IOS: Learning notes, Swift extensions

////Yyextension.swift////Created by Yao_yu on 14-7-18.//Copyright (c) 2014 Yao_yu. All rights reserved.//import foundationextension string{func strip ()-string{returnStringbytrimmingcharactersinset (Nscharacterset.whitespaceandnewlinecharacterset ())} func split (separator: String)-[string]{returncomponentsseparatedbystring (separator)} var doublevalue:double!{    return(Self asnsstring). Doublevalue} var length:int{returncountelements (self)} func Isvalidrange (Range:range<Int>)bool{returnRange.startindex >=0&& Range.endindex <=length} subscript (Range:range<Int>)String {assert (Isvalidrange (range),"range exceeds string length") var startIndex:String.Index=Advance (Self.startindex, Range.startindex) var endIndex:String.Index= Advance (StartIndex, Range.endindex-Range.startindex)returnSelf.substringwithrange (Range (Start:startindex, End:endindex))} subscript (Index:int)-string{returnSelf[index...index]} func at (Indexes:int ...)-[String] {returnIndexes.map {self[$0]}} func at (indexes: [Int])-[String] {returnIndexes.map {self[$0]}}}extension Array {func take (n:int)-slicetype{returnself[0.. <N]} func tail (n:int)-Slicetype {returnself[(count-n): <Count]} Func Skip (N:int)-Slicetype {returnSelf[n. < Count]}}