On Oct 15, 3:03 pm, "Matt McCredie" <[EMAIL PROTECTED]> wrote: > On 10/15/07, Dmitri O.Kondratiev <[EMAIL PROTECTED]> wrote: > > > > > 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 > > The example you posted won't work with tuples either because they, > like strings, are also immutable. So, the best way to get the posted > code to work (which is a bad way to go about reversing a string, but I > digress) is to cast the input parameter to a list first. The returned > value will always be a list, but you will simply have to convert it > back to the appropriate type when you are done. > > What is the purpose if immutability? It allows a value to be hashed. I > don't want to get into a discussion about methods for hashing mutable > types, if you are interested just do a search on the list archives. > Hashing allows for quick comparisons of values, but more importantly > it allows for values to be used as keys for the dict type. This is > very important because, as you will soon find out if you keep learning > the language, all namespaces in python are implemented as dicts. Good explanation, but basically strings are immutable so they can be used in dicts. > > So... if you want a mutable string, just cast it to a list, do your > operations and cast it back to a string. > > Incidentally, the proper method for converting a list of characters to > a string is by using the join method on an empty string. > > >>> s = "I am a string" > >>> x = list(s) > >>> x > > ['I', ' ', 'a', 'm', ' ', 'a', ' ', 's', 't', 'r', 'i', 'n', 'g']>>> > "".join(x) > > 'I am a string' > > Matt
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