Dave, perhaps we could use "^" as an anchor point for the start index and $ as the anchor point for the end index? It's familiar to anyone who knows a bit of regex, and all vim users. My main worry would be ^ is already infix xor operator.
On Fri, Dec 18, 2015 at 5:43 PM Paul Ossenbruggen via swift-evolution < swift-evolution@swift.org> wrote: > 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@swift.org> wrote: > > > On Dec 18, 2015, at 1:46 PM, Joe Groff via swift-evolution < > swift-evolution@swift.org> wrote: > > > On Dec 18, 2015, at 4:42 AM, Amir Michail via swift-evolution < > swift-evolution@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 > > > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution > > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution >
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