+1. One of those things where you wonder why this wasn't the default behavior.
~Robert Widmann
2016/09/28 14:23、Nevin Brackett-Rozinsky via swift-evolution
<swift-evolution@swift.org> のメッセージ:
> +1, I have been mildly surprised that this was not already present.
>
> My workaround heretofore has been:
>
> for idx in abc.indices {
> let val = abc[i]
> // do something with idx and val
> }
>
> Nevin
>
>
>> On Wed, Sep 28, 2016 at 1:55 PM, Erica Sadun via swift-evolution
>> <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
>>
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>>
>
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