddanelseclausethatsetsadifferentgreetingifoptionalNameisnil.

















Iftheoptionalvalueisnil,theconditionalisfalseandthecodeinbracesisskipped.Otherwise,theoptionalvalueisunwrappedandassignedtotheconstantafterlet,whichmakestheunwrappedvalueavailableinsidetheblockofcode.

Anotherwaytohandleoptionalvaluesistoprovideadefaultvalueusingthe??operator.Iftheoptionalvalueismissing,thedefaultvalueisusedinstead.

1 letnickName:String?=nil

2 letfullName:String="JohnAppleseed"

3 letinformalGreeting="Hi\(nickName??fullName)"

Switchessupportanykindofdataandawidevarietyofcomparisonoperations—theyaren’tlimitedtointegersandtestsforequality.

1 letvegetable="redpepper"

2 switchvegetable{

3 case"celery":

4 print("Addsomeraisinsandmakeantsonalog.")

5 case"cucumber","watercress":

6 print("Thatwouldmakeagoodteasandwich.")

7 caseletxwherex.hasSuffix("pepper"):

8 print("Isitaspicy\(x)?")

9 default:

print("Everythingtastesgoodinsoup.")

}

EXPERIMEN T

Tryremovingthedefaultcase.Whaterrordoyouget?

Noticehowletcanbeusedinapatterntoassignthevaluethatmatchedthepatterntoaconstant.

Afterexecutingthecodeinsidetheswitchcasethatmatched,theprogramexitsfromtheswitchstatement.Executiondoesn’tcontinuetothenextcase,sothereisnoneedtoexplicitlybreakoutoftheswitchattheendofeachcase’scode.































Youusefor-intoiterateoveritemsinadictionarybyprovidingapairofnamestouseforeachkey-valuepair.Dictionariesareanunorderedcollection,sotheirkeysandvaluesareiteratedoverinanarbitraryorder.

1 letinterestingNumbers=[

2 "Prime":[2,3,5,7,11,13],

3 "Fibonacci":[1,1,2,3,5,8],

4 "Square":[1,4,9,16,25],

5 ]

6 varlargest=0

7 for(kind,numbers)ininterestingNumbers{

8 fornumberinnumbers{

9 ifnumber>largest{

largest=number

}

}

}

print(largest)

EXPERIMEN T

Addanothervariabletokeeptrackofwhichkindofnumberwasthelargest,aswellaswhatthatlargestnumberwas.

Usewhiletorepeatablockofcodeuntilaconditionchanges.Theconditionofaloopcanbeattheendinstead,ensuringthattheloopisrunatleastonce.










1 varn=2

2 whilen<100{

3 n=n*2

4 }

5 print(n)

6

7 varm=2

8 repeat{

9 m=m*2

}whilem<100

print(m)

Youcankeepanindexinaloopbyusing..<tomakearangeofindexes.

1 vartotal=0

2 foriin0..toseparatetheparameternamesandtypesfromthefunction’sreturntype.




















1 funcgreet(name:String,day:String)->String{

2 return"Hello\(name),todayis\(day)."

3 }

4 greet("Bob",day:"Tuesday")

EXPERIMEN T

Removethedayparameter.Addaparametertoincludetoday’slunchspecialinthegreeting.

Useatupletomakeacompoundvalue—forexample,toreturnmultiplevaluesfromafunction.Theelementsofatuplecanbereferredtoeitherbynameorbynumber.















1 funccalculateStatistics(scores:[Int])->(min:Int,max:Int,sum:Int){

2 varmin=scores[0]

3 varmax=scores[0]

4 varsum=0

5

6 forscoreinscores{

7 ifscore>max{

8 max=score

9 }elseifscore<min{

min=score

}

sum+=score

}

return(min,max,sum)

}

letstatistics=calculateStatistics([5,3,100,3,9])

print(statistics.sum)

print(statistics.2)

Functionscanalsotakeavariablenumberofarguments,collectingthemintoanarray.











1 funcsumOf(numbers:Int...)->Int{

2 varsum=0

3 fornumberinnumbers{

4 sum+=number

5 }

6 returnsum

7 }

8 sumOf()

9 sumOf(42,597,12)

EXPERIMEN T

Writeafunctionthatcalculatestheaverageofitsarguments.

Functionscanbenested.Nestedfunctionshaveaccesstovariablesthatweredeclaredintheouterfunction.Youcanusenestedfunctionstoorganizethecodeinafunctionthatislongorcomplex.

1 funcreturnFifteen()->Int{

2 vary=10

3 funcadd(){

4 y+=5

5 }

6 add()

7 returny

8 }

9 returnFifteen()

Functionsareafirst-classtype.Thismeansthatafunctioncanreturnanotherfunctionasitsvalue.












1 funcmakeIncrementer()->((Int)->Int){

2 funcaddOne(number:Int)->Int{

3 return1+number

4 }

5 returnaddOne

6 }

7 varincrement=makeIncrementer()

8 increment(7)

Afunctioncantakeanotherfunctionasoneofitsarguments.

1 funchasAnyMatches(list:[Int],condition:(Int)->Bool)->Bool{

2 foriteminlist{

3 ifcondition(item){

4 returntrue

5 }

6 }

7 returnfalse

8 }

9 funclessThanTen(number:Int)->Bool{

returnnumber<10

}

varnumbers=[20,19,7,12]

hasAnyMatches(numbers,condition:lessThanTen)

Functionsareactuallyaspecialcaseofclosures:blocksofcodethatcanbecalledlater.Thecodeinaclosurehasaccesstothingslikevariablesandfunctionsthatwereavailableinthescopewheretheclosurewascreated,eveniftheclosureisinadifferentscopewhenitisexecuted—yousawanexampleofthisalreadywithnestedfunctions.Youcanwriteaclosurewithoutanamebysurroundingcodewithbraces({}).Useintoseparatetheargumentsandreturntypefromthebody.


















1 numbers.map({

2 (number:Int)->Intin

3 letresult=3*number

4 returnresult

5 })

EXPERIMEN T

Rewritetheclosuretoreturnzeroforalloddnumbers.

Youhaveseveraloptionsforwritingclosuresmoreconcisely.Whenaclosure’stypeisalreadyknown,suchasthecallbackforadelegate,youcanomitthetypeofitsparameters,itsreturntype,orboth.Singlestatementclosuresimplicitlyreturnthevalueoftheironlystatement.

1 letmappedNumbers=numbers.map({numberin3*number})

2 print(mappedNumbers)

Youcanrefertoparametersbynumberinsteadofbyname—thisapproachisespeciallyusefulinveryshortclosures.Aclosurepassedasthelastargumenttoafunctioncanappearimmediatelyaftertheparentheses.Whenaclosureistheonlyargumenttoafunction,youcanomittheparenthesesentirely.

1 letsortedNumbers=numbers.sort{$0>$1}

2 print(sortedNumbers)

ObjectsandClasses

Useclassfollowedbytheclass’snametocreateaclass.Apropertydeclarationinaclassiswrittenthesamewayasaconstantorvariabledeclaration,exceptthatitisinthecontextofaclass.Likewise,methodandfunctiondeclarationsarewrittenthesameway.
















1 classShape{

2 varnumberOfSides=0

3 funcsimpleDescription()->String{

4 return"Ashapewith\(numberOfSides)sides."

5 }

6 }

EXPERIMEN T

Addaconstantpropertywithlet,andaddanothermethodthattakesanargument.

Createaninstanceofaclassbyputtingparenthesesaftertheclassname.Usedotsyntaxtoaccessthepropertiesandmethodsoftheinstance.

1 varshape=Shape()

2 shape.numberOfSides=7

3 varshapeDescription=shape.simpleDescription()

ThisversionoftheShapeclassismissingsomethingimportant:aninitializertosetuptheclasswhenaninstanceiscreated.Useinittocreateone.














1 classNamedShape{

2 varnumberOfSides:Int=0

3 varname:String

4

5 init(name:String){

6 self.name=name

7 }

8


return"Ashapewith\(numberOfSides)sides."

}

}

Noticehowselfisusedtodistinguishthenamepropertyfromthenameargumenttotheinitializer.Theargumentstotheinitializerarepassedlikeafunctioncallwhenyoucreateaninstanceoftheclass.Everypropertyneedsavalueassigned—eitherinitsdeclaration(aswithnumberOfSides)orintheinitializer(aswithname).

Usedeinittocreateadeinitializerifyouneedtoperformsomecleanupbeforetheobjectisdeallocated.

Subclassesincludetheirsuperclassnameaftertheirclassname,separatedbyacolon.Thereisnorequirementforclassestosubclassanystandardrootclass,soyoucanincludeoromitasuperclassasneeded.

Methodsonasubclassthatoverridethesuperclass’simplementationaremarkedwithoverride—overridingamethodbyaccident,withoutoverride,isdetectedbythecompilerasanerror.Thecompileralsodetectsmethodswithoverridethatdon’tactuallyoverrideanymethodinthesuperclass.

















1 classSquare:NamedShape{

2 varsideLength:Double

3

4 init(sideLength:Double,name:String){

5 self.sideLength=sideLength

6 super.init(name:name)

7 numberOfSides=4

8 }

9

funcarea()->Double{

returnsideLength*sideLength

}

overridefuncsimpleDescription()->String{

return"Asquarewithsidesoflength\(sideLength)."

}

}

lettest=Square(sideLength:5.2,name:"mytestsquare")

test.area()

test.simpleDescription()

EXPERIMEN T

MakeanothersubclassofNamedShapecalledCirclethattakesaradiusandanameasargumentstoitsinitializer.Implementanarea()andasimpleDescription()methodontheCircleclass.

Inadditiontosimplepropertiesthatarestored,propertiescanhaveagetterandasetter.






1 classEquilateralTriangle:NamedShape{

2 varsideLength:Double=0.0

3

4 init(sideLength:Double,name:String){

5 self.sideLength=sideLength

6 super.init(name:name)

7 numberOfSides=3

8 }

9

varperimeter:Double{

get{

return3.0*sideLength

}

set{

sideLength=newValue/3.0

}

}

overridefuncsimpleDescription()->String{

return"Anequilateraltrianglewithsidesoflength\(sideLength)."

}

}

vartriangle=EquilateralTriangle(sideLength:3.1,name:"atriangle")

print(triangle.perimeter)

triangle.perimeter=9.9

print(triangle.sideLength)









Inthesetterforperimeter,thenewvaluehastheimplicitnamenewValue.Youcanprovideanexplicitnameinparenthesesafterset.

