查找字符串 fiand

两类函数:

1. find(),   rfind()
2. index(),   rindex()

找到了都返回下标.

find找不到返回-1,index找不到抛出ValueError.

带r的表示从右向左找.

都可以使用第二个参数表示从哪个下标开始找.

a='abcdab'
a.find('a')
Out[3]: 0
a.rfind('a')
Out[4]: 4
a.rfind('a',1)
Out[5]: 4
a.rfind('x')
Out[6]: -1
a.index('a')
Out[7]: 0
a.index('a',1)
Out[8]: 4
a.rindex('a')
Out[9]: 4
a.index('x')
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/IPython/core/interactiveshell.py", line 2882, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-10-51f0d5bb66b2>", line 1, in <module>
    a.index('x')
ValueError: substring not found