kotlin compareBy using IntArray, how do I get the 'a' and 'b' fields?-CodePudding

new to kotlin and struggle with the syntax of compareBy and its lambda to get the 'a' and 'b' parameters for a custom compare:

public inline fun <T> compareBy(crossinline selector: (T) -> Comparable<*>?): Comparator<T> =
    Comparator { a, b -> compareValuesBy(a, b, selector) }

Basically I want to compare 2d points that are stored as IntArray(x,y) but don't know how to access the a and b elements of the declaration. Here where I am stuck:

val compareByDistance: Comparator<IntArray> = compareBy {
    // b - a to origin = (bx2 - 0)^2 - (by2 - 0)^2 -> no need square root
    val distance = -1
    distance
}

val points = PriorityQueue<IntArray>(compareByDistance)
points.add(intArrayOf(1, 2))
points.add(intArrayOf(2, 3))

when I run the debugger I see the 'a' and 'b' parameters, but I have to idea how to access them.

If I do:

val k = it.first()

it does not give k = a = [2,3] just k =2, but still I can't access b.

What is the correct syntax for the lambda? I looked in this thread by did not help:

https://www.bezkoder.com/kotlin-priority-queue/

thank you.

CodePudding user response：

Try this.

   val compareByDistance = Comparator<IntArray> { a:IntArray, b:IntArray ->
            // Return -1 ,  1 ,0 - Based on your Formula
            0
        }

Here in your code , when you see compareBy will be accepting lambda of type Comparable<T> which has only one parameter. It will only return default Comparator.

The solution is to create object for Comparator Interface like above . In which compare has two arguments.

CodePudding user response：

I’m assuming from the formula you have in your code comment that you are wanting to compare your points by their distance from the origin (0, 0).

The compareBy lambda has a single parameter since you use it to convert each individual IntArray into a number that will be used to compare it to other IntArrays.

When a lambda has a single parameter and you leave it blank like in your code above, it has the implicit name it, so you could use it[0] and it[1] for a and b. But you can name it using a destructuring declaration to conveniently get named a and b variables.

val compareByDistance: Comparator<IntArray> = compareBy { (a, b) ->
    a * a   b * b
}