Swift enumerated equivalent in Kotlin

Question

In a 3x3 matrix representation, i can find the sum of both diagonals with one liners in Swift as below,

let array = [
   [1, 2, 3],
   [4, 5, 6],
   [-7, 8, 9]
]
let d1 = array.enumerated().map({ $1[$0] }).reduce(0, +)
let d2 = array.reversed().enumerated().map({ $1[$0] }).reduce(0, +)
print(d1) // prints 15
print(d2) // prints 1

I am able to find map and reduce equivalents in Kotlin as flatMap and fold but couldn't find for enumerated.

How can we achieve similar with higher order functions in Kotlin?

Answer 1

Starting with this input:

val input: Array<Array<Int>> = arrayOf(
        arrayOf(1, 2, 3),
        arrayOf(4, 5, 6),
        arrayOf(-7, 8, 9)
)

this is how I'd phrase the diagonal sums:

val mainDiagonalSum = input.indices
        .map { input[it][it] }
        .reduce(Int::plus)
val counterDiagonalSum = input.indices
        .map { input[input.size - 1 - it][it] }
        .reduce(Int::plus)

Note that this is an improvement over your solution because it doesn't have to create the reversed array. It improves the time complexity from O(n²) to O(n).

If you're dealing with large matrices, it would pay to reduce the space complexity from O(n) to O(1) as well, by using fold instead of reduce:

val mainDiagonalSum = input.indices
        .fold(0) { sum, i -> sum + input[i][i] }
val counterDiagonalSum = input.indices
        .fold(0) { sum, i -> sum + input[input.size - 1 - i][i] }