Swift：Map，FlatMap，Filter，Reduce 理解

原文链接

Swift是支持一门函数式编程的语言，拥有Map，FlatMap,Filter,Reduce针对集合类型的操作.本文主要根据官方文档举例了解Swift中的Map，FlatMap,Filter,Reduce

Map

首先我们来看一下map在Swift中的的定义，我们看到它可以用在 Optionals 和 SequenceType 上（如：数组、词典等）

public enum Optional : _Reflectable, NilLiteralConvertible {
    /// If `self == nil`, returns `nil`.  Otherwise, returns `f(self!)`.
    @warn_unused_result
    public func map(@noescape f: (Wrapped) throws -> U) rethrows -> U?
}

extension CollectionType {
    /// Returns an `Array` containing the results of mapping `transform`
    /// over `self`.
    ///
    /// - Complexity: O(N).
    @warn_unused_result
    public func map(@noescape transform: (Self.Generator.Element) throws -> T) rethrows -> [T]
}

我们可以用For-in完成类似的操作

var values = [1,3,5,7]
var results = [Int]()
for var value in values {
    value *= 2
    results.append(value)
}
//"[2, 6, 10, 14]"
print(results)

Map操作

var values = [1,3,5,7]
let results = values.map (){ $0 * 2 }
//"[2, 6, 10, 14]"

相比较而言，代码精简了很多。可以理解为 map是直接对当前的数组进行处理，返回的还是当前的数组样式

FlatMap

与map一样，它可以用在 Optionals和 SequenceType 上（如：数组、词典等）。

public enum Optional : _Reflectable, NilLiteralConvertible {
    /// Returns `nil` if `self` is `nil`, `f(self!)` otherwise.
    @warn_unused_result
    public func flatMap(@noescape f: (Wrapped) throws -> U?) rethrows -> U?
}

extension SequenceType {
    /// 返回一个将变换结果连接起来的数组
    /// `transform` over `self`.
    ///     s.flatMap(transform)
    /// is equivalent to
    ///     Array(s.map(transform).flatten())
    @warn_unused_result
    public func flatMap(transform: (Self.Generator.Element) throws -> S) rethrows -> [S.Generator.Element]
}

extension SequenceType {
    /// 返回一个包含非空值的映射变换结果
    @warn_unused_result
    public func flatMap(@noescape transform: (Self.Generator.Element) throws -> T?) rethrows -> [T]
}

一：空值过滤

var values:[Int?] = [1,3,5,7,9,nil]
let flattenResult = values.flatMap{ $0 }
/// [1, 3, 5, 7, 9]

二：压平,（降维）

var values = [[1,3,5,7],[9]]
let flattenResult = values.flatMap{ $0 }
/// [1, 3, 5, 7, 9]

FlatMap可以理解为强制空值过滤，剔除空值。强制压平\强制降维。

Filter

同样，我先来看看Swift中的定义：

extension SequenceType {
    /// 返回包含原数组中符合条件的元素的数组
    /// Returns an `Array` containing the elements of `self`,
    /// in order, that satisfy the predicate `includeElement`.
    @warn_unused_result
    public func filter(@noescape includeElement: (Self.Generator.Element) throws -> Bool) rethrows -> [Self.Generator.Element]
}

eg: 我们向flatMap传入了一个闭包，筛选出了能被3整除的数据

var values = [1,3,5,7,9]
let flattenResults = values.filter{ $0 % 3 == 0}
//[3, 9]

Filter 可以理解为条件过滤，返回包含原数组中符合条件的元素的数组

Reduce

我们先来看下Swift中的定义：

extension SequenceType {
    /// Returns the result of repeatedly calling `combine` with an
    /// accumulated value initialized to `initial` and each element of
    /// `self`, in turn, i.e. return
    /// `combine(combine(...combine(combine(initial, self[0]),
    /// self[1]),...self[count-2]), self[count-1])`.
    @warn_unused_result
    public func reduce(initial: T, @noescape combine: (T, Self.Generator.Element) throws -> T) rethrows -> T
}

eg

let fiveArray = [1,2,3,4,5]
let sum = fiveArray.reduce(0) { $0 + $1 } //求数组元素的和
let sum1 = fiveArray.reduce(0, +) // 简写
let sum2 = fiveArray.filter{ $0 > 2 }.reduce(0, +)  // 数组中元素大于2 的数据求和
print(sum,sum1,sum2)
// 15 15 12