[swift-users] Subsequences and shared indices

[Dave Abrahams]

on Thu Oct 13 2016, Tim Vermeulen <swift-users-AT-swift.org> wrote:

> Is it a requirement that collections share indices with its
> subsequence? 

Yes.

> Array and ArraySlice do share indices, which is why ArraySlice isn’t
> zero-based, and I think this is convenient. But String.CharacterView
> doesn’t seem to share indices with its subsequence (which is
> String.CharacterView as well). 

That's a bug.

> Consider this example:
>
> let foo = "foobar".characters
>
> let index = foo.index(foo.startIndex, offsetBy: 3)
> let bar = foo.suffix(from: index)                   // "bar"
>
> foo[index]                                          // "b" :)
> foo[bar.startIndex]                                 // "f" :(
>
> So does this mean that we can’t assume that collections and their
> subsequences share their indices (which could be very handy), or is
> this just a bug related to String.CharacterView?

The latter.

-- 
-Dave