Compound tasks (Playground)

Description #

Implement Customer.getMostExpensiveDeliveredProduct() and Shop.getNumberOfTimesProductWasOrdered() using functions from the Kotlin standard library.

Kotlin標準ライブラリの関数を使用して、Customer.getMostExpensiveDeliveredProduct() および Shop.getNumberOfTimesProductWasOrdered() を実装してください。

Code #

// Return the most expensive product among all delivered products
// (use the Order.isDelivered flag)
fun Customer.getMostExpensiveDeliveredProduct(): Product? {
    return orders.filter { it.isDelivered }.flatMap { it.products }.maxBy { it.price }
}

// Return how many times the given product was ordered.
// Note: a customer may order the same product for several times.
fun Shop.getNumberOfTimesProductWasOrdered(product: Product): Int {
    return customers.flatMap { it.getOrderedProductsList() }.count { it == product }
}

fun Customer.getOrderedProductsList(): List<Product> {
    return orders.flatMap { it.products }
}

Memo #

• これまでのコレクション関連の総おさらい問題
  • Kotlin Koans 20 Collections Introduction
  • Kotlin Koans 21 Filter; map
  • Kotlin Koans 22 All, Any and other predicates
  • Kotlin Koans 23 FlatMap
  • Kotlin Koans 24 Max; min
  • Kotlin Koans 25 Sort
  • Kotlin Koans 26 Sum
  • Kotlin Koans 27 Group By
  • Kotlin Koans 28 Partition
  • Kotlin Koans 29 Fold