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From: Joan Miller on 5 Mar 2010 13:01
What does a slice as [N::-1] ?

It looks that in the first it reverses the slice and then it shows
only N items, right?

Could you add an example to get the same result without use `::` to
see it more clear?

Thanks in advance
From: Arnaud Delobelle on 5 Mar 2010 13:12
Joan Miller <peloko45(a)gmail.com> writes:

> What does a slice as [N::-1] ?
>
> It looks that in the first it reverses the slice and then it shows
> only N items, right?
>
> Could you add an example to get the same result without use `::` to
> see it more clear?
>
> Thanks in advance

>>> l = range(10)
>>> l
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> l[7::-1]
[7, 6, 5, 4, 3, 2, 1, 0]
>>> [l[i] for i in range(7, -1, -1)]
[7, 6, 5, 4, 3, 2, 1, 0]

--
Arnaud
From: Steven D'Aprano on 5 Mar 2010 13:22
On Fri, 05 Mar 2010 10:01:40 -0800, Joan Miller wrote:

> What does a slice as [N::-1] ?

Why don't you try it?

>>> s = "abcdefgh"
>>> s[4::-1]
'edcba'

The rules for extended slicing are not explained very well in the docs,
and can be confusing. In my experience, apart from [::-1] it is best to
always use a positive stride (the third number).


--
Steven
From: Steven D'Aprano on 5 Mar 2010 13:28
On Fri, 05 Mar 2010 18:12:05 +0000, Arnaud Delobelle wrote:

>>>> l = range(10)
>>>> l
> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>> l[7::-1]
> [7, 6, 5, 4, 3, 2, 1, 0]
>>>> [l[i] for i in range(7, -1, -1)]
> [7, 6, 5, 4, 3, 2, 1, 0]

Where does the first -1 come from? Slices are supposed to have default
values of 0 and len(seq):

>>> l[7::1]
[7, 8, 9]
>>> [l[i] for i in range(7, len(l), 1)]
[7, 8, 9]
>>> [l[i] for i in range(7, len(l), -1)]
[]


I don't believe the actual behaviour is documented anywhere.


--
Steven
From: Mensanator on 5 Mar 2010 13:33
On Mar 5, 12:01 pm, Joan Miller <pelok...(a)gmail.com> wrote:
> What does a slice as [N::-1] ?

Starts at position N and returns all items to the start of the
list in reverse order.

>
> It looks that in the first it reverses the slice and then it shows
> only N items, right?

Wrong. It shows N+1 items. Remember, counting starts from 0.

>
> Could you add an example to get the same result without use `::` to
> see it more clear?

for i in range(8,-1,-1):print(a[i],end=' ')

although I doubt this is more clear.

>
> Thanks in advance

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