On Sat, Jan 16, 2016 at 01:19:16PM +0100, Peter Otten wrote:
> Steven D'Aprano wrote:
>
> > But slices are slightly different. When you provide two indexes in a
> > slice, they mark the gaps BETWEEN items:
>
> The other explanation that Python uses half-open intervals works for me.
Half-open at the start or the end? :-)
You're right, "half-open intervals" (open at the end) is also a good way
of thinking about it. It certainly helps with things like range(), which
don't involve slicing/cutting. You just memorize the the start value is
included and the end value is not.
If you think of indexes falling between elements, and slicing along
those gaps, then the natural consequence is a half-open interval. Take
mylist[1:6] for example:
0 1 2 3 4 5 6 7 8
mylist = [ a b c d e f g h ]
| |
cut here and here
By cutting *between* a and b, and f and g, you naturally get a half open
interval: mylist[1] is included, but mylist[6] is not.
But if you think of indexes being aligned with the items themselves, you
need to memorize a special rule "do I include the item or not?":
0 1 2 3 4 5 6 7
mylist = [ a b c d e f g h ]
| |
include exclude
this this
> > Now, what happens with *negative* indexes?
> >
> > mylist = [ 100, 200, 300, 400, 500 ]
> > indexes: ^ ^ ^ ^ ^ ^
> > -6 -5 -4 -3 -2 -1
> >
> > mylist[-5:-2] will be [200, 300, 400]. Easy.
>
> >>> mylist = [ 100, 200, 300, 400, 500 ]
> >>> mylist[-5:-2]
> [100, 200, 300]
>
> Off by one, you picked the wrong gaps.
Oops, so I did. You're absolutely right.
mylist = [ 100, 200, 300, 400, 500 ]
indexes: ^ ^ ^ ^ ^ ^
-5 -4 -3 -2 -1
The last index ought to be "-0" in some sense, but of course that's just
zero which is the first index. So as you say:
> Slightly related is a problem that comes up in practice; you cannot specify
> "including the last item" with negative indices:
But you can do so by leaving the end index blank:
py> mylist = [ 100, 200, 300, 400, 500 ]
py> mylist[-2:-1]
[400]
py> mylist[-2:0]
[]
py> mylist[-2:]
[400, 500]
--
Steve
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