minOf / maxOf / sumOf / averageOf

data class Order(val id: Int, val amount: Double, val qty: Int)

fun main() {
    val orders = listOf(
        Order(1, 99.99,  2),
        Order(2, 12.50,  5),
        Order(3, 45.00,  3),
    )

    val totalRevenue: Double = orders.sumOf { it.amount * it.qty }
    println(totalRevenue)                               // 412.48

    val biggest: Order = orders.maxBy { it.amount }
    println(biggest.id)                                 // 1

    val highestAmount: Double = orders.maxOf { it.amount }
    println(highestAmount)                              // 99.99

    val avgQty: Double = orders.map { it.qty }.average()
    println(avgQty)                                     // 3.33

    // Safer null-aware versions for possibly-empty lists
    val maybeBiggest = orders.maxByOrNull { it.amount }
    val maybeMax     = orders.maxOfOrNull { it.amount }
}