NoticethattheinitializerfortheEquilateralTriangleclasshasthreedifferentsteps:

1. Settingthevalueofpropertiesthatthesubclassdeclares.2. Callingthesuperclass’sinitializer.3. Changingthevalueofpropertiesdefinedbythesuperclass.Anyadditionalsetupworkthatuses

methods,getters,orsetterscanalsobedoneatthispoint.

Ifyoudon’tneedtocomputethepropertybutstillneedtoprovidecodethatisrunbeforeandaftersettinganewvalue,usewillSetanddidSet.Thecodeyouprovideisrunanytimethevaluechangesoutsideofaninitializer.Forexample,theclassbelowensuresthatthesidelengthofitstriangleisalwaysthesameasthesidelengthofitssquare.










1 classTriangleAndSquare{

2 vartriangle:EquilateralTriangle{

3 willSet{

4 square.sideLength=newValue.sideLength

5 }

6 }

7 varsquare:Square{

8 willSet{

9 triangle.sideLength=newValue.sideLength

}

}

init(size:Double,name:String){

square=Square(sideLength:size,name:name)

triangle=EquilateralTriangle(sideLength:size,name:name)

}

}

vartriangleAndSquare=TriangleAndSquare(size:10,name:"anothertestshape")

print(triangleAndSquare.square.sideLength)

print(triangleAndSquare.triangle.sideLength)

triangleAndSquare.square=Square(sideLength:50,name:"largersquare")

print(triangleAndSquare.triangle.sideLength)

Whenworkingwithoptionalvalues,youcanwrite?beforeoperationslikemethods,properties,andsubscripting.Ifthevaluebeforethe?isnil,everythingafterthe?isignoredandthevalueofthewholeexpressionisnil.Otherwise,theoptionalvalueisunwrapped,andeverythingafterthe?actsontheunwrappedvalue.Inbothcases,thevalueofthewholeexpressionisanoptionalvalue.

1 letoptionalSquare:Square?=Square(sideLength:2.5,name:"optionalsquare")

2 letsideLength=optionalSquare?.sideLength







EnumerationsandStructures

Useenumtocreateanenumeration.Likeclassesandallothernamedtypes,enumerationscanhavemethodsassociatedwiththem.

1 enumRank:Int{

2 caseAce=1

3 caseTwo,Three,Four,Five,Six,Seven,Eight,Nine,Ten

4 caseJack,Queen,King


6 switchself{

7 case.Ace:

8 return"ace"

9 case.Jack:

return"jack"

case.Queen:

return"queen"

case.King:

return"king"

default:

returnString(self.rawValue)

}

}

}

letace=Rank.Ace

letaceRawValue=ace.rawValue















EXPERIMEN T

WriteafunctionthatcomparestwoRankvaluesbycomparingtheirrawvalues.

Bydefault,Swiftassignstherawvaluesstartingatzeroandincrementingbyoneeachtime,butyoucanchangethisbehaviorbyexplicitlyspecifyingvalues.Intheexampleabove,Aceisexplicitlygivenarawvalueof1,andtherestoftherawvaluesareassignedinorder.Youcanalsousestringsorfloating-pointnumbersastherawtypeofanenumeration.UsetherawValuepropertytoaccesstherawvalueofanenumerationcase.

Usetheinit?(rawValue:)initializertomakeaninstanceofanenumerationfromarawvalue.

1 ifletconvertedRank=Rank(rawValue:3){

2 letthreeDescription=convertedRank.simpleDescription()

3 }

Thecasevaluesofanenumerationareactualvalues,notjustanotherwayofwritingtheirrawvalues.Infact,incaseswherethereisn’tameaningfulrawvalue,youdon’thavetoprovideone.











1 enumSuit{

2 caseSpades,Hearts,Diamonds,Clubs


4 switchself{

5 case.Spades:

6 return"spades"

7 case.Hearts:

8 return"hearts"

9 case.Diamonds:

return"diamonds"

case.Clubs:

return"clubs"

}

}

}

lethearts=Suit.Hearts

letheartsDescription=hearts.simpleDescription()

EXPERIMEN T

Addacolor()methodtoSuitthatreturns“black”forspadesandclubs,andreturns“red”forheartsanddiamonds.

NoticethetwowaysthattheHeartscaseoftheenumerationisreferredtoabove:Whenassigningavaluetotheheartsconstant,theenumerationcaseSuit.Heartsisreferredtobyitsfullnamebecausetheconstantdoesn’thaveanexplicittypespecified.Insidetheswitch,theenumerationcaseisreferredtobytheabbreviatedform.Heartsbecausethevalueofselfisalreadyknowntobeasuit.Youcanusetheabbreviatedformanytimethevalue’stypeisalreadyknown.

Usestructtocreateastructure.Structuressupportmanyofthesamebehaviorsasclasses,includingmethodsandinitializers.Oneofthemostimportantdifferencesbetweenstructuresandclassesisthatstructuresarealwayscopiedwhentheyarepassedaroundinyourcode,butclassesarepassedbyreference.







1 structCard{

2 varrank:Rank

3 varsuit:Suit


5 return"The\(rank.simpleDescription())of\(suit.simpleDescription())"

6 }

7 }

8 letthreeOfSpades=Card(rank:.Three,suit:.Spades)

9 letthreeOfSpadesDescription=threeOfSpades.simpleDescription()

EXPERIMEN T

AddamethodtoCardthatcreatesafulldeckofcards,withonecardofeachcombinationofrankandsuit.

Aninstanceofanenumerationcasecanhavevaluesassociatedwiththeinstance.Instancesofthesameenumerationcasecanhavedifferentvaluesassociatedwiththem.Youprovidetheassociatedvalueswhenyoucreatetheinstance.Associatedvaluesandrawvaluesaredifferent:Therawvalueofanenumerationcaseisthesameforallofitsinstances,andyouprovidetherawvaluewhenyoudefinetheenumeration.

Forexample,considerthecaseofrequestingthesunriseandsunsettimefromaserver.Theservereitherrespondswiththeinformationoritrespondswithsomeerrorinformation.

1 enumServerResponse{

2 caseResult(String,String)

3 caseFailure(String)

4 }

5

6 letsuccess=ServerResponse.Result("6:00am","8:09pm")

7 letfailure=ServerResponse.Failure("Outofcheese.")

8

9 switchsuccess{

caselet.Result(sunrise,sunset):

print("Sunriseisat\(sunrise)andsunsetisat\(sunset).")

caselet.Failure(message):

print("Failure...\(message)")

}

EXPERIMEN T

AddathirdcasetoServerResponseandtotheswitch.

NoticehowthesunriseandsunsettimesareextractedfromtheServerResponsevalueaspartofmatchingthevalueagainsttheswitchcases.

ProtocolsandExtensions

Useprotocoltodeclareaprotocol.

1 protocolExampleProtocol{

2 varsimpleDescription:String{get}

3 mutatingfuncadjust()

4 }

Classes,enumerations,andstructscanalladoptprotocols.

1 classSimpleClass:ExampleProtocol{

2 varsimpleDescription:String="Averysimpleclass."

3 varanotherProperty:Int=69105

4 funcadjust(){

5 simpleDescription+="Now100%adjusted."

6 }

7 }

8 vara=SimpleClass()

9 a.adjust()

letaDescription=a.simpleDescription

structSimpleStructure:ExampleProtocol{

varsimpleDescription:String="Asimplestructure"

mutatingfuncadjust(){

simpleDescription+="(adjusted)"

}

}

varb=SimpleStructure()

b.adjust()

letbDescription=b.simpleDescription

EXPERIMEN T

Writeanenumerationthatconformstothisprotocol.

NoticetheuseofthemutatingkeywordinthedeclarationofSimpleStructuretomarkamethodthatmodifiesthestructure.ThedeclarationofSimpleClassdoesn’tneedanyofitsmethodsmarkedasmutatingbecausemethodsonaclasscanalwaysmodifytheclass.

Useextensiontoaddfunctionalitytoanexistingtype,suchasnewmethodsandcomputedproperties.Youcanuseanextensiontoaddprotocolconformancetoatypethatisdeclaredelsewhere,oreventoatypethatyouimportedfromalibraryorframework.

1 extensionInt:ExampleProtocol{

2 varsimpleDescription:String{

3 return"Thenumber\(self)"

4 }

5 mutatingfuncadjust(){

6 self+=42

7 }

8 }

9 print(7.simpleDescription)

EXPERIMEN T

WriteanextensionfortheDoubletypethataddsanabsoluteValueproperty.

Youcanuseaprotocolnamejustlikeanyothernamedtype—forexample,tocreateacollectionofobjectsthathavedifferenttypesbutthatallconformtoasingleprotocol.Whenyouworkwithvalueswhosetypeisaprotocoltype,methodsoutsidetheprotocoldefinitionarenotavailable.

1 letprotocolValue:ExampleProtocol=a

2 print(protocolValue.simpleDescription)

3 //print(protocolValue.anotherProperty)//Uncommenttoseetheerror

EventhoughthevariableprotocolValuehasaruntimetypeofSimpleClass,thecompilertreatsitasthegiventypeofExampleProtocol.Thismeansthatyoucan’taccidentallyaccessmethodsorpropertiesthattheclassimplementsinadditiontoitsprotocolconformance.

ErrorHandling

YourepresenterrorsusinganytypethatadoptstheErrorTypeprotocol.

1 enumPrinterError:ErrorType{

2 caseOutOfPaper

3 caseNoToner

4 caseOnFire

5 }

Usethrowtothrowanerrorandthrowstomarkafunctionthatcanthrowanerror.Ifyouthrowanerrorinafunction,thefunctionreturnsimmediatelyandthecodethatcalledthefunctionhandlestheerror.

1 funcsendToPrinter(printerName:String)throws->String{

2 ifprinterName=="NeverHasToner"{

3 throwPrinterError.NoToner

4 }

5 return"Jobsent"

6 }

Thereareseveralwaystohandleerrors.Onewayistousedo-catch.Insidethedoblock,youmarkcodethatcanthrowanerrorbywritingtryinfrontofit.Insidethecatchblock,theerrorisautomaticallygiventhenameerrorunlessyoucangiveitadifferentname.

1 do{

2 letprinterResponse=trysendToPrinter("BiSheng")

3 print(printerResponse)

4 }catch{

5 print(error)

6 }

EXPERIMEN T

Changetheprinternameto"NeverHasToner",sothatthesendToPrinter(_:)functionthrowsanerror.

Youcanprovidemultiplecatchblocksthathandlespecificerrors.Youwriteapatternaftercatchjustasyoudoaftercaseinaswitch.

