Get all diagonals of a matrix

diagonals.kt

// Displays the coordinates of all elements of the secondary diagonal and of all diagonals parallel to the secondary diagonal
fun printSecondaryDiagonals(numColumns: Int, numRows: Int) {
    val diagonal = mutableListOf<Pair<Int, Int>>()
    for (k in 0..numColumns + numRows - 2) {
        for (j in 0..k) {
            val i = k - j
            if (i < numRows && j < numColumns) {
                diagonal.add(Pair(i, j))
            }
        }
        diagonal.clear()
    }
}

// Displays the coordinates of all elements of the main diagonal and of all diagonals parallel to the main diagonal
fun printMainDiagonals(numColumns: Int, numRows: Int) {
    val diagonal = mutableListOf<Pair<Int, Int>>()
    for (k in 0..numColumns + numRows - 2) {
        for (j in k downTo 0) {
            val i = numColumns - 1 - k + j
            if (j < numRows && i >= 0) {
                diagonal.add(Pair(j, i))
            }
        }
        diagonal.clear()
    }
}