python切片总结

切片的结构

object[star:end:step]

star:开始 end：结束 step：步长的默认值是1，可为负数，默认方向是从左往右，附属代表从右往左

1.获取单个值

 a[0]
0
 a[-4]
6

2.切割完整对象

>>> a[:]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[::]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[::-1]#-1 从右往左
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

3.star和end全为正数的情况下

>>> a[1:6]
[1, 2, 3, 4, 5]
>>> a[1:6:-1] 
[]
>>> a[:6]
[0, 1, 2, 3, 4, 5]
>>> a[6:]
[6, 7, 8, 9]
>>> a[6::-1]
[6, 5, 4, 3, 2, 1, 0]

4.star和end全部为负数的情况下

>>> a[-1:-6]
[]
>>> a[-1:-6:-1]
[9, 8, 7, 6, 5]
>>> a[:-6]
[0, 1, 2, 3]
>>> a[-6:-1]
[4, 5, 6, 7, 8]
>>> a[-6:]
[4, 5, 6, 7, 8, 9]
>>> a[-6::-1]
[4, 3, 2, 1, 0]

5.star和end分别为正数和负数的情况下

>>> a = [0,1,2,3,4,5,6,7,8,9]
>>> a[1:-6]
[1, 2, 3]
>>> a[-1:-6]
[]
>>> a[1:-6:-1]
[]
>>> a[-1:6]
[]
>>> a[-1:6:-1]
[9, 8, 7]

6.连续切片操作

>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[:8][2:5][-1:]
[4]
a[:8]---- [0 1 2 3 4 5 6 7]---[2:5] ---[2,3,4]--[-1:]---[4]

7.其他对象的切片操作

>>> t = (1,2,3,4)
>>> t[1:3]
(2, 3)
>>> s = "ABCDEFG"
>>> s[1:3]
'BC'
>>> for i in range(1,100)[2::3][-10:]:
... print(i)
...
72
75
78
81
8487
90
93
96
99

8.切片的三个参数是表达式

>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[2+1:3*2:7%3] #a[3:6:1]
[3, 4, 5]

9.取奇数位和偶数位

a[1::2]
[1, 3, 5, 7, 9]
>>> a[::2]
[0, 2, 4, 6, 8]

list=[1,2,3,4,5,6,7]

list[3] ---- [4]
list[2:4] ----[3,4]
list[:5] ----[1,2,3,4,5]
list[3:] -----[4,5,6,7]
list[::-1] ------[7,6,5,4,3,2,1]