코틀린 Count, <associateBy,groupBy,partition>

1. Count

val numbers = listOf(1, -2, 3, -4, 5, -6)

val totalCount = numbers.count()
val evenCount = numbers.count { it % 2 == 0 }
val oddCount = numbers.count  { it % 2 != 0 }


fun main() {

    // count() : elements의 개수를 return

    println("totalCount : $totalCount")         // 총 개수
    println("evenCount : $evenCount")           // 짝수의 개수
    println("oddCount : $oddCount")             // 홀수의 개수

}

2. associateBy,groupBy,partition

data class Person(val name: String, val city: String, val phone: String) // 1

val people = listOf(                                                     // 2
    Person("John", "Boston", "+1-888-123456"),
    Person("Sarah", "Munich", "+49-777-789123"),
    Person("Svyatoslav", "Saint-Petersburg", "+7-999-456789"),
    Person("Vasilisa", "Saint-Petersburg", "+7-999-123456"))

val phoneBook = people.associateBy { it.phone }                          // 3
val cityBook = people.associateBy(Person::phone, Person::city)           // 4
val peopleCities = people.groupBy(Person::city, Person::name)            // 5
val lastPersonCity = people.associateBy(Person::city, Person::name)      // 6
val part = people.partition { it.city == "Boston" }


fun main() {


    println("phoneBook : $phoneBook")
    println("cityBook : $cityBook")
    println("peopleCities : $peopleCities")
    println("lastPersonCity : $lastPersonCity")
    println("part : $part")

}

 

 

2-1. associateBy : 지정된 key로 Map을 반환

2-2. groupBy : 지정된 key로 Map을 반환

2-3. partition : 조건에 의해 배열로 나누어 반환

2-3. partition

fun main() {

    val numbers = listOf(1, -2, 3, -4, 5, -6)                // 1

    val evenOdd = numbers.partition { it % 2 == 0 }           // 2
    val (positives, negatives) = numbers.partition { it > 0 } // 3

    println("Numbers: $numbers")
    println("Even numbers: ${evenOdd.first}")
    println("Odd numbers: ${evenOdd.second}")
    println("Positive numbers: $positives")
    println("Negative numbers: $negatives")




}