Python学习--字符串slicing

Found this great table at http://wiki.python.org/moin/MovingToPythonFromOtherLanguages

Python indexes and slices for a six-element list. Indexes enumerate the elements, slices enumerate the spaces between the elements. Index from rear: -6 -5 -4 -3 -2 -1 a=[0,1,2,3,4,5] a[1:]==[1,2,3,4,5] Index from front: 0 1 2 3 4 5 len(a)==6 a[:5]==[0,1,2,3,4] +---+---+---+---+---+---+ a[0]==0 a[:-2]==[0,1,2,3] | a | b | c | d | e | f | a[5]==5 a[1:2]==[1] +---+---+---+---+---+---+ a[-1]==5 a[1:-1]==[1,2,3,4] Slice from front: : 1 2 3 4 5 : a[-2]==4 Slice from rear: : -5 -4 -3 -2 -1 : b=a[:] b==[0,1,2,3,4,5] (shallow copy of a)

In python 2.7

Slicing in python

[a:b:c]

len = length of string, tuple or list

c -- default is +1. sign of c indicates forward or backward, absolute value of c indicates steps. Default is forward with step size 1. Positive means forward, negative means backward.

a -- when c is positive or blank, default is 0. when c is negative, default is -1.

b -- when c is positive or blank, default is len. when c is negative, default is -(len+1).

Understanding index assignment is very important.

In forward direction, starts at 0 and ends at len-1

In backward direction, starts at -1 and ends at -len

when you say [a:b:c] you are saying depending on sign of c (forward or backward), start at a and end at b ( excluding element at bth index). Use the indexing rule above and remember you will only find elements in this range

-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1

but this range continues in both directions infinitely

...,-len -2 ,-len-1,-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1, len, len +1, len+2 , ....

e.g.

 0 1 2 3 4 5 6 7 8 9 10 11 a s t r i n g -9 -8 -7 -6 -5 -4 -3 -2 -1

if your choice of a , b and c allows overlap with the range above as you traverse using rules for a,b,c above you will either get a list with elements (touched during traversal) or you will get an empty list.

One last thing: if a and b are equal , then also you get an empty list

>>> l1
[2, 3, 4]

>>> l1[:]
[2, 3, 4]

>>> l1[::-1] # a default is -1 , b default is -(len+1)
[4, 3, 2]

>>> l1[:-4:-1] # a default is -1
[4, 3, 2]

>>> l1[:-3:-1] # a default is -1 
[4, 3]

>>> l1[::] # c default is +1, so a default is 0, b default is len
[2, 3, 4]

>>> l1[::-1] # c is -1 , so a default is -1 and b default is -(len+1)
[4, 3, 2]


>>> l1[-100:-200:-1] # interesting
[]

>>> l1[-1:-200:-1] # interesting
[4, 3, 2]


>>> l1[-1:5:1]
[4]

>>> l1[-1:-1:1]
[]


>>> l1[-1:5:1] # interesting
[4]


>>> l1[1:-7:1]
[]

>>> l1[1:-7:-1] # interesting
[3, 2]