Kotlin has array.indexOf but I cannot figure out how to do array.indexOfBy { lambda }

Question

Kotlin has array.indexOf(item) but I cannot figure out how to do array.indexOfBy { lambda }. Does it not exist? I can find an item, but I cannot get its index at the same time.

Am I missing a function in the stdlib?

I can create a function with a loop that chekcs the items and returns when it finds the target. Like this:

fun <T : Any> indexOfBy(items: Array<T>, predicate: (T) -> Boolean): Int {
    for (i in items.indices) { // or (i in 0..items.size-1)
        if (predicate(items[i])) {
            return i
        }
    }
    return -1
}

Then I tried to make it a little more functional using forEach:

fun <T : Any> indexOfBy(items: Array<T>, predicate: (T) -> Boolean): Int {
    (items.indices).forEach {
        if (predicate(items[it])) {
            return it
        }
    }
    return -1
}

Or I can do something silly like this which isn't very performant:

val slowAndSilly = people.indexOf(people.find { it.name == "David" })

And what looks best is maybe as extension functions:

fun <T: Any> Array<T>.indexOfBy(predicate: (T)->Boolean): Int =
        this.withIndex().find { predicate(it.value) }?.index ?: -1
fun <T: Any> Collection<T>.indexOfBy(predicate: (T)->Boolean): Int =
        this.withIndex().find { predicate(it.value) }?.index ?: -1
fun <T: Any> Sequence<T>.indexOfBy(predicate: (T)->Boolean): Int =
        this.withIndex().find { predicate(it.value) }?.index ?: -1

Is there a more elegant and idiomatic way to accomplish this?!? I also don't see a function like this for lists, collections nor sequences.

(this question is derived from the comment on another post)

Answer 1

You can use indexOfFirst

arrayOf(1, 2, 3).indexOfFirst { it == 2 } // returns 1
arrayOf(4, 5, 6).indexOfFirst { it < 3 } // returns -1