How to add an array to the member of array in swift 3

I don't think you can alter an array in that way because new array's type which is [[Int]] will be different than old one's [Int].

You can use map to do that:

let array = [[1, 2], [3, 4]]

let newArray = array.map { inner in
  return [inner, [5, 6]]
}

Less verbose but more compact version: (thanks to Honey)

let newArray = array.map { [$0, [5,6]] }

If you want to update in-place no matter what, downcast your array to [Any]:

var array: [Any] = [[1, 2], [3, 4]]

for (index, inner) in array.enumerated() {
  array[index] = [inner, [5, 6]]
}

which is not ideal as you have to cast to Int or [Int] every time you want to access integers in the lists:

for inner in array {
  if let inner = inner as? [Int] {
    ..
  }
}