Swift 函数Count,Filter,Map,Reduce

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blog.csdn.net/hello_hwc

前言：和OC不同，Swift有很多全局的函数，这些全局函数对简化代码来说很有用，目前Swift出到了2.0,不过我这篇文章还是用Swift1.2写的示例代码。

Count－统计数量

文档

func count<T : _CollectionType>(x: T) -> T.Index.Distance

Description 
Return the number of elements in x.
O(1) if T.Index is RandomAccessIndexType; O(N) otherwise.

示例

 let arr = [1,2,3,4]
 let dic = [1:2,"a":"b"]
 let str = "Wenchenhuang"
 println(count(arr))//4
 println(count(dic))//2
 println(count(str))//12

Filter－条件过滤

文档

func filter<S : SequenceType>(source: S, includeElement: (S.Generator.Element) -> Bool) -> [S.Generator.Element]

Description 
Return an Array containing the elements of source, in order, that satisfy the predicate includeElement.

示例－过滤长度大于4的字符串

 let array = ["Wen","Chen","Huang"]
 let filteredArray =  filter(array, { (element:String) -> Bool in return count(element)>4; }) println(filteredArray)

也可以简化

let filteredArray =  filter(array) {count($0)>4}

Map - 映射集合类型，返回数组

文档

func map<C : CollectionType, T>(source: C, transform: (C.Generator.Element) -> T) -> [T]
Description 

Return an Array containing the results of mapping transform over source.

示例

let array = ["Wen","Chen","Huang"]
let mapedAray = map(array, { (element:String) -> Int in return count(element) }) println(mapedAray) //［3，4，5］

同样可以简化

let mapedAray = map(array){count($0)}

Reduce - 把数组结合到一起

文档

func reduce<S : SequenceType, U>(sequence: S, initial: U, combine: @noescape (U, S.Generator.Element) -> U) -U

Description 
Return the result of repeatedly calling combine with an accumulated value initialized to initial and each element of `sequence`, in turn.

示例

 let array = ["Wen","Chen","Huang"]
 let reduceResult = reduce(array, "Hello ") { (originValue:String, element:String) -> String in
    return originValue + element;
 }
 println(reduceResult) //Hello WenChenHuang

可以简化

 let reduceResult = reduce(array, "Hello ") { $0 + $1}

进一步简化

let reduceResult = reduce(array, "Hello ",+)