Reversing a list using recursion does not give the expected output

Question

I'm looking to reverse a list using recursion in python. I succeeded in doing so but the output isn't exactly what I wanted. trying to figure out where i'm going wrong.

def revlist(ls):
    newlist = []
    n = len(ls)
    if n == 1:
        return ls[-1]
    else:
        return ls[-1],revlist(ls[:-1])

This is my output.

revlist([1,2,3,4])
(4, (3, (2, 1)))

What i'm really hoping to get is this:

revlist([1,2,3,4])
(4,3,2,1)

You're getting exactly what you asked for; your function returns either a single element or a tuple of the current last element and the rest of the iterable reversed. If you want to flatten it out, you will need to do so explicitly. — jonrsharpe
– jonrsharpe, Commented May 24, 2015 at 12:40

roippi · Accepted Answer · 2015-05-24 12:40:58Z

1

You're closeish.

def revlist(ls):
    newlist = []
    n = len(ls)
    if n == 1:
        return [ls[-1]]
    else:
        return [ls[-1]] + revlist(ls[:-1])

Or cleaned up:

def revlist(ls):
    if len(ls) < 1:
        return []
    else:
        return [ls[-1]] + revlist(ls[:-1])

Basically, you need to always return a list from your function, and append recursive calls appropriately.

answered May 24, 2015 at 12:40

roippi

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