1 do{

2 letprinterResponse=trysendToPrinter("Gutenberg")

3 print(printerResponse)

4 }catchPrinterError.OnFire{

5 print("I'lljustputthisoverhere,withtherestofthefire.")

6 }catchletprinterErrorasPrinterError{

7 print("Printererror:\(printerError).")

8 }catch{

9 print(error)

}

EXPERIMEN T

Addcodetothrowanerrorinsidethedoblock.Whatkindoferrordoyouneedtothrowsothattheerrorishandledbythefirstcatchblock?Whataboutthesecondandthirdblocks?

Anotherwaytohandleerrorsistousetry?toconverttheresulttoanoptional.Ifthefunctionthrows

anerror,thespecificerrorisdiscardedandtheresultisnil.Otherwise,theresultisanoptionalcontainingthevaluethatthefunctionreturned.

1 letprinterSuccess=try?sendToPrinter("Mergenthaler")

2 letprinterFailure=try?sendToPrinter("NeverHasToner")

Usedefertowriteablockofcodethatisexecutedafterallothercodeinthefunction,justbeforethefunctionreturns.Thecodeisexecutedregardlessofwhetherthefunctionthrowsanerror.Youcanusedefertowritesetupandcleanupcodenexttoeachother,eventhoughtheyneedtobeexecutedatdifferenttimes.

1 varfridgeIsOpen=false

2 letfridgeContent=["milk","eggs","leftovers"]

3

4 funcfridgeContains(itemName:String)->Bool{

5 fridgeIsOpen=true

6 defer{

7 fridgeIsOpen=false

8 }

9

letresult=fridgeContent.contains(itemName)

returnresult

}

fridgeContains("banana")

print(fridgeIsOpen)

Generics

Writeanameinsideanglebracketstomakeagenericfunctionortype.

1 funcrepeatItem(item:Item,numberOfTimes:Int)->[Item]{

2 varresult=[Item]()

3 for_in0..

1 funcanyCommonElements(lhs:T,_rhs:U)-

>Bool{

2 forlhsIteminlhs{

3 forrhsIteminrhs{

4 iflhsItem==rhsItem{

5 returntrue

6 }

7 }

8 }

9 returnfalse

}

anyCommonElements([1,2,3],[3])

EXPERIMEN T

ModifytheanyCommonElements(_:_:)functiontomakeafunctionthatreturnsanarrayoftheelementsthatanytwosequenceshaveincommon.

Writingisthesameaswriting.

LanguageGuide

TheBasics

SwiftisanewprogramminglanguageforiOS,OSX,watchOS,andtvOSappdevelopment.Nonetheless,manypartsofSwiftwillbefamiliarfromyourexperienceofdevelopinginCandObjective-C.

SwiftprovidesitsownversionsofallfundamentalCandObjective-Ctypes,includingIntforintegers,DoubleandFloatforfloating-pointvalues,BoolforBooleanvalues,andStringfortextualdata.Swiftalsoprovidespowerfulversionsofthethreeprimarycollectiontypes,Array,Set,andDictionary,asdescribedinCollectionTypes.

LikeC,Swiftusesvariablestostoreandrefertovaluesbyanidentifyingname.Swiftalsomakesextensiveuseofvariableswhosevaluescannotbechanged.Theseareknownasconstants,andaremuchmorepowerfulthanconstantsinC.ConstantsareusedthroughoutSwifttomakecodesaferandclearerinintentwhenyouworkwithvaluesthatdonotneedtochange.

Inadditiontofamiliartypes,SwiftintroducesadvancedtypesnotfoundinObjective-C,suchastuples.Tuplesenableyoutocreateandpassaroundgroupingsofvalues.Youcanuseatupletoreturnmultiplevaluesfromafunctionasasinglecompoundvalue.

Swiftalsointroducesoptionaltypes,whichhandletheabsenceofavalue.Optionalssayeither“thereisavalue,anditequalsx”or“thereisn’tavalueatall”.UsingoptionalsissimilartousingnilwithpointersinObjective-C,buttheyworkforanytype,notjustclasses.NotonlyareoptionalssaferandmoreexpressivethannilpointersinObjective-C,theyareattheheartofmanyofSwift’smostpowerfulfeatures.

Swiftisatype-safelanguage,whichmeansthelanguagehelpsyoutobeclearaboutthetypesofvaluesyourcodecanworkwith.IfpartofyourcodeexpectsaString,typesafetypreventsyoufrompassingitanIntbymistake.Likewise,typesafetypreventsyoufromaccidentallypassinganoptionalStringtoapieceofcodethatexpectsanonoptionalString.Typesafetyhelpsyoucatchandfixerrorsasearlyaspossibleinthedevelopmentprocess.

ConstantsandVariables

Constantsandvariablesassociateaname(suchasmaximumNumberOfLoginAttemptsorwelcomeMessage)withavalueofaparticulartype(suchasthenumber10orthestring"Hello").Thevalueofaconstantcannotbechangedonceitisset,whereasavariablecanbesettoadifferentvalueinthefuture.

DeclaringConstantsandVariables

Constantsandvariablesmustbedeclaredbeforetheyareused.Youdeclareconstantswiththeletkeywordandvariableswiththevarkeyword.Here’sanexampleofhowconstantsandvariablescanbeusedtotrackthenumberofloginattemptsauserhasmade:

1 letmaximumNumberOfLoginAttempts=10

2 varcurrentLoginAttempt=0

Thiscodecanbereadas:

“DeclareanewconstantcalledmaximumNumberOfLoginAttempts,andgiveitavalueof10.Then,declareanewvariablecalledcurrentLoginAttempt,andgiveitaninitialvalueof0.”

Inthisexample,themaximumnumberofallowedloginattemptsisdeclaredasaconstant,becausethemaximumvalueneverchanges.Thecurrentloginattemptcounterisdeclaredasavariable,becausethisvaluemustbeincrementedaftereachfailedloginattempt.

Youcandeclaremultipleconstantsormultiplevariablesonasingleline,separatedbycommas:

varx=0.0,y=0.0,z=0.0

NOTE

Ifastoredvalueinyourcodeisnotgoingtochange,alwaysdeclareitasaconstantwiththeletkeyword.Usevariablesonlyforstoringvaluesthatneedtobeabletochange.

TypeAnnotations

Youcanprovideatypeannotationwhenyoudeclareaconstantorvariable,tobeclearaboutthekindofvaluestheconstantorvariablecanstore.Writeatypeannotationbyplacingacolonaftertheconstantorvariablename,followedbyaspace,followedbythenameofthetypetouse.

ThisexampleprovidesatypeannotationforavariablecalledwelcomeMessage,toindicatethatthevariablecanstoreStringvalues:

varwelcomeMessage:String

Thecoloninthedeclarationmeans“…oftype…,”sothecodeabovecanbereadas:

“DeclareavariablecalledwelcomeMessagethatisoftypeString.”

Thephrase“oftypeString”means“canstoreanyStringvalue.”Thinkofitasmeaning“thetypeofthing”(or“thekindofthing”)thatcanbestored.

ThewelcomeMessagevariablecannowbesettoanystringvaluewithouterror:

welcomeMessage="Hello"

Youcandefinemultiplerelatedvariablesofthesametypeonasingleline,separatedbycommas,withasingletypeannotationafterthefinalvariablename:

varred,green,blue:Double

NOTE

Itisrarethatyouneedtowritetypeannotationsinpractice.Ifyouprovideaninitialvalueforaconstantorvariableatthepointthatitisdefined,Swiftcanalmostalwaysinferthetypetobeusedforthatconstantorvariable,asdescribedinTypeSafetyandTypeInference.InthewelcomeMessageexampleabove,noinitialvalueisprovided,andsothetypeofthewelcomeMessagevariableisspecifiedwithatypeannotationratherthanbeinginferredfromaninitialvalue.

NamingConstantsandVariables

Constantandvariablenamescancontainalmostanycharacter,includingUnicodecharacters:

1 letπ=3.14159

2 let你好="你好世界"

3 let ="dogcow"

Constantandvariablenamescannotcontainwhitespacecharacters,mathematicalsymbols,arrows,private-use(orinvalid)Unicodecodepoints,orline-andbox-drawingcharacters.Norcantheybeginwithanumber,althoughnumbersmaybeincludedelsewherewithinthename.

Onceyou’vedeclaredaconstantorvariableofacertaintype,youcan’tredeclareitagainwiththesamename,orchangeittostorevaluesofadifferenttype.Norcanyouchangeaconstantintoavariableoravariableintoaconstant.

NOTE

IfyouneedtogiveaconstantorvariablethesamenameasareservedSwiftkeyword,surroundthekeywordwithbackticks(`)whenusingitasaname.However,avoidusingkeywordsasnamesunlessyouhaveabsolutelynochoice.

Youcanchangethevalueofanexistingvariabletoanothervalueofacompatibletype.Inthis

example,thevalueoffriendlyWelcomeischangedfrom"Hello!"to"Bonjour!":

1 varfriendlyWelcome="Hello!"

2 friendlyWelcome="Bonjour!"

3 //friendlyWelcomeisnow"Bonjour!"

Unlikeavariable,thevalueofaconstantcannotbechangedonceitisset.Attemptingtodosoisreportedasanerrorwhenyourcodeiscompiled:

1 letlanguageName="Swift"

2 languageName="Swift++"

3 //thisisacompile-timeerror-languageNamecannotbechanged

PrintingConstantsandVariables

Youcanprintthecurrentvalueofaconstantorvariablewiththeprint(_:separator:terminator:)function:

1 print(friendlyWelcome)

2 //Prints"Bonjour!"

Theprint(_:separator:terminator:)functionisaglobalfunctionthatprintsoneormorevaluestoanappropriateoutput.InXcode,forexample,theprint(_:separator:terminator:)functionprintsitsoutputinXcode’s“console”pane.Theseparatorandterminatorparameterhavedefaultvalues,soyoucanomitthemwhenyoucallthisfunction.Bydefault,thefunctionterminatesthelineitprintsbyaddingalinebreak.Toprintavaluewithoutalinebreakafterit,passanemptystringastheterminator—forexample,print(someValue,terminator:"").Forinformationaboutparameterswithdefaultvalues,seeDefaultParameterValues.

Swiftusesstringinterpolationtoincludethenameofaconstantorvariableasaplaceholderinalongerstring,andtopromptSwifttoreplaceitwiththecurrentvalueofthatconstantorvariable.Wrapthenameinparenthesesandescapeitwithabackslashbeforetheopeningparenthesis:

1 print("ThecurrentvalueoffriendlyWelcomeis\(friendlyWelcome)")

2 //Prints"ThecurrentvalueoffriendlyWelcomeisBonjour!"

