To clarify my point: reverse() is a lucky one - Python has variants of *this particular* function both for lists and strings. Yet what about other list functions? How in general, can I write a function that works both on list and string types? Both are sequences, right? Why string is not a subtype of a list then? The advantage of string being a list of elements, where element is a char is that all list functions will work *without any modifications* on strings as well as on other types of lists. So, I am trying to understand: what is a rational for strings not being lists in Python?
Thanks, -- Dmitri O. Kondratiev [EMAIL PROTECTED] http://www.geocities.com/dkondr On 10/15/07, Dmitri O.Kondratiev <[EMAIL PROTECTED]> 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: > > 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? > > Thanks, > > Dmitri O. Kondratiev > [EMAIL PROTECTED] > http://www.geocities.com/dkondr > > On 10/15/07, 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: > > ... > > > > > > Gary Herron > > > > > > > > > > Thanks, > > > Dima > > > > > > >
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