필터와 맵

val fruits: List<Fruit> = listOf(
    Fruit(1, "사과", 1000, 1500),
    Fruit(2, "사과", 1200, null),
    Fruit(3, "사과", 1100, null),
    Fruit(4, "바나나", 1000, 2500),
    Fruit(5, "바나나", 1500, 2500),
    Fruit(6, "수박", 1500, 3500),
    Fruit(7, "자두", 1000, 2500),
    Fruit(8, "포도", 800, 1500),
)

val apples = fruits.filter { fruit -> fruit.name == "사과" }

val apples2 = fruits.filterIndexed { idx, fruit ->
    println(idx)
    fruit.name == "사과"
}

val applePrices = fruits.filter { fruit -> fruit.name == "사고" }
    .map { fruit -> fruit.currentPrice }

val applePrices2 = fruits.filter { fruit -> fruit.name == "사과" }
    .mapIndexed { idx, fruit ->
    println(idx)
    fruit.currentPrice
}

val values = fruits.filter { fruit -> fruit.name == "사과" }
    .mapNotNull { fruit -> fruit.currentPrice }

private fun filterFruits(
    fruits: List<Fruit>, 
    filter: (Fruit) -> Boolean
): List<Fruit> {
    return fruits.filter(filter)
}

다양한 컬렉션 처리 기능

val isAllApple = fruits.all { fruit -> fruit.name == "사과" }
val isNoApple = fruits.none { fruit -> fruit.name == "사과 "}
val isAnyApple = fruits.any { fruit -> fruit.factoryPrice >= 10000 }
val fruitCount = fruits.count()
val sortedFruitByCurrentPriceByAscending = fruits.sortedBy { fruit -> fruit.currentPrice }
val sortedFruitByCurrentPriceByDescending = fruits.sortedByDescending { fruit -> fruit.currentPrice }
val distinctFruitNames = fruits.distinctBy { fruit -> fruit.name }.map { fruit -> fruit.name }
val firstFruit = fruits.first()
val firstOrNullFruit = fruits.firstOrNull()
val lastFruit = fruits.last()
val lastOrNullFruit = fruits.lastOrNull()

List를 Map으로

val map: Map<String, List<Fruit>> = fruits.groupBy { fruit -> fruit.name }
val map2: Map<Long, Fruit> = fruits.associateBy { fruit -> fruit.id }
val map3: Map<String, List<Long>> = fruits.groupBy(
    { fruit -> fruit.name}, { fruit -> fruit.factoryPrice}
)

val map4: Map<String, List<Fruit>> = fruits.groupBy { fruit -> fruit.name }
    .filter { (key, value) -> key == "사과" }

중첩된 컬렉션 처리

val fruitsInList: List<List<Fruit>> = listOf(
    listOf(
        Fruit(1, "사과", 1000, 1500),
        Fruit(2, "사과", 1200, 1500),
        Fruit(3, "사과", 1200, 1500),
        Fruit(4, "사과", 1500, 1500),
    ),
    listOf(
        Fruit(5, "바나나", 3000, 3200),
        Fruit(6, "바나나", 3200, 3200),
        Fruit(7, "바나나", 2500, 3200),
    ),
    listOf(
        Fruit(8, "수박", 10000, 10000)
    )
)

val samePriceFruits = fruitsInList.flatMap { list ->
    list.filter { fruit -> fruit.factoryPrice == fruit.currentPrice }
}
val flatFruitList = fruitsInList.flatten()