lang comparison: in-place algorithm for reversing a list in Perl, Python, Lisp

Steven D'Aprano steve+comp.lang.python at pearwood.info
Thu Mar 1 06:01:30 CET 2012


On Wed, 29 Feb 2012 20:07:49 -0800, Xah Lee wrote:

> Here's in-place algorithm for reversing a list:
> 
> # python
> # in-place algorithm for reversing a list
> 
> list_a = ["a", "b", "c", "d", "e", "f", "g"]
> list_length = len(list_a)
> for i in range(list_length/2):
>     x = list_a[i]
>     list_a[i] = list_a[ list_length -1 - i] 
>     list_a[ list_length -1 - i] = x
> 
> print list_a

This is a good example of code written by somebody not very familiar with 
Python idioms. You don't need a temporary variable to swap two values in 
Python. A better way to reverse a list using more Pythonic idioms is:

for i in range(len(list_a)//2):
    list_a[i], list_a[-i-1] = list_a[-i-1], list_a[i]


But the best way (even more idiomatic and much, much faster) is this:

list_a.reverse()



-- 
Steven



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