NOTE

AlloptionsyoucanusewithstringinterpolationaredescribedinStringInterpolation.

Comments

Usecommentstoincludenonexecutabletextinyourcode,asanoteorremindertoyourself.CommentsareignoredbytheSwiftcompilerwhenyourcodeiscompiled.

CommentsinSwiftareverysimilartocommentsinC.Single-linecommentsbeginwithtwoforward-slashes(//):

//thisisacomment

Multilinecommentsstartwithaforward-slashfollowedbyanasterisk(/*)andendwithanasteriskfollowedbyaforward-slash(*/):

1 /*thisisalsoacomment,

2 butwrittenovermultiplelines*/

UnlikemultilinecommentsinC,multilinecommentsinSwiftcanbenestedinsideothermultilinecomments.Youwritenestedcommentsbystartingamultilinecommentblockandthenstartingasecondmultilinecommentwithinthefirstblock.Thesecondblockisthenclosed,followedbythefirstblock:

1 /*thisisthestartofthefirstmultilinecomment

2 /*thisisthesecond,nestedmultilinecomment*/

3 thisistheendofthefirstmultilinecomment*/

Nestedmultilinecommentsenableyoutocommentoutlargeblocksofcodequicklyandeasily,evenifthecodealreadycontainsmultilinecomments.

Semicolons

Unlikemanyotherlanguages,Swiftdoesnotrequireyoutowriteasemicolon(;)aftereachstatementinyourcode,althoughyoucandosoifyouwish.However,semicolonsarerequiredifyouwanttowritemultipleseparatestatementsonasingleline:

1 letcat=" ";print(cat)

2 //Prints" "

Integers

Integersarewholenumberswithnofractionalcomponent,suchas42and-23.Integersareeithersigned(positive,zero,ornegative)orunsigned(positiveorzero).

Swiftprovidessignedandunsignedintegersin8,16,32,and64bitforms.TheseintegersfollowanamingconventionsimilartoC,inthatan8-bitunsignedintegerisoftypeUInt8,anda32-bitsignedintegerisoftypeInt32.LikealltypesinSwift,theseintegertypeshavecapitalizednames.

IntegerBounds

Youcanaccesstheminimumandmaximumvaluesofeachintegertypewithitsminandmaxproperties:

1 letminValue=UInt8.min//minValueisequalto0,andisoftypeUInt8

2 letmaxValue=UInt8.max//maxValueisequalto255,andisoftypeUInt8

Thevaluesofthesepropertiesareoftheappropriate-sizednumbertype(suchasUInt8intheexampleabove)andcanthereforebeusedinexpressionsalongsideothervaluesofthesametype.

Int

Inmostcases,youdon’tneedtopickaspecificsizeofintegertouseinyourcode.Swiftprovidesanadditionalintegertype,Int,whichhasthesamesizeasthecurrentplatform’snativewordsize:

Unlessyouneedtoworkwithaspecificsizeofinteger,alwaysuseIntforintegervaluesinyourcode.Thisaidscodeconsistencyandinteroperability.Evenon32-bitplatforms,Intcanstoreanyvaluebetween-2,147,483,648and2,147,483,647,andislargeenoughformanyintegerranges.

Ona32-bitplatform,IntisthesamesizeasInt32.•Ona64-bitplatform,IntisthesamesizeasInt64.•

UInt

Swiftalsoprovidesanunsignedintegertype,UInt,whichhasthesamesizeasthecurrentplatform’snativewordsize:

NOTE

UseUIntonlywhenyouspecificallyneedanunsignedintegertypewiththesamesizeastheplatform’snativewordsize.Ifthisisnotthecase,Intispreferred,evenwhenthevaluestobestoredareknowntobenon-negative.AconsistentuseofIntforintegervaluesaidscodeinteroperability,avoidstheneedtoconvertbetweendifferentnumbertypes,andmatchesintegertypeinference,asdescribedinTypeSafetyandTypeInference.

Floating-PointNumbers

Floating-pointnumbersarenumberswithafractionalcomponent,suchas3.14159,0.1,and-273.15.

Floating-pointtypescanrepresentamuchwiderrangeofvaluesthanintegertypes,andcanstorenumbersthataremuchlargerorsmallerthancanbestoredinanInt.Swiftprovidestwosignedfloating-pointnumbertypes:

NOTE

Doublehasaprecisionofatleast15decimaldigits,whereastheprecisionofFloatcanbeaslittleas6decimaldigits.Theappropriatefloating-pointtypetousedependsonthenatureandrangeofvaluesyouneedtoworkwithinyourcode.Insituationswhereeithertypewouldbeappropriate,Doubleispreferred.

TypeSafetyandTypeInference

Swiftisatype-safelanguage.Atypesafelanguageencouragesyoutobeclearaboutthetypesofvaluesyourcodecanworkwith.IfpartofyourcodeexpectsaString,youcan’tpassitanIntbymistake.

Ona32-bitplatform,UIntisthesamesizeasUInt32.•Ona64-bitplatform,UIntisthesamesizeasUInt64.•

Doublerepresentsa64-bitfloating-pointnumber.•Floatrepresentsa32-bitfloating-pointnumber.•

BecauseSwiftistypesafe,itperformstypecheckswhencompilingyourcodeandflagsanymismatchedtypesaserrors.Thisenablesyoutocatchandfixerrorsasearlyaspossibleinthedevelopmentprocess.

Type-checkinghelpsyouavoiderrorswhenyou’reworkingwithdifferenttypesofvalues.However,thisdoesn’tmeanthatyouhavetospecifythetypeofeveryconstantandvariablethatyoudeclare.Ifyoudon’tspecifythetypeofvalueyouneed,Swiftusestypeinferencetoworkouttheappropriatetype.Typeinferenceenablesacompilertodeducethetypeofaparticularexpressionautomaticallywhenitcompilesyourcode,simplybyexaminingthevaluesyouprovide.

Becauseoftypeinference,SwiftrequiresfarfewertypedeclarationsthanlanguagessuchasCorObjective-C.Constantsandvariablesarestillexplicitlytyped,butmuchoftheworkofspecifyingtheirtypeisdoneforyou.

Typeinferenceisparticularlyusefulwhenyoudeclareaconstantorvariablewithaninitialvalue.Thisisoftendonebyassigningaliteralvalue(orliteral)totheconstantorvariableatthepointthatyoudeclareit.(Aliteralvalueisavaluethatappearsdirectlyinyoursourcecode,suchas42and3.14159intheexamplesbelow.)

Forexample,ifyouassignaliteralvalueof42toanewconstantwithoutsayingwhattypeitis,SwiftinfersthatyouwanttheconstanttobeanInt,becauseyouhaveinitializeditwithanumberthatlookslikeaninteger:

1 letmeaningOfLife=42

2 //meaningOfLifeisinferredtobeoftypeInt

Likewise,ifyoudon’tspecifyatypeforafloating-pointliteral,SwiftinfersthatyouwanttocreateaDouble:

1 letpi=3.14159

2 //piisinferredtobeoftypeDouble

SwiftalwayschoosesDouble(ratherthanFloat)wheninferringthetypeoffloating-pointnumbers.

Ifyoucombineintegerandfloating-pointliteralsinanexpression,atypeofDoublewillbeinferredfromthecontext:

1 letanotherPi=3+0.14159

2 //anotherPiisalsoinferredtobeoftypeDouble

Theliteralvalueof3hasnoexplicittypeinandofitself,andsoanappropriateoutputtypeofDoubleisinferredfromthepresenceofafloating-pointliteralaspartoftheaddition.

NumericLiterals

Integerliteralscanbewrittenas:

Alloftheseintegerliteralshaveadecimalvalueof17:

1 letdecimalInteger=17

2 letbinaryInteger=0b10001//17inbinarynotation

3 letoctalInteger=0o21//17inoctalnotation

4 lethexadecimalInteger=0x11//17inhexadecimalnotation

Floating-pointliteralscanbedecimal(withnoprefix),orhexadecimal(witha0xprefix).Theymustalwayshaveanumber(orhexadecimalnumber)onbothsidesofthedecimalpoint.Decimalfloatscanalsohaveanoptionalexponent,indicatedbyanuppercaseorlowercasee;hexadecimalfloatsmusthaveanexponent,indicatedbyanuppercaseorlowercasep.

Fordecimalnumberswithanexponentofexp,thebasenumberismultipliedby10exp:

Forhexadecimalnumberswithanexponentofexp,thebasenumberismultipliedby2exp:

Allofthesefloating-pointliteralshaveadecimalvalueof12.1875:

Adecimalnumber,withnoprefix•Abinarynumber,witha0bprefix•Anoctalnumber,witha0oprefix•Ahexadecimalnumber,witha0xprefix•

1.25e2means1.25x102,or125.0.•

1.25e-2means1.25x10-2,or0.0125.•

0xFp2means15x22,or60.0.•

0xFp-2means15x2-2,or3.75.•

1 letdecimalDouble=12.1875

2 letexponentDouble=1.21875e1

3 lethexadecimalDouble=0xC.3p0

Numericliteralscancontainextraformattingtomakethemeasiertoread.Bothintegersandfloatscanbepaddedwithextrazerosandcancontainunderscorestohelpwithreadability.Neithertypeofformattingaffectstheunderlyingvalueoftheliteral:

1 letpaddedDouble=000123.456

2 letoneMillion=1_000_000

3 letjustOverOneMillion=1_000_000.000_000_1

NumericTypeConversion

UsetheInttypeforallgeneral-purposeintegerconstantsandvariablesinyourcode,eveniftheyareknowntobenon-negative.Usingthedefaultintegertypeineverydaysituationsmeansthatintegerconstantsandvariablesareimmediatelyinteroperableinyourcodeandwillmatchtheinferredtypeforintegerliteralvalues.

Useotherintegertypesonlywhentheyarespecificallyneededforthetaskathand,becauseofexplicitly-sizeddatafromanexternalsource,orforperformance,memoryusage,orothernecessaryoptimization.Usingexplicitly-sizedtypesinthesesituationshelpstocatchanyaccidentalvalueoverflowsandimplicitlydocumentsthenatureofthedatabeingused.

