Hi,
AFAIK, these all would be very much sugar for:
1) (1 ..< 5).reversed()
2) The same as above: (1 ... 5).reversed() does exactly what you want.
3) Kind of makes sense to me to include >.., though to be thorough, you'd need
to add >.< as well - that's a weird looking operator...
4) There's a global function for this: stride(from: 1, to: 10, by: 2)
Charlie
> On Sep 21, 2016, at 9:49 AM, Mr Bee via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> Hi,
>
> I'm a Swift newbie so forgive me if my question sounds a bit silly, or
> perhaps had been asked previously. My question is, why Swift doesn't have
> built-in operator for backward or decremented range? Especially to be used in
> for-in loop.
>
> Swift only has ... (closed range) and ..< (half-opened range) that both go
> forward (incremented). To use backward (decremented) range, we have to use
> several techniques (stride, sequence, etc). Of course those techniques work
> well. But I don't think they're very swifty, so to say.
>
> So, is it possible to propose a new operator and a modification to existing
> operator regarding range? Would such proposal be considered and implemented
> after Swift 3? If it is, here's my pre-proposal about it.
>
> 1. Add new ..> operator that goes backward (decremented) as companion to ..<
> operator that goes forward (incremented).
>
> Example:
> for i in 1 ..< 5 { print(i) }
> will prints: 1 2 3 4
> for i in 5 ..> 1 { print(i) }
> will prints: 5 4 3 2
>
> 2. Modify the ... operator to be able to go both ways. If the left operand is
> greater than the right, it goes backward (decremented). While if the left
> operand is less than the right, it goes forward (incremented).
>
> Example:
> for i in 1 ... 5 { print(i) }
> will prints: 1 2 3 4 5
> for i in 5 ... 1 { print(i) }
> will prints: 5 4 3 2 1
>
> 3. Add new pair of operator to accompany the ..> and ..< (half opened range
> on the right side) pair operator. It's >.. and <.. operators which are half
> opened range on the left side. As you might guess, the >.. is the opposite of
> ..> operator and the <.. is the opposite of the ..< operator.
>
> Example:
> for i in 1 <.. 5 { print(i) }
> will prints: 2 3 4 5
> for i in 5 >.. 1 { print(i) }
> will prints: 4 3 2 1
>
> 4. I would like to go even further by introducing a new attribute into for-in
> loop syntax. It's `step` keyword which is used to define the interval of the
> loop (like BASIC). Of course this additional attribute only works if the
> range is countable or indexable. If not, the compiler should complain.
>
> Example:
> for i in 1 ... 10 step 2 { print(i) }
> will prints: 1 3 5 7 9
> for i in 10 ... 0 step 2 { print(i) }
> will prints: 10 8 6 4 2 0
> for i in 1 ..< 9 step 3 { print(i) }
> will prints: 1 3 6
> // note: 9 is omitted since it's on the opened side.
> for i in 9 >.. 1 step 2 { print(i) }
> will prints: 7 5 3 1
> // note: 9 is omitted since it's on the opened side.
>
> I hope you get the idea. I think such a rich for-in loop syntax would make
> Swift smarter, more robust, and easier to be learned and understood. They're
> also required since the flexibiliy of c-style for-loop is no longer available
> from Swift 3 and on. Decremented or backward range is sometimes needed in
> some algorithms. Making it built into the language would be better, instead
> of using non-language solutions. This proposal also doesn't break old codes
> since it doesn't change the old behavior.
>
> If such proposal is possible to be implemented —or at least considered— in
> the next version of Swift, I'll do the work of the formal proposal on GitHub.
> If it's not, well, I don't want to spend my time doing something that will be
> ignored.
>
> Thank you.
>
> Regards,
>
> –Mr Bee
>
> PS. I apologize if my English isn't well enough. I hope you all understand
> what I meant. :)
>
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