swift开发--函数map、flatMap、filter、reduce

Swift相比于Objective-C又一个重要的优点，它对函数式编程提供了很好的支持，Swift提供了map、filter、reduce这三个高阶函数作为对容器的支持。

1 map：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"]
        //1.先定义返回字符串个数的方法，再map
        func stringCount(string: String) -> Int {
            return string.count
        }
        let count = stringArray.map(stringCount)
        Tprint(messsage: count)//[11, 5, 4, 3, 10]
        
        //2.直接使用map方法
        let count1 = stringArray.map{ (string) -> Int in
            return string.count
        }
        Tprint(messsage: count1)//[11, 5, 4, 3, 10]

        // 3.$0代表数组中的每一个元素
        let count2 = stringArray.map{
            return $0.count 
        }
        Tprint(messsage: count2)//[11, 5, 4, 3, 10]

2 flatMap与map不同之处：

（1）flatMap返回后的数组中不存在nil，同时它会把Optional解包

        let count3 = stringArray.map { (str) -> Int? in
            return str.count
        }
         Tprint(messsage: count3)//[Optional(11), Optional(5), Optional(4), Optional(3), Optional(10)]
        
        let count4 = stringArray.flatMap { (str) -> Int? in
            return str.count
        }
        Tprint(messsage: count4)//[11, 5, 4, 3, 10]

（2）flatMap还能把数组中存有数组的数组（二维数组、N维数组）一同打开变成一个新的数组

let array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

let arr1 = array.map{ $0 }
arr1 // [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

let arr2 = array.flatMap{ $0 }
arr2 // [1, 2, 3, 4, 5, 6, 7, 8, 9]

（3）flatMap也能把两个不同的数组合并成一个数组，这个合并的数组元素个数是前面两个数组元素个数的乘积

let fruits = ["Apple", "Orange", "Puple"]
let counts = [2, 3, 5]
let array = counts.flatMap { count in
    fruits.map ({ fruit in
         return fruit + "  \(count)"            
    })   
}
array // ["Apple 2", "Orange 2", "Puple 2", "Apple 3", "Orange 3", "Puple 3", "Apple 5", "Orange 5", "Puple 5"]

3 filter：过滤，可以对数组中的元素按照某种规则进行一次过滤

let stringArray = ["Objective-C", "Swift", "HTML", "CSS", "JavaScript"]
   // 筛选出字符串的长度小于10的字符串
        func stringCountLessThan10(str :String) -> Bool{
            return str.count < 10
        }
        let count5 = stringArray.filter(stringCountLessThan10(str:))
         Tprint(messsage: count5)
        
        let count6 = stringArray.filter { (str) -> Bool in
            return str.count < 10
        }
        Tprint(messsage: count6)
        
        let count7 = stringArray.filter { $0.count < 10}
        Tprint(messsage: count7)

4 reduce：计算，可以对数组的元素进行计算

// 将数组中的每个字符串用‘、’拼接
let stringArray = ["Objective-C", "Swift", "HTML", "CSS", "JavaScript"]

func appendString(string1: String, string2: String) -> String {
    return string1 == "" ? string2 : string1 + "、" + string2
}
// reduce方法中的第一个参数是初始值
stringArray.reduce("", appendString)

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
stringArray.reduce("", {
    return $0 == "" ? $1 : $0 + "、" + $1
})

好啦，介绍完了愿能与君共勉。