クロージャ内で変数をキャプチャして使う

capture.swift

var nums = (10...19)
let result = nums.enumerated().flatMap { [count = nums.count, x = 10] (offset: Int, element: Int) -> [Int] in
    if offset < count / 2 {
        return []
    }
    return [element, element-x]
}
print(result) // [15, 5, 16, 6, 17, 7, 18, 8, 19, 9]