Swift 4.2字符串截取(subString过期)

1.截取前三个字符

        //方法一        
        let str = "0123456789"
        let subStr = str.prefix(3)
        print(subStr)//输出012

        //方法二
        let str = "0123456789"
        let endIndex =  str.index(str.startIndex, offsetBy: 3)
        let newStr = String(str[..<endIndex])
        print(newStr)//输出012

2.截取第三个到第六个范围内的字符

        let str = "0123456789"
        let startIndex = str.index(str.startIndex, offsetBy: 3)
        let endIndex =  str.index(str.startIndex, offsetBy: 6)
        let newStr = String(str[startIndex..<endIndex])
        print(newStr)//输出345

3.截取后三个字符 

        //方法一
        let str = "0123456789"
        let subStr = str.suffix(3)
        print(subStr)//输出789
        
        //方法二
        let str = "0123456789"
        let startIndex =  str.index(str.endIndex, offsetBy: -3)
        let newStr = String(str[startIndex..<str.endIndex])
        print(newStr)//输出789

注意：以上方法在赋值的时候需要进行类型转换  label.text = String(subStr)