Swift 3.0 简单高阶函数 — Map, filter, reduce 和 flatMap笔记整理

Map

对集合进行循环，并对集合中的每个元素应用相同的操作, 然后将所有的变换后的元素组成新的集合.

例子:

普通数组映射:

    var numArr = [1, 2, 3, 4, 5]
        
    var emptyArr :[Int] = []
    for num in numArr {
        emptyArr.append(num * 2)
    }
    // emptyArr = [2, 4, 6, 8, 10]

使用map函数写法:

    var numArr = [1, 2, 3, 4, 5]
        
    var newArr1 = numArr.map { (value) -> Int in
        return value * 2
    }
    // newArr1 = [2, 4, 6, 8, 10]

    
    var newArr2 = numArr.map { (value) in
        return value * 2
    }
    // newArr2 = [2, 4, 6, 8, 10]

    // 闭包中单条语句可以省略return
    var newArr3 = numArr.map{value in value * 2}
    // newArr3 = [2, 4, 6, 8, 10]

    // $0 表示闭包第一个参数,$1以此类推
    var newArr4 = numArr.map{$0 * 2}
    // newArr4 = [2, 4, 6, 8, 10]

Filter

普通数组过滤:

    var numArr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
    var emptyArr : [Int] = []
    
    for num in numArr {
        if num % 2 == 0 {
            emptyArr.append(num)
        }
    }
    // emptyArr = [2,4,6,8]
         

使用filter函数写法:

    var numArr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
    // filter (其他写法跟上面类似, 不再啰嗦)
    let newArr = numArr.filter{$0 % 2 == 0}
    // newArr = [2,4,6,8]

Reduce

普通数组求和:

    let numArr = [1, 2, 3, 4, 5]
    var sum : Int = 0
    
    for num in numArr {
        sum += num
    }
    // sum = 15

reduce函数写法:

    // reduce函数:
    /* 参数:
     * initialResult : 初始值
     * nextPartialResult : 闭包 (上次结果, 下一个值) -> 下一次结果 
     */
    reduce(initialResult: Result , nextPartialResult: (Result, Int)  -> Result)

    let numArr = [1, 2, 3, 4, 5]
    let sum = numArr.reduce(0) { (result, nextNum) -> Int in
        return result + nextNum
    }
    // sum = 15
    
    // 简洁写法:
    let numArr = [1, 2, 3, 4, 5]
    let sum = numArr.reduce(0) { $0 + $1 }
    // or numArr.reduce(0, { $0 + $1 }) 
    // sum = 15


    let numArr = [1, 2, 3, 4, 5]
    let sum = numArr.reduce(0, +) 
    // sum = 15
    

FlatMap

合并两个数组 普通写法:

    let arrInArr = [[1,2,3],[4,5,6]]
    var emptyArr : [Int] = []
    for arr in arrInArr {
        emptyArr += arr
    }
    // emptyArr = [1, 2, 3, 4, 5, 6]

flatMap写法:

补充:

• Swift 烧脑体操（四） - map 和 flatMap
• Swift 烧脑体操

    let arrInArr = [[1,2,3],[4,5,6]]
    let flattendArr = arrInArr.flatMap { $0 }
    // flattendArr = [1, 2, 3, 4, 5, 6]

    // 过滤含optional类型值的数组
    let name : [String?] = ["zhangsan",nil,"lisi",nil,"wang"]
    let validName = name.flatMap{ $0 }
    // validName = ["zhangsan", "lisi", "wang"]

函数链式编程

以上这些简单的例子总结了高阶函数代替更常用的方法。使用这些函数可以编写更优雅、更易于维护的代码，可读性更好。

    let arrInArr = [[1,2,3],[4,5,6]]
    let newValue = arrInArr.flatMap{$0}.filter{$0 % 2 == 0}.map{$0 * 2}.map{"$\($0)"}
    // newValue = ["$4", "$8", "$12"]