On Oct 15, 2:30 am, Gary Herron <[EMAIL PROTECTED]> wrote:
> 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?
>
> If you are doing this as an python-learning exercise, then read on. If
> you are doing this reversal for real code, then try:
>
> xs.reverse() for in-place reversal of a list (but not a string), or
> result = xs[::-1] for creating a reversed copy of either a string or a
> list
>
> Your recursion stops when xs == [], but when you're stripping characters
> off a string, like 'abc', the remaining portion will be 'bc', then 'c',
> than '', but never [] so you 'll never stop.
>
> Try:
>
> if xs == []:
> return []
> elif xs == '':
> return ''
> else:
> ...
The 'else' clause also breaks for strings: the second operand is a
list, which cannot be concatenated to strings. Here's a version that
works for any type with the common slicing semantics:
def reverse(xs):
if not xs:
return xs
else:
return reverse(xs[1:]) + xs[:1]
print reverse([1,2,3])
print reverse((1,2,3))
print reverse('123')
George
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