On Thu, 4 May 2023 at 07:43, Thomas Passin <li...@tompassin.net> wrote:
>
> On 5/3/2023 3:46 PM, Oscar Benjamin wrote:
> > On Wed, 3 May 2023 at 18:52, Thomas Passin <li...@tompassin.net> wrote:
> >>
> >> On 5/3/2023 5:45 AM, fedor tryfanau wrote:
> >>> I've been using python as a tool to solve competitive programming problems
> >>> for a while now and I've noticed a feature, python would benefit from
> >>> having.
> >>> Consider "reversed(enumerate(a))". This is a perfectly readable code,
> >>> except it's wrong in the current version of python. That's because
> >>> enumerate returns an iterator, but reversed can take only a sequence type.
> >>
> >> Depending on what you want to give and receive, enumerate(reversed(a))
> >> will do the job here. Otherwise list() or tuple() can achieve some of
> >> the same things.
> >
> > I don't think that is equivalent to the intended behaviour:
> >
> > reversed(enumerate(a)) # zip(reversed(range(len(a))), reversed(a))
> > enumerate(reversed(a)) # zip(range(len(a)), reversed(a))
>
> I don't think we know the intended behavior here. The OP did not say
> what type of object should be returned. He only wanted an expression
> that would run. Apparently the result should be an enumeration of
> variable "a" but with "a" reversed. Is "a" supposed to be a sequence?
> An iterator? Presumably the result was expected to be an enumeration,
> which is to say an iterator, and enumerate(reversed(a)) would return an
> iterator.
The part you're probably missing here can be easily shown:
>>> a = [10, 20, 30, 40, 50]
>>> for i, n in enumerate(reversed(a)):
... print("E-R", i, n)
...
E-R 0 50
E-R 1 40
E-R 2 30
E-R 3 20
E-R 4 10
>>> for i, n in reversed(list(enumerate(a))):
... print("R-E", i, n)
...
R-E 4 50
R-E 3 40
R-E 2 30
R-E 1 20
R-E 0 10
So a closer equivalent would be:
>>> for i, n in zip(reversed(range(len(a))), reversed(a)):
... print("Zip", i, n)
...
Zip 4 50
Zip 3 40
Zip 2 30
Zip 1 20
Zip 0 10
Note that it's reversing both the indices and the values.
ChrisA
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