[swift-evolution] (Draft) Add last(where:) and lastIndex(where:) methods
Dave Abrahams
dabrahams at apple.com
Sat May 21 16:53:49 CDT 2016
on Sat May 21 2016, Brent Royal-Gordon <brent-AT-architechies.com> wrote:
>>> Similarly, we shouldn't provide operations which are going to
>>> repeatedly seek elements near the tail of the list unless we're using
>>> a type which can access that tail efficiently. `last` is one
>>> thingâ€”it's only O(N). `lastIndex(of:)` is, I believe, O(n log n) in
>>> the case of an element that doesn't exist.
>>
>> I can't imagine where your O(N log N) comes from in this case, but even
>> if it is the right complexity that wouldn't be a reason not to provide
>> the operation.
>
> I'm imagining an implementation something like this:
>
> func lastIndex(of value: Element) -> Index? {
> let count = self.count
>
> for endOffset in 0..<count {
> let offset = count - endOffset - 1
> let i = index(startIndex, offsetBy: offset)
>
> if self[i] == value {
> return i
> }
> }
>
> return nil
> }
Oh, no!
extension Collection {
func lastIndex(where predicate: (Element)->Bool) -> Index? {
var result: Index? = nil
for i in indices {
if predicate(self[i]) { result = i }
}
return result
}
}
extension Collection where Element : Equatable {
func lastIndex(of value: Element) -> Index? {
return lastIndex(where: { $0 == value })
}
}
> `index(_:offsetBy:)` is O(N) for a ForwardCollection, and `offset`
> gets smaller as you execute the loop, so I *believe* this ends up
> being O(N log N). I was never that good at big-O notation, though, so
> I could be wrong about that. What I can say is that it would be O(N^2)
> if not for the decreasing size of `offset`, so it's smaller than that.
What you wrote is still (N*(N - 1))/2 steps, i.e. O(N^2), in the worst
case, unless I'm missing something.
>> I learned last week there's an O(N log N) in-place
>> reverse() for ForwardCollections, and I think we ought to have it. log
>> N is effectively a constant for most purposes.
>
> If you guys think it's okay, I'm not going to argue.
--
-Dave
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