IntegerConversion

Therangeofnumbersthatcanbestoredinanintegerconstantorvariableisdifferentforeachnumerictype.AnInt8constantorvariablecanstorenumbersbetween-128and127,whereasaUInt8constantorvariablecanstorenumbersbetween0and255.Anumberthatwillnotfitintoaconstantorvariableofasizedintegertypeisreportedasanerrorwhenyourcodeiscompiled:

1 letcannotBeNegative:UInt8=-1

2 //UInt8cannotstorenegativenumbers,andsothiswillreportanerror

3 lettooBig:Int8=Int8.max+1

4 //Int8cannotstoreanumberlargerthanitsmaximumvalue,

5 //andsothiswillalsoreportanerror

Becauseeachnumerictypecanstoreadifferentrangeofvalues,youmustoptintonumerictypeconversiononacase-by-casebasis.Thisopt-inapproachpreventshiddenconversionerrorsandhelpsmaketypeconversionintentionsexplicitinyourcode.

Toconvertonespecificnumbertypetoanother,youinitializeanewnumberofthedesiredtypewiththeexistingvalue.Intheexamplebelow,theconstanttwoThousandisoftypeUInt16,whereastheconstantoneisoftypeUInt8.Theycannotbeaddedtogetherdirectly,becausetheyarenotofthesametype.Instead,thisexamplecallsUInt16(one)tocreateanewUInt16initializedwiththevalueofone,andusesthisvalueinplaceoftheoriginal:

1 lettwoThousand:UInt16=2_000

2 letone:UInt8=1

3 lettwoThousandAndOne=twoThousand+UInt16(one)

BecausebothsidesoftheadditionarenowoftypeUInt16,theadditionisallowed.Theoutputconstant(twoThousandAndOne)isinferredtobeoftypeUInt16,becauseitisthesumoftwoUInt16values.

SomeType(ofInitialValue)isthedefaultwaytocalltheinitializerofaSwifttypeandpassinaninitialvalue.Behindthescenes,UInt16hasaninitializerthatacceptsaUInt8value,andsothisinitializerisusedtomakeanewUInt16fromanexistingUInt8.Youcan’tpassinanytypehere,however—ithastobeatypeforwhichUInt16providesaninitializer.Extendingexistingtypestoprovideinitializersthatacceptnewtypes(includingyourowntypedefinitions)iscoveredinExtensions.

IntegerandFloating-PointConversion

Conversionsbetweenintegerandfloating-pointnumerictypesmustbemadeexplicit:

1 letthree=3

2 letpointOneFourOneFiveNine=0.14159

3 letpi=Double(three)+pointOneFourOneFiveNine

4 //piequals3.14159,andisinferredtobeoftypeDouble

Here,thevalueoftheconstantthreeisusedtocreateanewvalueoftypeDouble,sothatbothsidesoftheadditionareofthesametype.Withoutthisconversioninplace,theadditionwouldnotbeallowed.

Floating-pointtointegerconversionmustalsobemadeexplicit.AnintegertypecanbeinitializedwithaDoubleorFloatvalue:

1 letintegerPi=Int(pi)

2 //integerPiequals3,andisinferredtobeoftypeInt

Floating-pointvaluesarealwaystruncatedwhenusedtoinitializeanewintegervalueinthisway.Thismeansthat4.75becomes4,and-3.9becomes-3.

NOTE

Therulesforcombiningnumericconstantsandvariablesaredifferentfromtherulesfornumericliterals.Theliteralvalue3canbeaddeddirectlytotheliteralvalue0.14159,becausenumberliteralsdonothaveanexplicittypeinandofthemselves.Theirtypeisinferredonlyatthepointthattheyareevaluatedbythecompiler.

TypeAliases

Typealiasesdefineanalternativenameforanexistingtype.Youdefinetypealiaseswiththetypealiaskeyword.

Typealiasesareusefulwhenyouwanttorefertoanexistingtypebyanamethatiscontextuallymoreappropriate,suchaswhenworkingwithdataofaspecificsizefromanexternalsource:

typealiasAudioSample=UInt16

Onceyoudefineatypealias,youcanusethealiasanywhereyoumightusetheoriginalname:

1 varmaxAmplitudeFound=AudioSample.min

2 //maxAmplitudeFoundisnow0

Here,AudioSampleisdefinedasanaliasforUInt16.Becauseitisanalias,thecalltoAudioSample.minactuallycallsUInt16.min,whichprovidesaninitialvalueof0forthemaxAmplitudeFoundvariable.

Booleans

SwifthasabasicBooleantype,calledBool.Booleanvaluesarereferredtoaslogical,becausetheycanonlyeverbetrueorfalse.SwiftprovidestwoBooleanconstantvalues,trueandfalse:

1 letorangesAreOrange=true

2 letturnipsAreDelicious=false

ThetypesoforangesAreOrangeandturnipsAreDelicioushavebeeninferredasBoolfromthefactthattheywereinitializedwithBooleanliteralvalues.AswithIntandDoubleabove,youdon’tneedtodeclareconstantsorvariablesasBoolifyousetthemtotrueorfalseassoonasyoucreatethem.TypeinferencehelpsmakeSwiftcodemoreconciseandreadablewhenitinitializesconstantsorvariableswithothervalueswhosetypeisalreadyknown.

Booleanvaluesareparticularlyusefulwhenyouworkwithconditionalstatementssuchastheifstatement:

1 ifturnipsAreDelicious{

2 print("Mmm,tastyturnips!")

3 }else{

4 print("Eww,turnipsarehorrible.")

5 }

6 //Prints"Eww,turnipsarehorrible."

ConditionalstatementssuchastheifstatementarecoveredinmoredetailinControlFlow.

Swift’stypesafetypreventsnon-BooleanvaluesfrombeingsubstitutedforBool.Thefollowingexamplereportsacompile-timeerror:

1 leti=1

2 ifi{

3 //thisexamplewillnotcompile,andwillreportanerror

4 }

However,thealternativeexamplebelowisvalid:

1 leti=1

2 ifi==1{

3 //thisexamplewillcompilesuccessfully

4 }

Theresultofthei==1comparisonisoftypeBool,andsothissecondexamplepassesthetype-check.Comparisonslikei==1arediscussedinBasicOperators.

AswithotherexamplesoftypesafetyinSwift,thisapproachavoidsaccidentalerrorsandensuresthattheintentionofaparticularsectionofcodeisalwaysclear.

Tuples

Tuplesgroupmultiplevaluesintoasinglecompoundvalue.Thevalueswithinatuplecanbeofanytypeanddonothavetobeofthesametypeaseachother.

Inthisexample,(404,"NotFound")isatuplethatdescribesanHTTPstatuscode.AnHTTPstatuscodeisaspecialvaluereturnedbyawebserverwheneveryourequestawebpage.Astatuscodeof404NotFoundisreturnedifyourequestawebpagethatdoesn’texist.

1 lethttp404Error=(404,"NotFound")

2 //http404Errorisoftype(Int,String),andequals(404,"NotFound")

The(404,"NotFound")tuplegroupstogetheranIntandaStringtogivetheHTTPstatuscodetwoseparatevalues:anumberandahuman-readabledescription.Itcanbedescribedas“atupleoftype(Int,String)”.

Youcancreatetuplesfromanypermutationoftypes,andtheycancontainasmanydifferenttypesasyoulike.There’snothingstoppingyoufromhavingatupleoftype(Int,Int,Int),or(String,Bool),orindeedanyotherpermutationyourequire.

Youcandecomposeatuple’scontentsintoseparateconstantsorvariables,whichyouthenaccessasusual:

1 let(statusCode,statusMessage)=http404Error

2 print("Thestatuscodeis\(statusCode)")

3 //Prints"Thestatuscodeis404"

4 print("Thestatusmessageis\(statusMessage)")

5 //Prints"ThestatusmessageisNotFound"

Ifyouonlyneedsomeofthetuple’svalues,ignorepartsofthetuplewithanunderscore(_)whenyoudecomposethetuple:

1 let(justTheStatusCode,_)=http404Error

2 print("Thestatuscodeis\(justTheStatusCode)")


Alternatively,accesstheindividualelementvaluesinatupleusingindexnumbersstartingatzero:

1 print("Thestatuscodeis\(http404Error.0)")


3 print("Thestatusmessageis\(http404Error.1)")

4 //Prints"ThestatusmessageisNotFound"

Youcannametheindividualelementsinatuplewhenthetupleisdefined:

lethttp200Status=(statusCode:200,description:"OK")

Ifyounametheelementsinatuple,youcanusetheelementnamestoaccessthevaluesofthoseelements:

1 print("Thestatuscodeis\(http200Status.statusCode)")


3 print("Thestatusmessageis\(http200Status.description)")

4 //Prints"ThestatusmessageisOK"

Tuplesareparticularlyusefulasthereturnvaluesoffunctions.Afunctionthattriestoretrieveawebpagemightreturnthe(Int,String)tupletypetodescribethesuccessorfailureofthepageretrieval.

Byreturningatuplewithtwodistinctvalues,eachofadifferenttype,thefunctionprovidesmoreusefulinformationaboutitsoutcomethanifitcouldonlyreturnasinglevalueofasingletype.Formoreinformation,seeFunctionswithMultipleReturnValues.

NOTE

Tuplesareusefulfortemporarygroupsofrelatedvalues.Theyarenotsuitedtothecreationofcomplexdatastructures.Ifyourdatastructureislikelytopersistbeyondatemporaryscope,modelitasaclassorstructure,ratherthanasatuple.Formoreinformation,seeClassesandStructures.

Optionals

Youuseoptionalsinsituationswhereavaluemaybeabsent.Anoptionalsays:

or

NOTE

Theconceptofoptionalsdoesn’texistinCorObjective-C.ThenearestthinginObjective-Cistheabilitytoreturnnilfromamethodthatwouldotherwisereturnanobject,withnilmeaning“theabsenceofavalidobject.”However,thisonlyworksforobjects—itdoesn’tworkforstructures,basicCtypes,orenumerationvalues.Forthesetypes,Objective-Cmethodstypicallyreturnaspecialvalue(suchasNSNotFound)toindicatetheabsenceofavalue.Thisapproachassumesthatthemethod’scallerknowsthereisaspecialvaluetotestagainstandrememberstocheckforit.Swift’soptionalsletyouindicatetheabsenceofavalueforanytypeatall,withouttheneedforspecialconstants.

Here’sanexampleofhowoptionalscanbeusedtocopewiththeabsenceofavalue.Swift’sInttypehasaninitializerwhichtriestoconvertaStringvalueintoanIntvalue.However,noteverystringcanbeconvertedintoaninteger.Thestring"123"canbeconvertedintothenumericvalue123,butthestring"hello,world"doesnothaveanobviousnumericvaluetoconvertto.

