연관과 평탄화

fun main() {
    val words = listOf("a", "bb", "ccc")
    val lengths = words.associateWith { it.length }
    println(lengths)
}

data class User(val id: Int, val name: String)

fun main() {
    val users = listOf(User(1, "Ann"), User(2, "Bob"))
    val byId = users.associateBy { it.id }
    println(byId[2]?.name)
}

fun main() {
    val names = listOf("Ann", "Bob", "Cy")
    val map = names.associate { it to it.length }
    println(map)
}

fun main() {
    val words = listOf("apple", "ant", "bee")
    val byFirst = words.associateBy { it.first() }
    println(byFirst)
}

fun main() {
    val nested = listOf(listOf(1, 2), listOf(3, 4), listOf(5))
    val flat = nested.flatten()
    println(flat)
}

fun main() {
    val words = listOf("hi", "ok")
    val chars = words.flatMap { it.toList() }
    println(chars)
}

data class Team(val members: List<String>)

fun main() {
    val teams = listOf(Team(listOf("Ann", "Bob")), Team(listOf("Cy")))
    val everyone = teams.flatMap { it.members }
    println(everyone)
}

fun main() {
    val numbers = listOf(1, 2, 3)
    val mapped = numbers.map { listOf(it, it * 10) }
    val flat = numbers.flatMap { listOf(it, it * 10) }
    println("map: $mapped")
    println("flatMap: $flat")
}

data class Order(val items: List<String>)

fun main() {
    val orders = listOf(Order(listOf("pen", "ink")), Order(listOf("pen")))
    val counts = orders.flatMap { it.items }.groupingBy { it }.eachCount()
    println(counts)
}