OC定义字符：

 char charValue = 'a';

swift定义字符：

  var charValue : Character = "a"

Unicode 国际标准的文本编码，他几乎可以表示所有国家的字符、
OC可以存储ASCII字符，swift可以存ASCII+Unicode

 var charValue :Character = "????" //正确的
 var charValue2:Character = "小玉" //正确的
 var charValue3:Character = "玉" //错误的

 字符串是可以存很多字符的集合。
 OC:NSString * stringValue = @"小玉";
 swift:let stringValue:String = "小玉"

 区别：NSString/String  @""/""

 //String -> NSString
 let string:NSString = NSString(string: "dahuan")

 OC是以‘\0’为字符串结束：NSString * stringValue = @"aaaa\0bbbb";
 swift不是以'\0'结束：let stringValue:String = "aaaa\0bbbb"

字符串的常用方法

• 1、初始化空字符串

    let emptyString = ""
    let emptyString2 = String()
  

• 2、遍历字符串

    let string2 = "康姐是我们的好朋友！"
    for a in string2.characters {
        print(a)
    }
  

• 3、获取字符串的长度

    let count = string2.characters.count
    print(count)
  

• 4、判断字符串是否为空

    let emptyStr = "1"
    if emptyStr.isEmpty {
        print("yes")
    }
  

• 5、字符串的拼接

    let str1 = "康姐"
    let str2 = "是我们的好朋友！"
    let result = str1 + str2
    
    var str3 = "haha"
    str3 = str3 + str2
    print(str3)
  

• 6、格式化字符串

    let intValue = 10
    let str4 = "爱情"
    let str5 = "\(intValue)、因为\(str4)，不会悲伤"
    print(str5)
    
    //取小数后两位
    let pi = 3.1415
    let str6 = String(format: "%.2f", pi)
    print(str6)
  

• 7、字符串比较

    //和C语言strcmp一样
    
    let str7 = "a"
    let str8 = "ab"
    
    if str7 > str8 {
        print(">")
    } else if str7 < str8 {
        print("<")
    }
  

• 8、是否有前后缀

    let str9 = "www.baidu.com"
    if str9.hasPrefix("www") {
        print("yes")
    } else {
        print("no")
    }
    if str9.hasSuffix("cm") {
        print("yes")
    } else {
        print("no")
    }
  

• 9、大小写

    let str10 = str9.uppercaseString
    let str11 = str9.lowercaseString
    
    print(str10)
  

• 10、String转基本数据类型

    let str12 = "5.2吗"
    let num = Float(str12)
    print(num)
  

• 11、截取字符串

    	let str13 = "康姐是我们的好朋友！"
    	//str13.startIndex  从0开始 Index不是int
    	let fromStr = str13.startIndex.advancedBy(2)
    	let toStr = str13.endIndex.advancedBy(-2)
    	let str14 = str13.substringFromIndex(toStr)
    	
    	
    	let range:Range <String.Index> = Range(start: fromStr, end: toStr)
    	let str15 = str13.substringWithRange(range)
    	
    	print(str15)
    	
    	let toStr1 = (str13 as NSString).substringToIndex(2);
    	print(toStr1)
    	
    	let range1 = NSRange(location: 2, length: 6)
    	
    	let toStr2 = (str13 as NSString).substringWithRange(range1)
    	print(toStr2)