列表切片问题

 

2018-01-03@望京

关于列表切片的注意事项

>>> a = [1,2,3,4,5]
>>> 
>>> b=a
>>> 
>>> id(a)
140353696988696
>>> id(b)
140353696988696
>>> 
>>> 
>>> a is b
True
>>> 
>>> 
>>> b.append(6)
>>> 
>>> a
[1, 2, 3, 4, 5, 6]
>>> b
[1, 2, 3, 4, 5, 6]
>>> 
>>> c = a[3:]
>>> 
>>> id(c)
140353697153824
>>> c
[4, 5, 6]
>>> 
>>> d = a[:2]
>>> 
>>> id(d)
140353697153896
>>> d
[1, 2]
>>> 
>>> 
>>> e = a[:]
>>> 
>>> id(e)
140353697154472
>>> id(a)
140353696988696
>>> 
>>> 
>>> e
[1, 2, 3, 4, 5, 6]
>>> 
>>> 
>>> import copy
>>> f = copy.deepcopy(a)
>>> id(f)
140353697153752
>>> id(a)
140353696988696
>>> 
>>> f
[1, 2, 3, 4, 5, 6]
>>> 
>>> 
>>> a
[1, 2, 3, 4, 5, 6]
>>> b
[1, 2, 3, 4, 5, 6]
>>> e
[1, 2, 3, 4, 5, 6]
>>> f
[1, 2, 3, 4, 5, 6]
>>> 
>>> 
>>> id(a),id(b),id(e),id(f)
(140353696988696, 140353696988696, 140353697154472, 140353697153752)
>>> 
>>> 
>>> a is b
True
>>> a is e
False
>>> a is f
False
>>> 

　　

补充示例1

>>> a
[1, 2, 3, 4, 5, 6]
>>> 
>>> g = a[-2:]
>>> g
[5, 6]
>>> 
>>> h = a[1:3]
>>> h
[2, 3]
>>> 
>>> i = a[3:1]
>>> i
[]
>>> 
>>> j = a[2:-1]
>>> j
[3, 4, 5]
>>>

 

用切片来删除序列里的某一段

>>> 
>>> a = [1, 2, 3, 4, 5, 6, 7]
>>> 
>>> a[1:4] = []
>>> 
>>> a
[1, 5, 6, 7]
>>> 

　　

 

 

列表反转

>>> 
>>> li = ['a','b','c','d']
>>> li.reverse()
>>> li
['d', 'c', 'b', 'a']
>>> print ''.join(li)
dcba
>>> 

 

 

关于字符串切片注意事项（不同于列表）

>>> 
>>> a = 'standby'
>>> a
'standby'
>>> 
>>> b = a[:]
>>> b
'standby'
>>> 
>>> a is b
True
>>> 
>>> id(a)
139908204403952
>>> id(b)
139908204403952
>>> 
>>>

 

切片实现字符串反转

>>> 
>>> a = 'standby'
>>> a[::-1]
'ybdnats'
>>> 
>>> 
>>> a = 'Life is short, use Python.'
>>> a[::3]
'Leshtu tn'
>>> 
>>> a[::-2]
'.otPeu,rh iei'
>>>　　

 

递归实现字符串反转

>>> 
>>> def str_reverse(string):
...     if '' == string:
...         return string
...     else:
...         return str_reverse(string[1:])+string[0]
... 
>>> str_reverse('Use Python.')
'.nohtyP esU'
>>> 

　　

 

赋值在字符串和列表中的区别 *****

>>> 
>>> a = 'standby'
>>> 
>>> b = 'standby'
>>> 
>>> a is b
True
>>> 
>>> id(a)
139908204403952
>>> id(b)
139908204403952
>>> 
>>> 
>>> 
>>> a = [1,2,3]
>>> b = [1,2,3]
>>> 
>>> a is b
False
>>> 
>>> id(a)
139908204413800
>>> id(b)
139908204248600
>>>