Flatten array in Kotlin

Question

I have a two dimensional array of Nodes that I want to flatten into a single array of all nodes using the flatten function of Kotlin arrays.

    val nodes = kotlin.Array(width, { width ->
    kotlin.Array(height, { height -> Node(width, height) })
})

I then try to call the flatten function on the 2D array

nodes.flatten()

but I get an error: Type mismatch: inferred type is Array<Array<Node>> but Array<Array<out ???>> was expected. Is there another way I should be doing this?

Answer 1

Arrays in Kotlin are invariant so an Array<Array<Node>> is not an Array<Array<out T>> (which is the receiver type for flatten).

It looks like this will be fixed in Kotlin 1.1: Relax generic variance in Array.flatten · JetBrains/kotlin@49ea0f5.

Until Kotlin 1.1 is released you can maintain your your own version of flatten:

/**
 * Returns a single list of all elements from all arrays in the given array.
 */
fun <T> Array<out Array<out T>>.flatten(): List<T> {
    val result = ArrayList<T>(sumBy { it.size })
    for (element in this) {
        result.addAll(element)
    }
    return result
}

Answer 2

Use a more universal flatMap:

nodes.flatMap {it.asIterable()}