Swift 高阶函数

1. 排序函数

func sorted(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> [Element]

        var names: [String] = ["61","95","8","248","42"]
        //一个包含字符串的数组
        names = names.sorted(by: { (s1, s2) -> Bool in
            return s1 < s2
        })
        print(names) //输出结果

该高阶函数内部大概实现:

extension Array {
    
    typealias  IncreasingOrder = (String,String) -> Bool
    
    mutating func mySorted(_ increasingOrder:IncreasingOrder) -> [String] {
        var newString:[String] = [String]()

        // 假设这里采用简单选择排序
        for n in 0..<self.count{
            var tamper:String = self[n] as! String
            for i in n+1..<self.count {  //
                var next:String = self[i] as! String
                guard !increasingOrder(tamper,next) else {
                    continue
                }
                swap(&tamper, &next)
                swap(&self[n], &self[i])
            }
            newString.append(tamper)
        }
        return newString
    }
}

2. First函数

func first(where predicate: (Element) throws -> Bool) rethrows -> Element?

        let names:[String] = ["61","95","8","248","42"]
        //第一个满足条件的值
        let name = names.first { (first) -> Bool in
            return first == "8"
            }!
        print(name) // 输出: 8

3. Map函数

func map<T>(_ transform: (Element) throws -> T) rethrows -> [T]

        var names:[String] = ["61","95","8","248","42"]
        // 在每个字符前加上$
        names = names.map { (str) -> String in
            "$" + str
        }
        print(names) // ["$61", "$95", "$8", "$248", "$42"]

4. compactMap函数

flatMap 方法同 map 方法比较类似，只不过它返回后的数组中不存在 nil（自动把 nil 给剔除掉），同时它会把 Optional 解包

        let array = ["Apple", "Orange", "Grape", ""]
        
        let arr1 = array.map { a -> Int? in
            let length = a.count
            guard length > 0 else { return nil }
            return length
        }
        print("arr1:\(arr1)") // [Optional(5), Optional(6), Optional(5), nil]
        
        let arr2 = array.compactMap { a-> Int? in
            let length = a.count
            guard length > 0 else { return nil }
            return length
        }
        print("arr2:\(arr2)")// [5, 6, 5]

5. filter函数

筛选满足条件的数据

        var names:[String] = ["61","95","8","248","42"]
        
        names = names.filter({ (str) -> Bool in
           return str.count > 1
        })
        print(names) // ["61", "95", "248", "42"]

6. reduce函数

计算, 可以对数组中的元素进行计算

let array = [1, 2, 3, 4]
        //计算数组array元素的和
        //在这里$0和$1的意义不同，$0代表元素计算后的结果，$1代表元素
        //0代表初始化值，在这里可以理解为 $0初始值 = 0
        let result = array.reduce(0){
            $0 + $1
        }
        print(result) // 10