How can I interleave two arrays?

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If both arrays have the same length then this is a possible solution:

let one = [1,3,5]
let two = [2,4,6]

let merged = zip(one, two).flatMap { [$0, $1] }

print(merged) // [1, 2, 3, 4, 5, 6]

Here zip() enumerates the arrays in parallel and returns a sequence
of pairs (2-element tuples) with one element from each array. flatMap() creates a 2-element array from each pair and concatenates the result.

If the arrays can have different length then you append the
extra elements of the longer array to the result:

func mergeFunction<T>(one: [T], _ two: [T]) -> [T] {
    let commonLength = min(one.count, two.count)
    return zip(one, two).flatMap { [$0, $1] } 
           + one.suffixFrom(commonLength)
           + two.suffixFrom(commonLength)
}

Update for Swift 3:

func mergeFunction<T>(_ one: [T], _ two: [T]) -> [T] {
    let commonLength = min(one.count, two.count)
    return zip(one, two).flatMap { [$0, $1] } 
           + one.suffix(from: commonLength)
           + two.suffix(from: commonLength)
}

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