Swift中高阶函数map、flatMap、filter、reduce

Swift相比于Objective-C又一个重要的优点，它对函数式编程提供了很好的支持，Swift提供了map、filter、reduce这三个高阶函数作为对容器的支持。

1.map 函数可以对数组的每一个元素进行处理

    // 计算字符串的长度
    let stringLength = ["Objective-C", "Swift"]
    // $0代表数组中的每一个元素
    stringLength.map{   
       return $0.characters.count
     }


    
  // 数组的每一个元素是相应的元祖
  let controllerInfo: [(String, String, UIViewController.Type)] = [
        ("夺宝", "duobao", MainController.self),
        ("最新揭晓", "zuixin", CompletedActivityController.self),
        ("晒单", "shaidan", CommunityController.self),
        ("购物车", "gouwuche", ShoppingController.self),
        ("我的", "geren", MineController.self)
    ]
    
    override func viewDidLoad() {
        super.viewDidLoad()
       // 通过map函数 获取每一个元祖
        self.viewControllers = controllerInfo.map({ (title, img, viewController) -> UINavigationController in
            let vc = viewController.init()
            vc.tabBarItem.image = UIImage(named: img)
            vc.tabBarItem.title = title
            vc.tabBarItem.setTitleTextAttributes([NSForegroundColorAttributeName: Theme.themeColor], for: .selected)
            return CommonNavController(rootViewController: vc)
        })
    }

2.flatMap 与 map 函数的不同处：
(1）flatMap返回后的数组中不存在nil，同时它会把Optional解包

    let oneArray = ["name", "age", "height", ""]

    let length = oneArray.map { $0 -> Int? in
        let stringLength = $0.characters.count
        guard stringLength > 0 else {return nil} 
        return length  
     }
     legth // [4, 3, 6, nil]

    let length = oneArray.flatMap { $0 -> Int? in
         let stringLength = $0.characters.count
         guard stringLength > 0 else { return nil}    
         return stringLength    
  }    
    length // [4, 3, 6]

3.filter 可以对数组中的元素按照某种规则进行一次过滤

let oneArray = ["name", "age", "height"]
// $0表示数组中的每一个元素
 
stringArray.filter{ $0 -> Void in   
   if $0.characters.count > 4 {
       print（$0）
       // height 
   }
}

4 reduce：计算，可以对数组的元素进行计算

let p1 = [20,20,10]
// $0 代表计算的值  $1代表元素
let sum = p1.reduce(0) { $0+$1 }
print(sum) //50