[swift-evolution] [Pitch] remove(at: Set<Index>)

[Saagar Jha]

This isn’t actually that complex, especially if you ditch the “C-style” for
loop algorithm and switch it to, as you mentioned, “filtering code”. filter,
enumerated (to get indices), and map (to go back to elements) are more than
up to the task. Plus, this is much more efficient.

var myArray = [0, 1, 2]
let indices: Set = [0, 1]
myArray = myArray.enumerated().filter {
    return !indices.contains($0.offset)
}.map {
    return $0.element // to get the elements back
}
print(myArray) // prints “[2]"

Adding it to the standard library might be useful, but it’s not as hard as
it looks.


On Sat, Jun 18, 2016 at 9:09 PM Karl via swift-evolution <
swift-evolution at swift.org> wrote:

> So like most good pitches, this one comes about because of a bug I
> recently fixed which seems common and subtle enough that I think it would
> be nice to include in the standard library.
>
> Removing a collection of elements at once from a collection. You might
> want to do this in some kind of advanced filtering code. In my case, our
> code was merging adjacent elements in-place, and storing a list of indexes
> that were no longer required because they were now part of a merged
> element, and then we cleaned up the duplicates by removing those indexes.
>
> A näive implementation might look like this:
>
> for index in indexesToRemove {
>     myArray.remove(at: index)
> }
>
> However, as the array is mutated, those indexes won’t make sense any more.
> You’ll end up with invalid results - for example, if you have the array
> [0,1,2] and indexesToRemove is [0,1], your resulting array will actually be
> [1] (not [2], as expected). Actually removing a batch of indexes is subtly
> more complex. Here’s my generic implementation:
>
> extension RangeReplaceableCollection where Index:Hashable,
> Self:BidirectionalIndexable {
>
>         mutating func remove(at indexes: Set<Index>) {
>                 var removed : IndexDistance = 0
>                 for idx in indexes.sorted() {
>                         remove(at: index(idx, offsetBy: -removed))
>                         removed = removed.advanced(by: 1)
>                 }
>         }
> }
>
> I think it would be nice to have this in the standard library. I think
> it’s a reasonably common problem and it’d be nice to make it easier for
> people.
>
> Karl
>
>
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>
-- 
-Saagar Jha
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