I’m fairly sure that MutableCollection mutations are not allowed to invalidate any indices (for this exact reason). It would be great if someone could confirm this.
> One (potentially dumb) question... > > If the actual indices are needed inside the loop, presumably this means they > will be used in the loop, perhaps to mutate it, and if the collection is > mutated won't that either invalidate (or change the relative correspondence > of) the others indices, because the collection accessed in the loop will be > the mutated one whereas the indices refer to the copy that was taken at the > start when the 'for' statement was evaluated? > > > > On Wed, 28 Sep 2016 at 19:34 Sean Heber via > swift-evolution<swift-evolution@swift.org(mailto:swift-evolution@swift.org)>wrote: > > This might just be me being silly, but is there any way to be able to do > > something like this instead: > > > > for (index, value) in sequence { > > } > > > > Maybe by adding another variant of makeIterator() that only differs by the > > return type or something like that? > > > > I sort of dislike that enumerated() and indexed() would co-exist and > > potentially lead to really subtle bugs when getting them confused. > > Obviously removing enumerated() would be a breaking change, though, and > > maybe it has valuable uses that I’m not really thinking about (although it > > seems to me that the index/value pair is what you want like, 99% of the > > time and plenty of people - myself included - have been using the index of > > enumerated() as an array index even though that’s technically maybe not > > quite ‘correct'). > > > > l8r > > Sean > > > > > > >On Sep 28, 2016, at 12:55 PM, Erica Sadun via > > >swift-evolution<swift-evolution@swift.org(mailto:swift-evolution@swift.org)>wrote: > > > > > >Gist here:https://gist.github.com/erica/2b2d92e6db787d001c689d3e37a7c3f2 > > > > > >Introducing indexed() collections > > > > > >• Proposal: TBD > > >• Author: Erica Sadun, Nate Cook, Jacob Bandes-Storch, Kevin Ballard > > >• Status: TBD > > >• Review manager: TBD > > >Introduction > > > > > >This proposal introduces indexed() to the standard library, a method on > > >collections that returns an (index, element) tuple sequence. > > > > > >Swift-evolution thread: TBD > > > > > >Motivation > > > > > >The standard library's enumerated() method returns a sequence of pairs > > >enumerating a sequence. The pair's first member is a monotonically > > >incrementing integer starting at zero, and the second member is the > > >corresponding element of the sequence. When working with arrays, the > > >integer is coincidentally the same type and value as an Array index but > > >the enumerated value is not generated with index-specific semantics. This > > >may lead to confusion when developers attempt to subscript a non-array > > >collection with enumerated integers. It can introduce serious bugs when > > >developers use enumerated()-based integer subscripting with non-zero-based > > >array slices. > > > > > >Indices have a specific, fixed meaning in Swift, which are used to create > > >valid collection subscripts. This proposal introduces indexed() to produce > > >a more semantically relevant sequence by pairing a collection's indices > > >with its members. While it is trivial to create a solution in Swift, the > > >most common developer approach shown here calculates indexes twice: > > > > > >extension Collection { > > >/// Returns a sequence of pairs (*idx*, *x*), where *idx* represents a > > >/// consecutive collection index, and *x* represents an element of > > >/// the sequence. > > >func indexed() ->Zip2Sequence<Self.Indices, Self>{ > > >return zip(indices, self) > > >} > > >} > > > > > >Incrementing an index in some collections can be unnecessarily costly. In > > >a lazy filtered collection, an index increment is potentially O(N). We > > >feel this is better addressed introducing a new function into the Standard > > >Library to provide a more efficient design that avoids the attractive > > >nuisance of the "obvious" solution. > > > > > >Detailed Design > > > > > >Our vision of indexed() bypasses duplicated index generation with their > > >potentially high computation costs. We'd create an iterator that > > >calculates each index once and then applies that index to subscript the > > >collection. Implementation would take place through IndexedSequence, > > >similar to EnumeratedSequence. > > > > > >Impact on Existing Code > > > > > >This proposal is purely additive and has no impact on existing code. > > > > > >Alternatives Considered > > > > > >Not yet > > >_______________________________________________ > > >swift-evolution mailing list > > >swift-evolution@swift.org(mailto:swift-evolution@swift.org) > > >https://lists.swift.org/mailman/listinfo/swift-evolution > > > > _______________________________________________ > > swift-evolution mailing list > > swift-evolution@swift.org(mailto: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
