map:

可以对集合类型中的每一个元素做一次处理，转换成一个新数组。

数组

let colors = ["red", "blue", "yellow", "white"]

let counts = colors.map { (string: String) -> Int in
    return string.characters.count
}
// 简化
let counts = colors.map { $0.characters.count }
// 结果 [3, 4, 6, 5]

class Color {
    var name: String
    init(name: String) {
        self.name = name
    }
}
// 转换为对象数组
let colorObjs = colors.map {
    return Color(name: $0)
}
for obj in colorOjbs {
    print(obj.name)
}
// red
// blue
// yellow
// white

集合

let ColorsSet: Set = ["red", "blue", "yellow", "white"]
let colorsCount = ColorsSet.map { $0.count }
// 结果: [3, 4, 6, 5]

字典

let dict = [1: "one", 2: "two", 3: "three"]
let keys = dict.map { $0.key }  // [2, 3, 1]
let values = dict.map { $0.value }  // ["two", "three", "one"]

flatMap:

不包含nil

flatMap返回后的数组中不存在nil，同时它会把Optional解包。

let colors = ["red", "blue", "yellow", ""]

let countsOfMap = colors.map { item -> Int? in
    let length = item.characters.count
    guard length > 0 else {
        return nil
    }
    return length
}
// 结果: [3, 4, 6, nil]

let counts = colors.flatMap { item -> Int? in
    let length = item.characters.count
    guard length > 0 else {
        return nil
    }
    return length
}
// 结果 [3, 4, 6]

打开数组

flatMap还能把数组中存有数组的数组（二维数组、N维数组）一同打开变成一个新的数组。

let array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

let arr1 = array.map{ $0 }
// arr1: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

let arr2 = array.flatMap{ $0 }
// arr2: [1, 2, 3, 4, 5, 6, 7, 8, 9]

合并数组

flatMap也能把两个不同的数组合并成一个数组，这个合并的数组元素个数是前面两个数组元素个数的乘积。

let fruits = ["Apple", "Orange", "Puple"]
let counts = [2, 3, 5]

let newArray = counts.flatMap { count in
    fruits.map ({ fruit in
        return fruit + "  \(count)"
    })
}
// ["Apple 2", "Orange 2", "Puple 2", "Apple 3", "Orange 3", "Puple 3", "Apple 5", "Orange 5", "Puple 5"]

filter:

过滤，可以对数组中的元素按照某种规则进行一次过滤。

let numbers = [1, 3, 5, 7, 9]
let filterNumbers = numbers.filter { $0 <= 5 }
// [1, 3, 5]

reduce:

计算，可以对数组的元素进行计算。

// 返回result，现在result是字符串
let fruits = ["Apple", "Orange", "Puple"]
let string = fruits.reduce("", {
    // $0: result, $1: 数组的值
    return $0 == "" ? $1 : $0 + "," + $1
})
// Apple,Orange,Puple