Map:可以對數(shù)組中的每一個(gè)元素做一次處理

• 如數(shù)組中存在nil,會(huì)判斷nil,如果存在則返回

    override func viewDidLoad() {
        super.viewDidLoad()
        //可以對數(shù)組中的每一個(gè)元素做一次處理
        let cast = ["Vivien", "Marlon", "Kim", "Karl",nil, nil, "liliguang"]
        
//      map(_ transform: (String) throws -> T)                  //映射閉包。 transform接受此序列的元素作為其參數(shù)鲁猩，并返回相同或不同類型的轉(zhuǎn)換值坎怪。
        
        let lowercaseNames = cast.map { $0.lowercased() }       //注意研究各種不同的映射閉包寫法
        print(lowercaseNames)
        let letterCounts = cast.map { $0.count }
        print(letterCounts)

        print(   stringArray.map( lowStr )   )
        print(   stringArray.map( stringCount )   )
        print("------")
        
        print(stringArray.map({ string -> Int in
            return string.count
        }))
    }
    //轉(zhuǎn)換為小寫
    func lowStr(str : String) -> String {
        return str.lowercased()
    }
    //獲取字符串長度
    func stringCount(string: String) -> Int {
        return string.count
    }
    

打印結(jié)果

[Optional("vivien"), Optional("marlon"), Optional("kim"), Optional("karl"), nil, nil, Optional("liliguang")]
[Optional(6), Optional(6), Optional(3), Optional(4), nil, nil, Optional(9)]
["aaaa", "bbbb", "cc", "ddd", "eeeee"]
[4, 4, 2, 3, 5]
------
[4, 4, 2, 3, 5]

compactMap : 以前是flatMap

• flatMap返回后的數(shù)組中不存在nil，同時(shí)它會(huì)把Optional解包

    override func viewDidLoad() {
        super.viewDidLoad()
        //可以對數(shù)組中的每一個(gè)元素做一次處理
        let cast = ["Vivien", "Marlon", "Kim", "Karl",nil, nil, "liliguang"]
        
        let lowercaseNames = cast.compactMap { $0?.lowercased() }       //理解 : 調(diào)用$0的對象方法
        print(lowercaseNames)
        let letterCounts = cast.compactMap { $0?.count }
        print(letterCounts)

        print(stringArray.compactMap( lowStr )   )
        print(stringArray.compactMap( stringCount )   )
        print(stringArray.compactMap({ string -> Int in
            return string.count
        }))
    }
    
    func lowStr(str : String) -> String {
        return str.lowercased()
    }
    
    func stringCount(string: String) -> Int {
        return string.count
    }

打印結(jié)果

["vivien", "marlon", "kim", "karl", "liliguang"]
[6, 6, 3, 4, 9]
["aaaa", "bbbb", "cc", "ddd", "eeeee"]
[4, 4, 2, 3, 5]
------
[4, 4, 2, 3, 5]

filer：過濾廓握，可以對數(shù)組中的元素按照某種規(guī)則進(jìn)行一次過濾

    override func viewDidLoad() {
        super.viewDidLoad()
        //可以對數(shù)組中的每一個(gè)元素做一次處理
        let cast = ["Vivien", "Marlon", "Kim", "Karl",nil, nil, "liliguang"]
        
        let newCase = cast.filter{ string -> Bool in
            return string == "liliguang"
        }
        print(newCase)
    }

打印結(jié)果

[Optional("liliguang")]

reduce : 計(jì)算搅窿，可以對數(shù)組的元素進(jìn)行計(jì)算

    override func viewDidLoad() {
        super.viewDidLoad()
        //可以對數(shù)組中的每一個(gè)元素做一次處理
        let stringArray = ["Objective-C", "Swift", "HTML", "CSS", "JavaScript"]
        // reduce方法中的第一個(gè)參數(shù)是初始值
        print(stringArray.reduce("", appendString))
        print(stringArray.reduce("", {(string1, string2) -> String in
            return string1 == "" ? string2 : string1 + "、" + string2
        }) )
        
        // $0表示計(jì)算后的結(jié)果, $1表示數(shù)組中的每一個(gè)元素   <======
        print(stringArray.reduce("", {
            return $0 == "" ? $1 : $0 + "隙券、" + $1
        }))

        //第一次 : $0  = ""                                        $1 =  "Objective-C"
        //第二次 : $0  = "Objective-C"                   $1 =  "Swift"
        //第三次 : $0  = "Objective-C男应、Swif"        $1 =  "HTML"
        //......
  
        let numbers = [1, 2, 3, 4]
        let numberSum = numbers.reduce(0, { x, y in
            x + y
        })
        print(numberSum)
    }
    
    func appendString(string1: String, string2: String) -> String {
        return string1 == "" ? string2 : string1 + "、" + string2
    }

打印結(jié)果

Objective-C娱仔、Swift沐飘、HTML、CSS牲迫、JavaScript
Objective-C耐朴、Swift、HTML盹憎、CSS隔箍、JavaScript
Objective-C、Swift脚乡、HTML、CSS、JavaScript
10

參考博文:
swift中高階函數(shù)map奶稠、flatMap俯艰、filter、reduce