[swift-evolution] Proposal: Filter split extension on Sequence to return tuple of sequences that meet criteria and that do not

[Thorsten Seitz]

+1 from me for adding this method to the standard library.
I would prefer the name `partition(by:)` for it, though, under which I know it from other languages and which is quite fitting, I think.

-Thorsten 

> Am 08.06.2016 um 22:59 schrieb gadiraju praneeth via swift-evolution <swift-evolution at swift.org>:
> 
> I added an extension to do this, something like this:
> extension Array {
>     func filterSplit(includeElement: (Element) -> Bool) -> ([Element], [Element]) {
>         var elementsSatisfyingCondition = [Element]()
>         var elementsNotSatisfyingCondition = [Element]()
>         
>         self.forEach { element in
>             if includeElement(element) { elementsSatisfyingCondition.append(element) }
>             else { elementsNotSatisfyingCondition.append(element) }
>         }
>         
>         return (elementsSatisfyingCondition, elementsNotSatisfyingCondition)
>     }
> }
> 
> that way you can right away do: values.filterSplit { <yourPredicateBlock> }
> 
> 
>> On Wed, Jun 8, 2016 at 3:40 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> on Wed Jun 08 2016, Dave Abrahams <swift-evolution at swift.org> wrote:
>> 
>> > on Wed Jun 08 2016, gadiraju praneeth <swift-evolution at swift.org> wrote:
>> >
>> >> Many times, I came across a scenario where I had to filter an array with a
>> >> condition and filter the same array with opposite of that condition. For
>> >> example:
>> >>
>> >> let values = [2, 4, 3, 5, 6, 9]
>> >>
>> >> let divisibleByTwo = values.filter { $0 % 2 == 0 }
>> >> let notDivisibleByTwo = values.filter { $0 % 2 != 0 }
>> >>
>> >> Is there a way currently where we can filter the array into two arrays
>> >> based on a condition?
>> >
>> > Well, you need a stable partition for this if you care about ordering
>> > (see
>> > https://github.com/apple/swift/blob/master/test/Prototypes/Algorithms.swift#L299)
>> > but then you can do
>> >
>> > var parts = values
>> > let mid = values.stablePartition { $0 % 2 == 0 }
>> > let divisibleByTwo = parts.prefix(upTo: mid)
>> > let notDivisibleByTwo = parts.suffix(from: mid)
>> >
>> > Nate Cook has an enhancement to the result of stablyPartitioned in that
>> > prototype that would let you write:
>> >
>> >   let parts = values.stablyPartitioned { $0 % 2 == 0 }
>> >   let divisibleByTwo = parts.prefix(upTo: parts.partitionPoint)
>> >   let notDivisibleByTwo = parts.suffix(from: parts.partitionPoint)
>> 
>> Hmm, come to think of it, Nate, maybe there should also be a more
>> convenient way to get the two partitions from the result.
>> 
>> --
>> Dave
>> 
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