indexOfFirst

Returns index of the first element matching the given predicate, or -1 if the array does not contain such element.

inline fun <T> Array<out T>.indexOfFirst(predicate: (T) -> Boolean): Int(source)

fun main() {
    val fruits = arrayOf("apple", "banana", "cherry", "date")
    val index = fruits.indexOfFirst { it.startsWith("c") }  // returns 2
    println("First fruit starting with 'c' is at index: $index")
}

Source