能接受函数作为参数或者返回值为函数的函数，就是高阶函数
1.map(映射)

        //定义一个数组，用map来遍历，并且把返回值重新组合成arr
        var a = [1,2,3,4,5]
        a = a.map{$0*$0}
        print("a===\(a)")
        
        //flatMap把二维数组降低为一维数组
        let b = [[1, 2, 3], [1, 2, 3]]
        let c = b.flatMap{$0}
        print("c===\(c)")

        //compactMap把数组中的可选值去掉，保留确定值
        let arr = [1, 2, 3, Optional(9), nil, 4, 5]
        print("arr===\(arr)")
        
        let newArr = arr.compactMap { $0 }
        print("newArr===\(newArr)")

a===[1, 4, 9, 16, 25]
c===[1, 2, 3, 1, 2, 3]
arr===[Optional(1), Optional(2), Optional(3), Optional(9), nil, Optional(4), Optional(5)]
newArr===[1, 2, 3, 9, 4, 5]

2. reduce

        let abc = [1,2,3,4,5]
        
        let abd = abc.reduce(0) { (value1, value2) -> Int in
            print("1==\(value1) 2===\(value2)")
            return value1 + value2
        }
        
        print(abd)

1==0 2===1
1==1 2===2
1==3 2===3
1==6 2===4
1==10 2===5
15

可以看到reduce接受一个参数0，并且把闭包里返回的值作为参数继续传入reduce，闭包里value1是reduce接受的第一个参数，value2是数组的下一个元素，当元素结束的时候，整个函数返回闭包的计算值

3. filter 数组过滤

        let strArr = ["张三", "李四", "王五", "赵六"]
        let str = strArr.filter { (str) -> Bool in
            return str.hasPrefix("张")
        }
        print(str)

["张三"]

4.sorted数组排序

        let strArr = ["张三", "李四", "王五", "赵六"]
        
        let str = strArr.sorted {
            return $0 > $1
        }
        print(str)

["赵六", "王五", "李四", "张三"]