How to create unique arrays of a given size from an array in Swift?

Question

Given an array of Ints, and a desired array size, myFunction should return an array of all possible unique arrays. All the Ints in the initial array are supposed to

Answer 1

If I understand your question correctly then you want to create all k-element subsets of a given set with n elements. This can be done recursively by

• combining the first element a[1] with all (k-1)-element subsets of the remaining elements a[2] ... a[n], and
• adding all (k)-element subsets of a[2] ... a[n].

Swift code (a bit generic so that it can be used not only with integers):

func allSubsetsOf<T>(elements: [T], withCardinality k : UInt,
    combinedWith prefix : [T] = [], startingWithIndex j : Int = 0) -> [[T]] {

        if k == 0 {
            return [prefix]
        }

        if j < elements.count  {
            let first = elements[j]
            return allSubsetsOf(elements, withCardinality: k-1, combinedWith: prefix + [first], startingWithIndex : j+1)
                + allSubsetsOf(elements, withCardinality: k, combinedWith: prefix, startingWithIndex: j+1)
        } else {
            return []
        }
}

Examples:

let result1 = allSubsetsOf([1, 2, 3, 4, 5], withCardinality: 3)
println(result1)
// [[1, 2, 3], [1, 2, 4], [1, 2, 5], [1, 3, 4], [1, 3, 5], [1, 4, 5], [2, 3, 4], [2, 3, 5], [2, 4, 5], [3, 4, 5]]

let result2 = allSubsetsOf(["a", "b", "c", "d"], withCardinality: 2)
println(result2)
// [[a, b], [a, c], [a, d], [b, c], [b, d], [c, d]]