Swift3:

let stings= "a, b,c"
var i = stings.characters.startIndex

swift4中直接操作字符串，无须通过characters：

let stings= "a, b,c"
var i = strings.startIndex

swift4中字符串也是一个集合了，所以可以有filter,map等集合的特性

swift4中采用了三引号来代表换行,首尾的三行号必须单独一行
无须通过\n来换行咯

let strings = '''
                        my name is hellos,
                        nice to meet you
                    '''
//      my name is hellos,
         nice to meet you

swift3中取字符串的某位

strings[i ..< strings.endIndex]

swift4中利用...来匹配

strings[i...]