TheexamplebelowusestheinitializertotrytoconvertaStringintoanInt:

1 letpossibleNumber="123"

2 letconvertedNumber=Int(possibleNumber)

3 //convertedNumberisinferredtobeoftype"Int?",or"optionalInt"

Thereisavalue,anditequalsx•

Thereisn’tavalueatall•

Becausetheinitializermightfail,itreturnsanoptionalInt,ratherthananInt.AnoptionalIntiswrittenasInt?,notInt.Thequestionmarkindicatesthatthevalueitcontainsisoptional,meaningthatitmightcontainsomeIntvalue,oritmightcontainnovalueatall.(Itcan’tcontainanythingelse,suchasaBoolvalueoraStringvalue.It’seitheranInt,orit’snothingatall.)

nil

Yousetanoptionalvariabletoavaluelessstatebyassigningitthespecialvaluenil:

1 varserverResponseCode:Int?=404

2 //serverResponseCodecontainsanactualIntvalueof404

3 serverResponseCode=nil

4 //serverResponseCodenowcontainsnovalue

NOTE

nilcannotbeusedwithnonoptionalconstantsandvariables.Ifaconstantorvariableinyourcodeneedstoworkwiththeabsenceofavalueundercertainconditions,alwaysdeclareitasanoptionalvalueoftheappropriatetype.

Ifyoudefineanoptionalvariablewithoutprovidingadefaultvalue,thevariableisautomaticallysettonilforyou:

1 varsurveyAnswer:String?

2 //surveyAnswerisautomaticallysettonil

NOTE

Swift’snilisnotthesameasnilinObjective-C.InObjective-C,nilisapointertoanonexistentobject.InSwift,nilisnotapointer—itistheabsenceofavalueofacertaintype.Optionalsofanytypecanbesettonil,notjustobjecttypes.

IfStatementsandForcedUnwrapping

Youcanuseanifstatementtofindoutwhetheranoptionalcontainsavaluebycomparingtheoptionalagainstnil.Youperformthiscomparisonwiththe“equalto”operator(==)orthe“notequalto”operator(!=).

Ifanoptionalhasavalue,itisconsideredtobe“notequalto”nil:

1 ifconvertedNumber!=nil{

2 print("convertedNumbercontainssomeintegervalue.")

3 }

4 //Prints"convertedNumbercontainssomeintegervalue."

Onceyou’resurethattheoptionaldoescontainavalue,youcanaccessitsunderlyingvaluebyaddinganexclamationmark(!)totheendoftheoptional’sname.Theexclamationmarkeffectivelysays,“Iknowthatthisoptionaldefinitelyhasavalue;pleaseuseit.”Thisisknownasforcedunwrappingoftheoptional’svalue:

1 ifconvertedNumber!=nil{

2 print("convertedNumberhasanintegervalueof\(convertedNumber!).")

3 }

4 //Prints"convertedNumberhasanintegervalueof123."

Formoreontheifstatement,seeControlFlow.

NOTE

Tryingtouse!toaccessanonexistentoptionalvaluetriggersaruntimeerror.Alwaysmakesurethatanoptionalcontainsanon-nilvaluebeforeusing!toforce-unwrapitsvalue.

OptionalBinding

Youuseoptionalbindingtofindoutwhetheranoptionalcontainsavalue,andifso,tomakethatvalueavailableasatemporaryconstantorvariable.Optionalbindingcanbeusedwithifandwhilestatementstocheckforavalueinsideanoptional,andtoextractthatvalueintoaconstantorvariable,aspartofasingleaction.ifandwhilestatementsaredescribedinmoredetailinControlFlow.

Writeanoptionalbindingforanifstatementasfollows:

iflet constantName = someOptional {

statements

}

YoucanrewritethepossibleNumberexamplefromtheOptionalssectiontouseoptionalbindingratherthanforcedunwrapping:

1 ifletactualNumber=Int(possibleNumber){

2 print("\"\(possibleNumber)\"hasanintegervalueof\(actualNumber)")

3 }else{

4 print("\"\(possibleNumber)\"couldnotbeconvertedtoaninteger")

5 }

6 //Prints""123"hasanintegervalueof123"

Thiscodecanbereadas:

“IftheoptionalIntreturnedbyInt(possibleNumber)containsavalue,setanewconstantcalledactualNumbertothevaluecontainedintheoptional.”

Iftheconversionissuccessful,theactualNumberconstantbecomesavailableforusewithinthefirstbranchoftheifstatement.Ithasalreadybeeninitializedwiththevaluecontainedwithintheoptional,andsothereisnoneedtousethe!suffixtoaccessitsvalue.Inthisexample,actualNumberissimplyusedtoprinttheresultoftheconversion.

Youcanusebothconstantsandvariableswithoptionalbinding.IfyouwantedtomanipulatethevalueofactualNumberwithinthefirstbranchoftheifstatement,youcouldwriteifvaractualNumberinstead,andthevaluecontainedwithintheoptionalwouldbemadeavailableasavariableratherthanaconstant.

YoucanincludemultipleoptionalbindingsinasingleifstatementanduseawhereclausetocheckforaBooleancondition.Ifanyofthevaluesintheoptionalbindingsarenilorthewhereclauseevaluatestofalse,thewholeoptionalbindingisconsideredunsuccessful.

1 ifletfirstNumber=Int("4"),secondNumber=Int("42")wherefirstNumber<

secondNumber{

2 print("\(firstNumber)<\(secondNumber)")

3 }

4 //Prints"4<42"

NOTE

Constantsandvariablescreatedwithoptionalbindinginanifstatement.areavailableonlywithinthebodyoftheifstatement.Incontrast,theconstantsandvariablescreatedwithaguardstatementareavailableinthelinesofcodethatfollowtheguardstatement,asdescribedinEarlyExit,

ImplicitlyUnwrappedOptionals

Asdescribedabove,optionalsindicatethataconstantorvariableisallowedtohave“novalue”.Optionalscanbecheckedwithanifstatementtoseeifavalueexists,andcanbeconditionallyunwrappedwithoptionalbindingtoaccesstheoptional’svalueifitdoesexist.

Sometimesitisclearfromaprogram’sstructurethatanoptionalwillalwayshaveavalue,afterthatvalueisfirstset.Inthesecases,itisusefultoremovetheneedtocheckandunwraptheoptional’svalueeverytimeitisaccessed,becauseitcanbesafelyassumedtohaveavalueallofthetime.

Thesekindsofoptionalsaredefinedasimplicitlyunwrappedoptionals.Youwriteanimplicitlyunwrappedoptionalbyplacinganexclamationmark(String!)ratherthanaquestionmark(String?)afterthetypethatyouwanttomakeoptional.

Implicitlyunwrappedoptionalsareusefulwhenanoptional’svalueisconfirmedtoexistimmediatelyaftertheoptionalisfirstdefinedandcandefinitelybeassumedtoexistateverypointthereafter.TheprimaryuseofimplicitlyunwrappedoptionalsinSwiftisduringclassinitialization,asdescribedinUnownedReferencesandImplicitlyUnwrappedOptionalProperties.

Animplicitlyunwrappedoptionalisanormaloptionalbehindthescenes,butcanalsobeusedlikeanonoptionalvalue,withouttheneedtounwraptheoptionalvalueeachtimeitisaccessed.ThefollowingexampleshowsthedifferenceinbehaviorbetweenanoptionalstringandanimplicitlyunwrappedoptionalstringwhenaccessingtheirwrappedvalueasanexplicitString:

1 letpossibleString:String?="Anoptionalstring."

2 letforcedString:String=possibleString!//requiresanexclamationmark

3

4 letassumedString:String!="Animplicitlyunwrappedoptionalstring."

5 letimplicitString:String=assumedString//noneedforanexclamationmark

Youcanthinkofanimplicitlyunwrappedoptionalasgivingpermissionfortheoptionaltobeunwrappedautomaticallywheneveritisused.Ratherthanplacinganexclamationmarkaftertheoptional’snameeachtimeyouuseit,youplaceanexclamationmarkaftertheoptional’stypewhenyoudeclareit.

NOTE

Ifanimplicitlyunwrappedoptionalisnilandyoutrytoaccessitswrappedvalue,you’lltriggeraruntimeerror.Theresultisexactlythesameasifyouplaceanexclamationmarkafteranormaloptionalthatdoesnotcontainavalue.

Youcanstilltreatanimplicitlyunwrappedoptionallikeanormaloptional,tocheckifitcontainsavalue:

1 ifassumedString!=nil{

2 print(assumedString)

3 }

4 //Prints"Animplicitlyunwrappedoptionalstring."

Youcanalsouseanimplicitlyunwrappedoptionalwithoptionalbinding,tocheckandunwrapitsvalueinasinglestatement:

1 ifletdefiniteString=assumedString{

2 print(definiteString)

3 }

4 //Prints"Animplicitlyunwrappedoptionalstring."

NOTE

Donotuseanimplicitlyunwrappedoptionalwhenthereisapossibilityofavariablebecomingnilatalaterpoint.Alwaysuseanormaloptionaltypeifyouneedtocheckforanilvalueduringthelifetimeofavariable.

ErrorHandling

Youuseerrorhandlingtorespondtoerrorconditionsyourprogrammayencounterduringexecution.

Incontrasttooptionals,whichcanusethepresenceorabsenceofavaluetocommunicatesuccessorfailureofafunction,errorhandlingallowsyoutodeterminetheunderlyingcauseoffailure,and,ifnecessary,propagatetheerrortoanotherpartofyourprogram.

Whenafunctionencountersanerrorcondition,itthrowsanerror.Thatfunction’scallercanthen

catchtheerrorandrespondappropriately.

1 funccanThrowAnError()throws{

2 //thisfunctionmayormaynotthrowanerror

3 }

Afunctionindicatesthatitcanthrowanerrorbyincludingthethrowskeywordinitsdeclaration.Whenyoucallafunctionthatcanthrowanerror,youprependthetrykeywordtotheexpression.

Swiftautomaticallypropagateserrorsoutoftheircurrentscopeuntiltheyarehandledbyacatchclause.

1 do{

2 trycanThrowAnError()

3 //noerrorwasthrown

4 }catch{

5 //anerrorwasthrown

6 }

Adostatementcreatesanewcontainingscope,whichallowserrorstobepropagatedtooneormorecatchclauses.

Here’sanexampleofhowerrorhandlingcanbeusedtorespondtodifferenterrorconditions:

1 funcmakeASandwich()throws{

2 //...

3 }

4

5 do{

6 trymakeASandwich()

7 eatASandwich()

8 }catchError.OutOfCleanDishes{

9 washDishes()

}catchError.MissingIngredients(letingredients){

buyGroceries(ingredients)

}

Inthisexample,themakeASandwich()functionwillthrowanerrorifnocleandishesareavailableorifanyingredientsaremissing.BecausemakeASandwich()canthrowanerror,thefunctioncalliswrappedinatryexpression.Bywrappingthefunctioncallinadostatement,anyerrorsthatarethrownwillbepropagatedtotheprovidedcatchclauses.

