[swift-evolution] Proposal: Python's indexing and slicing

[Paul Ossenbruggen]

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 <swift-evolution at swift.org> wrote:
> 
>> 
>> On Dec 18, 2015, at 1:46 PM, Joe Groff via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> 
>>> On Dec 18, 2015, at 4:42 AM, Amir Michail via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> 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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