CODEWITHSUNDEEP
python
Shi Libin
Shi Libin

Reputation: 29

Why can we reverse a string by writing like this: str = str[::-1]?

Somebody told me you can reverse a string like this:

a='123456'
a = a[::-1]
print a

I just don't know how it works.

Upvotes: 1

Views: 136

Answers (3)

Stephen Diehl
Stephen Diehl

Reputation: 8419

In Python negative slicing wraps around, so -1 is the last element.

[1,2,3,4][-1] = 4

In slice notation the first two elements are the bounds and the third is the index increment. By default the increment is 1.

[1,2,3,4,5][1:4] == [2,3,4]
[1,2,3,4,5][1:4:2] == [2,4]

Python also lets omit the bound indices to refer to the whole list [::]. 

[1,2,3,4,5][::1] == [1,2,3,4,5]

So if your increment is negative you reverse the list by indexing backwards from the end. 

[1,2,3,4,5][::-1] == [5,4,3,2,1]

Strings implement the same iterable protocol as lists so instead of reversing elements in a list you are reversing characters in a string.

Upvotes: 2

John La Rooy
John La Rooy

Reputation: 304355

The third parameter is the step size. Try some different step sizes to get the idea

>>> a = '123456'
>>> a[::2]
'135'
>>> a[::3]
'14'
>>> a[::-3]
'63'
>>> a[::-2]
'642'
>>> a[::-1]
'654321'

Since the start and stop are left empty, Python will choose them to step along the whole string.

Upvotes: 2

user2555451
user2555451

Reputation:

That takes advantage of Python's slice notation. Basically, it returns a new string created by stepping backwards through the original string. See a demonstration below:

>>> mystr = "abcde"    
>>> mystr[::-1]
'edcba'
>>> for i in mystr[::-1]:
...     print i
...
e
d
c
b
a
>>>

The format for slice notation is [start:stop:step].

Upvotes: 4

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