Apple Swift - Generate combinations with repetition

Question

I\'m trying to generate a nested array containing all combinations with repetition in Apple\'s Swift programming language.

An detailed explanation of combinations wi

Answer 1

You can get rid of var sub = subcombo by writing the loop as

for subcombo in subcombos {
    ret.append([head] + subcombo)
}

This can be further simplified using the map() function:

func combos(var array: Array, k: Int) -> Array> {
    if k == 0 {
        return [[]]
    }

    if array.isEmpty {
        return []
    }

    let head = [array[0]]
    let subcombos = combos(array, k: k - 1)
    var ret = subcombos.map {head + $0}
    array.removeAtIndex(0)
    ret += combos(array, k: k)

    return ret
}

---

Update for Swift 4:

func combos(elements: ArraySlice, k: Int) -> [[T]] {
    if k == 0 {
        return [[]]
    }

    guard let first = elements.first else {
        return []
    }

    let head = [first]
    let subcombos = combos(elements: elements, k: k - 1)
    var ret = subcombos.map { head + $0 }
    ret += combos(elements: elements.dropFirst(), k: k)

    return ret
}

func combos(elements: Array, k: Int) -> [[T]] {
    return combos(elements: ArraySlice(elements), k: k)
}

Now array slices are passed to the recursive calls to avoid the creation of many temporary arrays.

Example:

print(combos(elements: [1, 2, 3], k: 2))
// [[1, 1], [1, 2], [1, 3], [2, 2], [2, 3], [3, 3]]