Swift 高阶函数map、flatMap、filter、reduce

Map:可以对数组中的每一个元素做一次处理

• 如数组中存在nil,会判断nil,如果存在则返回

    override func viewDidLoad() {
        super.viewDidLoad()
        //可以对数组中的每一个元素做一次处理
        let cast = ["Vivien", "Marlon", "Kim", "Karl",nil, nil, "liliguang"]
        
//      map(_ transform: (String) throws -> T)                  //映射闭包。 transform接受此序列的元素作为其参数，并返回相同或不同类型的转换值。
        
        let lowercaseNames = cast.map { $0.lowercased() }       //注意研究各种不同的映射闭包写法
        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
        }))
    }
    //转换为小写
    func lowStr(str : String) -> String {
        return str.lowercased()
    }
    //获取字符串长度
    func stringCount(string: String) -> Int {
        return string.count
    }
    

打印结果

[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返回后的数组中不存在nil，同时它会把Optional解包

    override func viewDidLoad() {
        super.viewDidLoad()
        //可以对数组中的每一个元素做一次处理
        let cast = ["Vivien", "Marlon", "Kim", "Karl",nil, nil, "liliguang"]
        
        let lowercaseNames = cast.compactMap { $0?.lowercased() }       //理解 : 调用$0的对象方法
        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
    }

打印结果

["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：过滤，可以对数组中的元素按照某种规则进行一次过滤

    override func viewDidLoad() {
        super.viewDidLoad()
        //可以对数组中的每一个元素做一次处理
        let cast = ["Vivien", "Marlon", "Kim", "Karl",nil, nil, "liliguang"]
        
        let newCase = cast.filter{ string -> Bool in
            return string == "liliguang"
        }
        print(newCase)
    }

打印结果

[Optional("liliguang")]

---

---

reduce : 计算，可以对数组的元素进行计算

    override func viewDidLoad() {
        super.viewDidLoad()
        //可以对数组中的每一个元素做一次处理
        let stringArray = ["Objective-C", "Swift", "HTML", "CSS", "JavaScript"]
        // reduce方法中的第一个参数是初始值
        print(stringArray.reduce("", appendString))
        print(stringArray.reduce("", {(string1, string2) -> String in
            return string1 == "" ? string2 : string1 + "、" + string2
        }) )
        
        // $0表示计算后的结果, $1表示数组中的每一个元素   <======
        print(stringArray.reduce("", {
            return $0 == "" ? $1 : $0 + "、" + $1
        }))

        //第一次 : $0  = ""                                        $1 =  "Objective-C"
        //第二次 : $0  = "Objective-C"                   $1 =  "Swift"
        //第三次 : $0  = "Objective-C、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
    }

打印结果

Objective-C、Swift、HTML、CSS、JavaScript
Objective-C、Swift、HTML、CSS、JavaScript
Objective-C、Swift、HTML、CSS、JavaScript
10

参考博文:
swift中高阶函数map、flatMap、filter、reduce