<OT>answering to Dmitri O.Kondratiev<OT>
> On Sunday 14 October 2007 5:06:19 pm Dmitri O.Kondratiev wrote:
>
>>The function I wrote (below) reverses lists all right:
>>
>>def reverse(xs):
>> if xs == []:
>> return []
>> else:
>> return (reverse (xs[1:])) + [xs[0]]
>>
>>>>>reverse ([1,2,3])
>>[3, 2, 1]
>>
>>Yet when I try to reverse a string I get:
>>>>>reverse ("abc")
>> File "C:\wks\python-wks\reverse.py", line 5, in reverse
>> return (reverse (xs[1:])) + [xs[0]]
>> File "C:\wks\python-wks\reverse.py", line 5, in reverse
>> return (reverse (xs[1:])) + [xs[0]]
>> File "C:\wks\python-wks\reverse.py", line 2, in reverse
>> if xs == []:
>>RuntimeError: maximum recursion depth exceeded in cmp
>>
>>
>>What's wrong? Why recursion never stops?
Because an empty string doesn't compare equal to an empty list. So if xs
is not a list, the test always fails, and you always go to the 'else'
branch.
Now since in Python, most (and at least all builtins) empty containers
and/or sequence types evals to false in a boolen context, there's an
obvious fix to your function:
def reverse(xs):
if xs:
return xs
else:
return (reverse (xs[1:])) + [xs[0]]
Anyway, there's a much simpler solution:
reverse = lambda x : x[::-1]
which is so simple it doesn't even justify a function call !-)
Oh, and, yes, while we're at it:
"""
Help on class reversed in module __builtin__:
class reversed(object)
| reversed(sequence) -> reverse iterator over values of the sequence
"""
HTH
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