Kotlin Koans 22 All, Any and other predicates

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All, Any and other predicates (Playground)

Description

Implement all the functions below using all, any, count, find.

all, any, count, findを使用して、以下のすべての関数を実装してください。

val numbers = listOf(-1, 0, 2)
val isZero: (Int) -> Boolean = { it == 0 }
numbers.any(isZero) == true
numbers.all(isZero) == false
numbers.count(isZero) == 1
numbers.find { it > 0 } == 2

Code 

// Return true if all customers are from the given city
fun Shop.checkAllCustomersAreFrom(city: City): Boolean = customers.all { it.city == city }

// Return true if there is at least one customer from the given city
fun Shop.hasCustomerFrom(city: City): Boolean = customers.any { it.city == city }

// Return the number of customers from the given city
fun Shop.countCustomersFrom(city: City): Int = customers.count { it.city == city }

// Return a customer who lives in the given city, or null if there is none
fun Shop.findAnyCustomerFrom(city: City): Customer? = customers.find { it.city == city }

Memo

  • all ... 全要素がラムダ式にマッチする(trueである)場合、trueを返す
  • any ... 1つ以上の要素がラムダ式にマッチする(trueである)場合、trueを返す
  • count ... ラムダ式にマッチする(trueである)要素の数を返す
  • find ... ラムダ式にマッチする(trueである)最初の要素を返す

ラムダ式についてはLambdasをおさらい。

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