let str = "Hello, playground"

let str1 = ""
let str2 = String()

if str1.isEmpty {
  print("this String Object is Empty")
}

var str3 = "Hello"
str3 += " "+"World"http://str3 = Hello World

let multiplier = 3
let message = "\(multiplier) times 2.5 is \(Double(multiplier) * 2.5)"http://3 times 2.5 is 7.5

str3.characters.count

let comStr1 = "one two"
let comStr2 = "one two"
comStr1==comStr2//true

let tmp3 = "thank you"
tmp3.hasPrefix("thank")//true
tmp3.hasSuffix("you")//true

for chara in str3.characters {
  print(chara)
}

let char1 = ""
var cgar2 = "HS"

let chars:[Character] = ["H","e","l","l","o"]
let str4 = String(chars)

var str5 = ""
let ca:Character = "a"
str5.append(ca)

"\0"http://"" 空白符
"\\"http://"\"反斜杠符號(hào)
"\t"http://" "制表符
"\n"http://換行符
"\r"http://回車符
"\'"http://"'"單引號(hào)
"\""http://"""雙引號(hào)
"\u{24}"http://"$"unicode字符

let tmp = "Hello Swift"
//獲取字符開始的下標(biāo)值 0
let indexStart = tmp.startIndex
//獲取某個(gè)下標(biāo)后一個(gè)字符的下標(biāo) 1
let next = indexStart.successor()
//獲取最后一個(gè)字符的下標(biāo)值 注意有\(zhòng)0的存在 
let indexEnd = tmp.endIndex
//獲取某個(gè)下標(biāo)前一個(gè)字符的下標(biāo)
let pre = indexEnd.predecessor()
//通過(guò)下標(biāo)獲取字符串中的字符 t
var c = tmp[pre]
//進(jìn)行下標(biāo)移動(dòng) o
var c2 = tmp[indexStart.advancedBy(4)]
//通過(guò)遍歷下標(biāo)來(lái)遍歷字符 H e l l o S w i f t
for index in tmp.characters.indices {
   print("\(tmp[index]) ", terminator: "")
}

var tmp2 = "Hello"
tmp2.insert("!", atIndex: tmp2.endIndex)

tmp2.insertContentsOf(" Swift".characters, at: tmp2.endIndex)

tmp2.removeAtIndex(tmp2.endIndex.predecessor())

let range = tmp2.endIndex.advancedBy(-4)..<tmp2.endIndex
tmp2.removeRange(range)