Ifnoerroristhrown,theeatASandwich()functioniscalled.IfanerroristhrownanditmatchestheError.OutOfCleanDishescase,thenthewashDishes()functionwillbecalled.IfanerroristhrownanditmatchestheError.MissingIngredientscase,thenthebuyGroceries(_:)functioniscalledwiththeassociated[String]valuecapturedbythecatchpattern.

Throwing,catching,andpropagatingerrorsiscoveredingreaterdetailinErrorHandling.

Assertions

Insomecases,itissimplynotpossibleforyourcodetocontinueexecutionifaparticularconditionisnotsatisfied.Inthesesituations,youcantriggeranassertioninyourcodetoendcodeexecutionandtoprovideanopportunitytodebugthecauseoftheabsentorinvalidvalue.

DebuggingwithAssertions

AnassertionisaruntimecheckthataBooleanconditiondefinitelyevaluatestotrue.Literallyput,an

assertion“asserts”thataconditionistrue.Youuseanassertiontomakesurethatanessentialconditionissatisfiedbeforeexecutinganyfurthercode.Iftheconditionevaluatestotrue,codeexecutioncontinuesasusual;iftheconditionevaluatestofalse,codeexecutionends,andyourappisterminated.

Ifyourcodetriggersanassertionwhilerunninginadebugenvironment,suchaswhenyoubuildandrunanappinXcode,youcanseeexactlywheretheinvalidstateoccurredandquerythestateofyourappatthetimethattheassertionwastriggered.Anassertionalsoletsyouprovideasuitabledebugmessageastothenatureoftheassert.

YouwriteanassertionbycallingtheSwiftstandardlibraryglobalassert(_:_:file:line:)function.Youpassthisfunctionanexpressionthatevaluatestotrueorfalseandamessagethatshouldbedisplayediftheresultoftheconditionisfalse:

1 letage=-3

2 assert(age>=0,"Aperson'sagecannotbelessthanzero")

3 //thiscausestheassertiontotrigger,becauseageisnot>=0

Inthisexample,codeexecutionwillcontinueonlyifage>=0evaluatestotrue,thatis,ifthevalueofageisnon-negative.Ifthevalueofageisnegative,asinthecodeabove,thenage>=0evaluatestofalse,andtheassertionistriggered,terminatingtheapplication.

Theassertionmessagecanbeomittedifdesired,asinthefollowingexample:

assert(age>=0)

NOTE

Assertionsaredisabledwhenyourcodeiscompiledwithoptimizations,suchaswhenbuildingwithanapptarget’sdefaultReleaseconfigurationinXcode.

WhentoUseAssertions

Useanassertionwheneveraconditionhasthepotentialtobefalse,butmustdefinitelybetrueinorderforyourcodetocontinueexecution.Suitablescenariosforanassertioncheckinclude:

Anintegersubscriptindexispassedtoacustomsubscriptimplementation,butthesubscriptindexvaluecouldbetoolowortoohigh.

•

Avalueispassedtoafunction,butaninvalidvaluemeansthatthefunctioncannotfulfillitstask.

•

SeealsoSubscriptsandFunctions.

NOTE

Assertionscauseyourapptoterminateandarenotasubstitutefordesigningyourcodeinsuchawaythatinvalidconditionsareunlikelytoarise.Nonetheless,insituationswhereinvalidconditionsarepossible,anassertionisaneffectivewaytoensurethatsuchconditionsarehighlightedandnoticedduringdevelopment,beforeyourappispublished.

Anoptionalvalueiscurrentlynil,butanon-nilvalueisessentialforsubsequentcodetoexecutesuccessfully.

•

BasicOperators

Anoperatorisaspecialsymbolorphrasethatyouusetocheck,change,orcombinevalues.Forexample,theadditionoperator(+)addstwonumbers,asinleti=1+2,andthelogicalANDoperator(&&)combinestwoBooleanvalues,asinifenteredDoorCode&&passedRetinaScan.

SwiftsupportsmoststandardCoperatorsandimprovesseveralcapabilitiestoeliminatecommoncodingerrors.Theassignmentoperator(=)doesnotreturnavalue,topreventitfrombeingmistakenlyusedwhentheequaltooperator(==)isintended.Arithmeticoperators(+,-,*,/,%andsoforth)detectanddisallowvalueoverflow,toavoidunexpectedresultswhenworkingwithnumbersthatbecomelargerorsmallerthantheallowedvaluerangeofthetypethatstoresthem.YoucanoptintovalueoverflowbehaviorbyusingSwift’soverflowoperators,asdescribedinOverflowOperators.

UnlikeC,Swiftletsyouperformremainder(%)calculationsonfloating-pointnumbers.Swiftalsoprovidestworangeoperators(a..

Theassignmentoperator(a=b)initializesorupdatesthevalueofawiththevalueofb:

1 letb=10

2 vara=5

3 a=b

4 //aisnowequalto10

Iftherightsideoftheassignmentisatuplewithmultiplevalues,itselementscanbedecomposedintomultipleconstantsorvariablesatonce:

1 let(x,y)=(1,2)

2 //xisequalto1,andyisequalto2

UnliketheassignmentoperatorinCandObjective-C,theassignmentoperatorinSwiftdoesnotitselfreturnavalue.Thefollowingstatementisnotvalid:

1 ifx=y{

2 //thisisnotvalid,becausex=ydoesnotreturnavalue

3 }

Thisfeaturepreventstheassignmentoperator(=)frombeingusedbyaccidentwhentheequaltooperator(==)isactuallyintended.Bymakingifx=yinvalid,Swifthelpsyoutoavoidthesekindsoferrorsinyourcode.

ArithmeticOperators

Swiftsupportsthefourstandardarithmeticoperatorsforallnumbertypes:

Addition(+)•Subtraction(-)•Multiplication(*)•Division(/)•

1 1+2//equals3

2 5-3//equals2

3 2*3//equals6

4 10.0/2.5//equals4.0

UnlikethearithmeticoperatorsinCandObjective-C,theSwiftarithmeticoperatorsdonotallowvaluestooverflowbydefault.YoucanoptintovalueoverflowbehaviorbyusingSwift’soverflowoperators(suchasa&+b).SeeOverflowOperators.

TheadditionoperatorisalsosupportedforStringconcatenation:

"hello,"+"world"//equals"hello,world"

RemainderOperator

Theremainderoperator(a%b)worksouthowmanymultiplesofbwillfitinsideaandreturnsthevaluethatisleftover(knownastheremainder).

NOTE

Theremainderoperator(%)isalsoknownasamodulooperatorinotherlanguages.However,itsbehaviorinSwiftfornegativenumbersmeansthatitis,strictlyspeaking,aremainderratherthanamodulooperation.

Here’showtheremainderoperatorworks.Tocalculate9%4,youfirstworkouthowmany4swillfitinside9:

Youcanfittwo4sinside9,andtheremainderis1(showninorange).

InSwift,thiswouldbewrittenas:

9%4//equals1

Todeterminetheanswerfora%b,the%operatorcalculatesthefollowingequationandreturnsremainderasitsoutput:

a=(bxsomemultiplier)+remainder

wheresomemultiplieristhelargestnumberofmultiplesofbthatwillfitinsidea.

Inserting9and4intothisequationyields:

9=(4x2)+1

Thesamemethodisappliedwhencalculatingtheremainderforanegativevalueofa:

-9%4//equals-1

Inserting-9and4intotheequationyields:

-9=(4x-2)+-1

givingaremaindervalueof-1.

Thesignofbisignoredfornegativevaluesofb.Thismeansthata%banda%-balwaysgivethesameanswer.

Floating-PointRemainderCalculations

UnliketheremainderoperatorinCandObjective-C,Swift’sremainderoperatorcanalsooperateonfloating-pointnumbers:

8%2.5//equals0.5

Inthisexample,8dividedby2.5equals3,witharemainderof0.5,sotheremainderoperatorreturnsaDoublevalueof0.5.

UnaryMinusOperator

Thesignofanumericvaluecanbetoggledusingaprefixed-,knownastheunaryminusoperator:

1 letthree=3

2 letminusThree=-three//minusThreeequals-3

3 letplusThree=-minusThree//plusThreeequals3,or"minusminusthree"

Theunaryminusoperator(-)isprependeddirectlybeforethevalueitoperateson,withoutanywhitespace.

UnaryPlusOperator

Theunaryplusoperator(+)simplyreturnsthevalueitoperateson,withoutanychange:

1 letminusSix=-6

2 letalsoMinusSix=+minusSix//alsoMinusSixequals-6

Althoughtheunaryplusoperatordoesn’tactuallydoanything,youcanuseittoprovidesymmetryinyourcodeforpositivenumberswhenalsousingtheunaryminusoperatorfornegativenumbers.

CompoundAssignmentOperators

LikeC,Swiftprovidescompoundassignmentoperatorsthatcombineassignment(=)withanotheroperation.Oneexampleistheadditionassignmentoperator(+=):

1 vara=1

2 a+=2

3 //aisnowequalto3

Theexpressiona+=2isshorthandfora=a+2.Effectively,theadditionandtheassignmentarecombinedintooneoperatorthatperformsbothtasksatthesametime.

NOTE

Thecompoundassignmentoperatorsdonotreturnavalue.Forexample,youcannotwriteletb=a+=2.

AcompletelistofcompoundassignmentoperatorscanbefoundinExpressions.

ComparisonOperators

SwiftsupportsallstandardCcomparisonoperators:

NOTE

Swiftalsoprovidestwoidentityoperators(===and!==),whichyouusetotestwhethertwoobjectreferencesbothrefertothesameobjectinstance.Formoreinformation,seeClassesandStructures.

EachofthecomparisonoperatorsreturnsaBoolvaluetoindicatewhetherornotthestatementistrue:

1 1==1//truebecause1isequalto1

2 2!=1//truebecause2isnotequalto1

3 2>1//truebecause2isgreaterthan1

4 1<2//truebecause1islessthan2

5 1>=1//truebecause1isgreaterthanorequalto1

6 2b)•Lessthan(a<b)•Greaterthanorequalto(a>=b)•Lessthanorequalto(a

1 letname="world"

2 ifname=="world"{

3 print("hello,world")

4 }else{

5 print("I'msorry\(name),butIdon'trecognizeyou")

6 }

7 //prints"hello,world",becausenameisindeedequalto"world"

Formoreontheifstatement,seeControlFlow.

