//字符串的创建有两种

//不可变字符串

let str = "I'm a string"

//可变字符串

var string = "I'm a mutableString1"

//可变字符串初始化再赋值

var mutableString : String

mutableString = "I'm a initMutableString"

print("str = \(str)")

print("string = \(string)")

print("mutableString = \(mutableString)")

str = I'm a string

string = I'm a mutableString1

mutableString = I'm a initMutableString

let string = ""

let string1 = String()

print("voidString = \(string)")

print("voidString1 = \(string1)")

voidString =

voidString1 = 

let string = ""

if string == ""{

    print("string 是空字符")

}else{

    print("string 不是是空字符")

}

//第二种

if string.isEmpty {

    print("string 是空字符")

}else{

    print("string 不是是空字符")

}

string 是空字符

string 是空字符

//字符串的拼接

//直接 +

var string = "abc"

 string += "def"

print("string = \(string)")

//第二种拼接\()

let str = "hij"

let str1 = "\(str)klm"  //字符串的拼接   用    \(要拼接的内容)

print("str1 = \(str1)"

string = abcdef

str1 = hijklm

let number =

let number1 = 2.1

let sum = "\(number) 加上 \(number1) 等于 \(Double(number) + number1)"

print("sum = \(sum)")

sum =  加上 2.1 等于 5.1

//字符串的长度

let string = "China,America"

let length = string.characters.count

print("length = \(length)")

length = 

let str = "abcdefghij"

let str2 = "nmgghjkjhghj"

if str == str2 {

    print("str 和 str2 是相等的")

}else{

    print("str 和 str2 是不相等的")

}

str 和 str2 是不相等的