Why does swapping fail in python?

Question

I was trying to write a program that would reverse a list in python3. I first tried:

def reverse(lst):
    """ Reverses lst in place.
    >>> x = [3, 2, 4, 5, 1]
    >>> reverse(x)
    >>> x
    [1, 5, 4, 2, 3]
    """
    n = len(lst)
    for i in range(n//2):
        lst[i], lst[n-i-1] = lst[n-i-1], lst[i]

It failed and the x I got was the original value. However, when I changed my code to this, it worked:

def reverse(lst):
    """ Reverses lst in place.
    >>> x = [3, 2, 4, 5, 1]
    >>> reverse(x)
    >>> x
    [1, 5, 4, 2, 3]
    """
    n = len(lst)
    for i in range(n//2):
        temp = lst[i]
        lst[i] = lst[n-i-1]
        lst[n-i-1] = temp

Your original code works just fine.

Lennart Regebro
– Lennart Regebro

2013-10-09 08:49:48 +00:00
Commented Oct 9, 2013 at 8:49 — Lennart Regebro
– Lennart Regebro, Commented Oct 9, 2013 at 8:49

falsetru · Accepted Answer · 2013-10-09 08:49:36Z

2

It works as expected:

>>> def reverse(lst):
...     n = len(lst)
...     for i in range(n//2):
...         lst[i], lst[n-i-1] = lst[n-i-1], lst[i]
...
>>> lst = [1,2,3,4,5]
>>> reverse(lst)
>>> print(lst)
[5, 4, 3, 2, 1]

BTW, why don't you use list.reverse?

>>> lst = [1,2,3]
>>> lst.reverse()
>>> lst
[3, 2, 1]

answered Oct 9, 2013 at 8:49

falsetru

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1 Comment

xiadaomike Over a year ago

Yeah...worked for me now..I don't know what happened there.. This was just a problem I tried to solve.

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