swift - Split large Array in Array of two elements

Question

I have large list of objects and I need to split them in a group of two elements for UI propouse.

Example:

[0, 1, 2, 3, 4, 5, 6]

Becomes an array with these four arrays

[[0, 1], [2, 3], [4, 5], [6]]

There are a ton of ways to split an array. But, what is the most efficient (least costly) if the array is huge.

Answer 1

If you want an array of subslices, you can use the split function to generate it using a closure that captures a state variable and increments it as it passes over each element, splitting only on every nth element. As an extension of Sliceable (Swift 2.0 only, would need to be a free function in 1.2):

extension Sliceable {
    func splitEvery(n: Index.Distance) -> [SubSlice] {
        var i: Index.Distance = 0
        return split(self) { _ in ++i % n == 0 }
    }
}

Subslices are very efficient in so much as they usually share internal storage with the original sliceable entity. So no new memory will be allocated for storing the elements - only memory for tracking the subslices' pointers into the original array.

Note, this will work on anything sliceable, like strings:

"Hello, I must be going"
    .characters
    .splitEvery(3)
    .map(String.init)

returns ["He", "lo", " I", "mu", "t ", "e ", "oi", "g"].

If you want to lazily split the array up (i.e. generate a sequence that only serves up subslices on demand) you could write it using anyGenerator:

extension Sliceable {
    func lazilySplitEvery(n: Index.Distance) -> AnySequence<SubSlice> {

        return AnySequence { () -> AnyGenerator<SubSlice> in
            var i: Index = self.startIndex
            return anyGenerator {
                guard i != self.endIndex else { return nil }
                let j = advance(i, n, self.endIndex)
                let r = i..<j
                i = j
                return self[r]
            }
        }
    }
}


for x in [1,2,3,4,5,6,7].lazilySplitEvery(3) {
    print(x)
}
// prints [1, 2, 3]
//        [4, 5, 6]
//        [7]