swift 高阶函数 map filter reduce

        //针对 数组 字典 集合
        //map简写法
        let values = [2.0,3.0,5.0,7.0]
        let squares = values.map{$0 * $0}
        print(squares)//[4.0, 9.0, 25.0, 49.0]
        //map正常写法
        let squares2 = values.map({
            (value: Double) -> Double in
            return value * value
        })
        print(squares2)//[4.0, 9.0, 25.0, 49.0]
        //filter
        let digits = [1, 4, 10, 15]
        let even = digits.filter { $0 % 2 == 0 }
        print(even)//[4, 10]
        
        let even1 = digits.filter{
            (number) -> Bool in
            return number % 2 == 0
        }
        print(even1)//[4, 10]
        //reduce
        let reduceSouce = [2,3,5,7]
        let total = reduceSouce.reduce(10, +)
        print(total)//27
        
        let codes = ["abc", "def", "ghi"]
        let total1 =  codes.reduce("", +)
        print(total1)//abcdefghi
        
        //value1 代表初始值 "===="以及后面拼接后的值
        //value2 代表string 数组里面的元素
        let string = ["wo","de","who"]
        let total2 = string.reduce("===="){
            value1,value2 in "\(value1),\(value2)"
        }
        print(total2)//====,wo,de,who

        //flatMap  合并数组 集合
        let collection = [[2,4,5],["a","b"],[9,5]]
        let resultCollection = collection.flatMap{$0}
        print(resultCollection)//[2, 4, 5, "a", "b", 9, 5]
        
        //flatMap还“知道”可选值的概念，它将会把集合中的nil元素移除掉
        let people: [String?] = ["Tom", nil, "Peter", nil, "Harry"]
        let valid = people.flatMap { $0 }
        // ["Tom", "Peter", "Harry"]
        
   //https://www.jianshu.com/p/1b4a8b4517bd