I would like to avoid what you currently have to do for iterating a
subcontainer.
for a in container[0..container.count-4] {
// do something.
}
The slicing syntax would certainly help in these common situations. Maybe there
are easy ways that I am not aware of.
- Paul
> On Dec 18, 2015, at 2:39 PM, Dave Abrahams via swift-evolution
> <[email protected]> wrote:
>
>>
>> On Dec 18, 2015, at 1:46 PM, Joe Groff via swift-evolution
>> <[email protected] <mailto:[email protected]>> wrote:
>>
>>>
>>> On Dec 18, 2015, at 4:42 AM, Amir Michail via swift-evolution
>>> <[email protected] <mailto:[email protected]>> wrote:
>>>
>>> Examples:
>>>
>>> >>> l=[1,2,3,4,5]
>>> >>> l[-1]
>>> 5
>>> >>> l[-2]
>>> 4
>>> >>> l[2:4]
>>> [3, 4]
>>> >>> l[2:]
>>> [3, 4, 5]
>>> >>> l[-2:]
>>> [4, 5]
>>> >>> l[:3]
>>> [1, 2, 3]
>>> >>> l[::2]
>>> [1, 3, 5]
>>> >>> l[::]
>>> [1, 2, 3, 4, 5]
>>
>> Accepting negative indices is problematic for two reasons: it imposes
>> runtime overhead in the index operation to check the sign of the index;
>> also, it masks fencepost errors, since if you do foo[m-n] and n is
>> accidentally greater than m, you'll quietly load the wrong element instead
>> of trapping. I'd prefer something like D's `$-n` syntax for explicitly
>> annotating end-relative indexes.
>
> Yes, we already have facilities to do most of what Python can do here, but
> one major problem IMO is that the “language” of slicing is so non-uniform: we
> have [a..<b], dropFirst, dropLast, prefix, and suffix. Introducing “$” for
> this purpose could make it all hang together and also eliminate the “why does
> it have to be so hard to look at the 2nd character of a string?!” problem.
> That is, use the identifier “$” (yes, that’s an identifier in Swift) to
> denote the beginning-or-end of a collection. Thus,
>
> c[c.startIndex.advancedBy(3)] => c[$+3] // Python: c[3]
> c[c.endIndex.advancedBy(-3)] => c[$-3] // Python: c[-3]
> c.dropFirst(3) => c[$+3...] // Python: c[3:]
> c.dropLast(3) => c[..<$-3] // Python: c[:-3]
> c.prefix(3) => c[..<$+3] // Python: c[:3]
> c.suffix(3) => c[$-3...] // Python: c[-3:]
>
> It even has the nice connotation that, “this might be a little more expen$ive
> than plain indexing” (which it might, for non-random-access collections). I
> think the syntax is still a bit heavy, not least because of “..<“ and “...”,
> but the direction has potential.
>
> I haven’t had the time to really experiment with a design like this; the
> community might be able to help by prototyping and using some alternatives.
> You can do all of this outside the standard library with extensions.
>
> -Dave
>
>
>
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