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]