on Wed Sep 28 2016, Robert Widmann <swift-evolution@swift.org> wrote:
> +1. One of those things where you wonder why this wasn't the default
> behavior.
FWIW, it's because what Nevin did below is easy to write, but getting
integer offsets isn't easy, at least not without introducing your own
counter state.
> ~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
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution@swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>
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--
-Dave
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