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

[Dave Abrahams]

on Wed Jun 08 2016, gadiraju praneeth <praneethgadiraju-AT-gmail.com> wrote:

> 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> }

Yes, it's convenient.  Its efficiency is suboptimal, though, unless you
really need the results to be Arrays (rather than, say ArraySlices).

>
>
> 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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>>

-- 
Dave