Youcanalsocomparetuplesthathavethesamenumberofvalues,aslongaseachofthevaluesinthetuplecanbecompared.Forexample,bothIntandStringcanbecompared,whichmeanstuplesofthetype(Int,String)canbecompared.Incontrast,Boolcan’tbecompared,whichmeanstuplesthatcontainaBooleanvaluecan’tbecompared.

Tuplesarecomparedfromlefttoright,onevalueatatime,untilthecomparisonfindstwovaluesthataren’tequal.Ifalltheelementsareequal,thenthetuplesthemselvesareequal.Forexample:

1 (1,"zebra")<(2,"apple")//truebecause1islessthan2

2 (3,"apple")<(3,"bird")//truebecause3isequalto3,and"apple"isless

than"bird"

3 (4,"dog")==(4,"dog")//truebecause4isequalto4,and"dog"isequal

to"dog"

NOTE

TheSwiftstandardlibraryincludestuplecomparisonoperatorsfortupleswithlessthansevenelements.Tocomparetupleswithsevenormoreelements,youmustimplementthecomparisonoperatorsyourself.

TernaryConditionalOperator

Theternaryconditionaloperatorisaspecialoperatorwiththreeparts,whichtakestheformquestion?answer1:answer2.Itisashortcutforevaluatingoneoftwoexpressionsbasedonwhetherquestionistrueorfalse.Ifquestionistrue,itevaluatesanswer1andreturnsitsvalue;otherwise,itevaluatesanswer2andreturnsitsvalue.

Theternaryconditionaloperatorisshorthandforthecodebelow:

1 ifquestion{

2 answer1

3 }else{

4 answer2

5 }

Here’sanexample,whichcalculatestheheightforatablerow.Therowheightshouldbe50pointstallerthanthecontentheightiftherowhasaheader,and20pointstalleriftherowdoesn’thaveaheader:

1 letcontentHeight=40

2 lethasHeader=true

3 letrowHeight=contentHeight+(hasHeader?50:20)

4 //rowHeightisequalto90

Theprecedingexampleisshorthandforthecodebelow:

1 letcontentHeight=40

2 lethasHeader=true

3 letrowHeight:Int

4 ifhasHeader{

5 rowHeight=contentHeight+50

6 }else{

7 rowHeight=contentHeight+20

8 }

9 //rowHeightisequalto90

Thefirstexample’suseoftheternaryconditionaloperatormeansthatrowHeightcanbesettothecorrectvalueonasinglelineofcode.Thisismoreconcisethanthesecondexample,andremovestheneedforrowHeighttobeavariable,becauseitsvaluedoesnotneedtobemodifiedwithinanifstatement.

Theternaryconditionaloperatorprovidesanefficientshorthandfordecidingwhichoftwoexpressionstoconsider.Usetheternaryconditionaloperatorwithcare,however.Itsconcisenesscanleadtohard-to-readcodeifoverused.Avoidcombiningmultipleinstancesoftheternaryconditionaloperatorintoonecompoundstatement.

NilCoalescingOperator

Thenilcoalescingoperator(a??b)unwrapsanoptionalaifitcontainsavalue,orreturnsadefaultvaluebifaisnil.Theexpressionaisalwaysofanoptionaltype.Theexpressionbmustmatchthetypethatisstoredinsidea.

Thenilcoalescingoperatorisshorthandforthecodebelow:

a!=nil?a!:b

Thecodeaboveusestheternaryconditionaloperatorandforcedunwrapping(a!)toaccessthevaluewrappedinsideawhenaisnotnil,andtoreturnbotherwise.Thenilcoalescingoperatorprovidesamoreelegantwaytoencapsulatethisconditionalcheckingandunwrappinginaconciseandreadableform.

NOTE

Ifthevalueofaisnon-nil,thevalueofbisnotevaluated.Thisisknownasshort-circuitevaluation.

Theexamplebelowusesthenilcoalescingoperatortochoosebetweenadefaultcolornameandanoptionaluser-definedcolorname:

1 letdefaultColorName="red"

2 varuserDefinedColorName:String?//defaultstonil

3

4 varcolorNameToUse=userDefinedColorName??defaultColorName

5 //userDefinedColorNameisnil,socolorNameToUseissettothedefaultof"red"

TheuserDefinedColorNamevariableisdefinedasanoptionalString,withadefaultvalueofnil.BecauseuserDefinedColorNameisofanoptionaltype,youcanusethenilcoalescingoperatortoconsideritsvalue.Intheexampleabove,theoperatorisusedtodetermineaninitialvalueforaStringvariablecalledcolorNameToUse.BecauseuserDefinedColorNameisnil,theexpressionuserDefinedColorName??defaultColorNamereturnsthevalueofdefaultColorName,or"red".

Ifyouassignanon-nilvaluetouserDefinedColorNameandperformthenilcoalescingoperatorcheckagain,thevaluewrappedinsideuserDefinedColorNameisusedinsteadofthedefault:

1 userDefinedColorName="green"

2 colorNameToUse=userDefinedColorName??defaultColorName

3 //userDefinedColorNameisnotnil,socolorNameToUseissetto"green"

RangeOperators

Swiftincludestworangeoperators,whichareshortcutsforexpressingarangeofvalues.

ClosedRangeOperator

Theclosedrangeoperator(a...b)definesarangethatrunsfromatob,andincludesthevaluesaandb.Thevalueofamustnotbegreaterthanb.

Theclosedrangeoperatorisusefulwheniteratingoverarangeinwhichyouwantallofthevaluestobeused,suchaswithafor-inloop:

1 forindexin1...5{

2 print("\(index)times5is\(index*5)")

3 }

4 //1times5is5

5 //2times5is10

6 //3times5is15

7 //4times5is20

8 //5times5is25

Formoreonfor-inloops,seeControlFlow.

Half-OpenRangeOperator

Thehalf-openrangeoperator(a..

saidtobehalf-openbecauseitcontainsitsfirstvalue,butnotitsfinalvalue.Aswiththeclosedrangeoperator,thevalueofamustnotbegreaterthanb.Ifthevalueofaisequaltob,thentheresultingrangewillbeempty.

Half-openrangesareparticularlyusefulwhenyouworkwithzero-basedlistssuchasarrays,whereitisusefultocountupto(butnotincluding)thelengthofthelist:

1 letnames=["Anna","Alex","Brian","Jack"]

2 letcount=names.count

3 foriin0..

on,withoutanywhitespace.Itcanbereadas“nota”,asseeninthefollowingexample:

1 letallowedEntry=false

2 if!allowedEntry{

3 print("ACCESSDENIED")

4 }

5 //Prints"ACCESSDENIED"

Thephraseif!allowedEntrycanbereadas“ifnotallowedentry.”Thesubsequentlineisonlyexecutedif“notallowedentry”istrue;thatis,ifallowedEntryisfalse.

Asinthisexample,carefulchoiceofBooleanconstantandvariablenamescanhelptokeepcodereadableandconcise,whileavoidingdoublenegativesorconfusinglogicstatements.

LogicalANDOperator

ThelogicalANDoperator(a&&b)createslogicalexpressionswherebothvaluesmustbetruefortheoverallexpressiontoalsobetrue.

Ifeithervalueisfalse,theoverallexpressionwillalsobefalse.Infact,ifthefirstvalueisfalse,thesecondvaluewon’tevenbeevaluated,becauseitcan’tpossiblymaketheoverallexpressionequatetotrue.Thisisknownasshort-circuitevaluation.

ThisexampleconsiderstwoBoolvaluesandonlyallowsaccessifbothvaluesaretrue:

1 letenteredDoorCode=true

2 letpassedRetinaScan=false

3 ifenteredDoorCode&&passedRetinaScan{

4 print("Welcome!")

5 }else{


7 }

8 //Prints"ACCESSDENIED"

LogicalOROperator

ThelogicalORoperator(a||b)isaninfixoperatormadefromtwoadjacentpipecharacters.Youuseittocreatelogicalexpressionsinwhichonlyoneofthetwovalueshastobetruefortheoverallexpressiontobetrue.

LiketheLogicalANDoperatorabove,theLogicalORoperatorusesshort-circuitevaluationtoconsideritsexpressions.IftheleftsideofaLogicalORexpressionistrue,therightsideisnotevaluated,becauseitcannotchangetheoutcomeoftheoverallexpression.

Intheexamplebelow,thefirstBoolvalue(hasDoorKey)isfalse,butthesecondvalue(knowsOverridePassword)istrue.Becauseonevalueistrue,theoverallexpressionalsoevaluatestotrue,andaccessisallowed:

1 lethasDoorKey=false

2 letknowsOverridePassword=true

3 ifhasDoorKey||knowsOverridePassword{

4 print("Welcome!")

5 }else{


7 }

8 //Prints"Welcome!"

CombiningLogicalOperators

Youcancombinemultiplelogicaloperatorstocreatelongercompoundexpressions:

1 ifenteredDoorCode&&passedRetinaScan||hasDoorKey||knowsOverridePassword{

2 print("Welcome!")

3 }else{


5 }


Thisexampleusesmultiple&&and||operatorstocreatealongercompoundexpression.However,the&&and||operatorsstilloperateononlytwovalues,sothisisactuallythreesmallerexpressionschainedtogether.Theexamplecanbereadas:

Ifwe’veenteredthecorrectdoorcodeandpassedtheretinascan,orifwehaveavaliddoorkey,orifweknowtheemergencyoverridepassword,thenallowaccess.

BasedonthevaluesofenteredDoorCode,passedRetinaScan,andhasDoorKey,thefirsttwosubexpressionsarefalse.However,theemergencyoverridepasswordisknown,sotheoverallcompoundexpressionstillevaluatestotrue.

NOTE

TheSwiftlogicaloperators&&and||areleft-associative,meaningthatcompoundexpressionswithmultiplelogicaloperatorsevaluatetheleftmostsubexpressionfirst.

ExplicitParentheses

Itissometimesusefultoincludeparentheseswhentheyarenotstrictlyneeded,tomaketheintentionofacomplexexpressioneasiertoread.Inthedooraccessexampleabove,itisusefultoaddparenthesesaroundthefirstpartofthecompoundexpressiontomakeitsintentexplicit:

1 if(enteredDoorCode&&passedRetinaScan)||hasDoorKey||knowsOverridePassword{

2 print("Welcome!")

3 }else{


5 }


Theparenthesesmakeitclearthatthefirsttwovaluesareconsideredaspartofaseparatepossiblestateintheoveralllogic.Theoutputofthecompoundexpressiondoesn’tcha

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