Velocity Reviews > Re: Newbi Q: Recursively reverse lists but NOT strings?

# Re: Newbi Q: Recursively reverse lists but NOT strings?

Kurt Smith
Guest
Posts: n/a

 10-15-2007
On 10/15/07, Gary Herron <(E-Mail Removed)> wrote:
> Dmitri O.Kondratiev wrote:
> > Gary, thanks for lots of info!
> > Python strings are not lists! I got it now. That's a pity, I need two
> > different functions: one to reverse a list and one to reverse a string:

> True, they are not both lists, but they *are* both sequences, with some
> things in common. In particular xs[::-1] will reverse both types of
> objects. And even if you roll you own reversal function, you don't need
> two. One will do.
>
> Gary Herron
>
> >
> > def reverseList(xs):
> > if xs == []:
> > return xs
> > else:
> > return (reverseList (xs[1:])) + [xs[0]]
> >
> > def reverseStr(str):
> > if str == "":
> > return str
> > else:
> > return (reverseStr (str[1:])) + str[0]
> >
> > Ok. Now regarding in-place reversal of a list:
> >
> > >>> l = [1,2,3]
> > >>> l

> > [1, 2, 3]
> > >>> l.reverse()
> > >>> l

> > [3, 2, 1]
> >
> > That was, as I expected. Good.
> >
> > Then why this ? :
> >
> > >>> ls = [1,2,3].reverse()
> > >>> ls
> > >>>
> > >>> print [1,2,3].reverse()

> > None
> > >>>

> > I mean, why ls is empty after assignment?
> >
> > Also, I couldn't find in the Python docs what this form of slicing means:
> > xs[::-1] ?
> >
> > It works for creating a reversed copy of either a string or a list,
> > but what does '::-1' syntax means?

mylist[::-1] is interpreted as mylist[slice(None,None,-1)], and the
slice object has a method, 'indices' that computes the right endpoints
for what you want to do.

So:

>>> myseq = (1,2,3)
>>> myseq[::-1]

(3, 2, 1)
>>> myseq[slice(None,None,-1)]

(3, 2, 1)
>>> myseq[None:None:-1]

(3, 2, 1)

etc.

> >
> > Thanks,
> >
> > Dmitri O. Kondratiev
> > http://www.velocityreviews.com/forums/(E-Mail Removed) <(E-Mail Removed)>
> > http://www.geocities.com/dkondr
> >
> > On 10/15/07, *Gary Herron* < (E-Mail Removed)
> > <(E-Mail Removed)>> 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:
> > ...
> >
> >
> > Gary Herron
> >
> >
> > >
> > > Thanks,
> > > Dima

> >
> >
> >
> >

>
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