swift中高阶函数

有些记不清了，随笔备忘，也参考了一些别人的

1.map

数组中的每一个元素调用一次该闭包函数，并返回该元素所映射的值。就是数组中每个元素通过某种规则（闭包实现）进行转换，最后返回一个新的数组，简单用法：

//写法1
var list = [10, 11, 12, 13]
list = list.map({ res in
    return res + 2
})
 print(list)//[12, 13, 14, 15]

//写法2
var list = [10, 11, 12, 13]
list = list.map({
    $0 + 2//$0表示第几个参数，当前闭包只有一个参数所以是$0
 })
print(list)//[12, 13, 14, 15]

2. flatMap

类似于map，区别就是，flatMap会把二维数组降为一维数组，其中有nil会移除

let list1 = [[10, 11, 12, 13], [10, 11, 12, 13], [10, 11, 12, 13]]
let list2 = list1.flatMap{$0}
 print(list2) //[10, 11, 12, 13, 10, 11, 12, 13, 10, 11, 12, 13]

3.sort：排序

let list1 = [13, 12, 14, 11]
let list2 = list1.sorted(by: <)
print(list2)//[11, 12, 13, 14]

4.forEach：遍历

// 准备一个数组，
var array = ["A", "B", "C", "D", "E"]
// 遍历
array.forEach{ str in   
    print(str)
}

5.reduce：合归

//求里面数组的和并排序
let array = [[5,2,7],[4,8],[9,1,3]]
let sumResult = array.map{$0.reduce(0, +)}.sorted(by: <)
print(sumResult)//[12, 13, 14]

//--------

let array = [[5,2,7],[4,8],[9,1,3]]
let sumResult = array.map{$0.reduce(0, {
    (res, ele) in
    return res + ele
})}.sorted(by: <)
print(sumResult)//[12, 13, 14]

//---------

let arr = ["a","b","c"]
let resultStr: String = arr.reduce("i") { (result, element) -> String in
    print(element)
    return result + ", " + element
}
print(resultStr)
//a
//b
//c
//i, a, b, c

6.filter： 对数组中元素进行条件筛选, 返回一个泛型数组

let listStr = ["I", "hate", "this", "city"]
let a = listStr.filter { res in
     return res != "I"
}
print(a)//["hate", "this", "city"]

7.compactMap/compactMapValues： 过滤空值

let listStr = ["1", "2", "d", "3", "4", "a"]
let a = listStr.compactMap {
    Int($0)
}
print(a)//[1, 2, 3, 4]

//-------
let listStr:[String:String] = ["1":"1", "d":"2", "3":"3", "2":"d"]
let a = listStr.compactMapValues { Int($0)}
print(a)//["d": 2, "3": 3, "1": 1]

8.mapValues转换value

let dic: [String : Int] = ["a": 1, "b": 2, "c": 3, "d": 4 ]
 // 字典中的函数, 对字典的value值执行操作, 返回改变value后的新的字典
let mapValues = dic.mapValues({ $0 + 2 })
print(mapValues)//["a": 3, "b": 4, "c": 5, "d": 6]