Given a 2d array of Ints, how do I find the highest numbers and their positions?

Question

Given a 2D array brightnessMap which contains [[1, 2, 3], [4, 5, 6], [7, 8, 9]], how do find both the highest x numbers, in order (say, x=3 to get 9, 8, 7) AND their positions (2,2 ; 2,1; 2,0)?

don't know. if the numbers were always unique I could do something like `array.index(of: array.max())` to get the highest, but I am not guaranteeing uniqueness. — cannyboy, Jan 17 '20 at 14:27
Well you can combine the indices with the elements: `zip(array.indices, array)`, and you can then find the max by the elements, while keeping the corresponding indices. — Alexander, Jan 17 '20 at 14:30
@Alexander-ReinstateMonica Isn't that what `enumerated()` does? :) — Sulthan, Jan 17 '20 at 14:37
@Sulthan Nope, enumerated is more like `zip(0..., array)`. The resulting indices aren't indices, they're offsets. They always start at 0, and can't be used to index back into the collection, in the general case. Try it on String, or Dictionary, for example. — Alexander, Jan 17 '20 at 14:42
@Alexander-ReinstateMonica Oh, you are right about that. In case of array they are the same though. — Sulthan, Jan 17 '20 at 14:44
@Sulthan Yep. But then you have to be careful with things like `a.dropFirst().enumerated()` vs `a.enumerated().dropFirst()`, because they're not the same. — Alexander, Jan 17 '20 at 14:57
having trouble getting the `zip` to work (does it need to be in a loop?), can u post a code snippet as an answer? — cannyboy, Jan 17 '20 at 14:58

Joakim Danielson · Answer 1 · 2020-01-17T15:11:08.360

Maybe not the most elegant solution but it does the job

func maxFromArrays(_ maxCount: Int ) -> [Int: [(Int,Int)]] {
    let max3 = array.flatMap {$0}.sorted().suffix(3)
    var positions: [Int: [(Int,Int)]] = [:]

    for (index, value) in array.enumerated() {
        for (index2, value2) in value.enumerated() {
            if max3.contains(value2) {
                positions[value2, default: []].append((index, index2))
            }
        }
    }
    return positions
}

Borrowing makeList from this answer we can test it with some larger arrays

func makeList(_ n: Int) -> [Int] {
    return (0..<n).map { _ in .random(in: 1...20) }
}

let array = [makeList(10), makeList(10), makeList(10)] 
print(array)
print(maxFromArrays(3))

[[11, 3, 15, 7, 1, 7, 5, 17, 13, 8], [13, 9, 7, 15, 4, 17, 7, 17, 17, 2], [9, 18, 13, 15, 12, 2, 11, 20, 4, 16]]
[18: [(2, 1)], 17: [(0, 7), (1, 5), (1, 7), (1, 8)], 20: [(2, 7)]]

score 1 · Accepted Answer · answered Jan 17 '20 at 15:57

You can use the enumerated method and the beauty of flat map:

let brightnessMap = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let count = 3
let result = brightnessMap
    .enumerated() 
    .map { x, elts in elts.enumerated().map { y, value in  (pos: (x,y), value: value) } }
    .flatMap { $0 }
    .sorted { $0.value > $1.value }
    .prefix(count)

The result will contain this array of tuples:

[(pos: (2, 2), value: 9), (pos: (2, 1), value: 8), (pos: (2, 0), value: 7)]

Given a 2d array of Ints, how do I find the highest numbers and their positions?

2 Answers2