Op 2005-12-11, Bengt Richter schreef <[EMAIL PROTECTED]>: > On 10 Dec 2005 12:07:12 -0800, "Devan L" <[EMAIL PROTECTED]> wrote: > >> >>Antoon Pardon wrote: >>> On 2005-12-10, Duncan Booth <[EMAIL PROTECTED]> wrote: >>[snip] >>> >> I also think that other functions could benefit. For instance suppose >>> >> you want to iterate over every second element in a list. Sure you >>> >> can use an extended slice or use some kind of while. But why not >>> >> extend enumerate to include an optional slice parameter, so you could >>> >> do it as follows: >>> >> >>> >> for el in enumerate(lst,::2) >>> > >>> > 'Why not'? Because it makes for a more complicated interface for something >>> > you can already do quite easily. >>> >>> Do you think so? This IMO should provide (0,lst[0]), (2,lst[2]), >>> (4,lst[4]) ... >>> >>> I haven't found a way to do this easily. Except for something like: >>> >>> start = 0: >>> while start < len(lst): >>> yield start, lst[start] >>> start += 2 >>> >>> But if you accept this, then there was no need for enumerate in the >>> first place. So eager to learn something new, how do you do this >>> quite easily? >> >>>>> lst = ['ham','eggs','bacon','spam','foo','bar','baz'] >>>>> list(enumerate(lst))[::2] >>[(0, 'ham'), (2, 'bacon'), (4, 'foo'), (6, 'baz')] >> >>No changes to the language necessary. >> > Or, without creating the full list intermediately, > > >>> lst = ['ham','eggs','bacon','spam','foo','bar','baz'] > >>> import itertools > >>> list(itertools.islice(enumerate(lst), 0, None, 2)) > [(0, 'ham'), (2, 'bacon'), (4, 'foo'), (6, 'baz')]
As far as I understand use of this idiom can turn an O(n) algorithm into an O(n^2) algorithm. Suppose I have a list with 10 000 elements and I want the sum of the first 100, the sum of the second 100 ... One way to do that would be: for i in xrange(0,10000,100): sum(itertools.islice(lst, i, i+100)) But itertools.islice would each time start from the begining of the list and iterate over i elements before giving 100 elements to sum. Which would make this implementation O(n^2) instead of O(n). -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
