セット特有のオペレーション

The Kotlin collections package contains extension functions for popular operations on sets: finding intersections, merging, or subtracting collections from each other.

To merge two collections into one, use the union() function. It can be used in the infix form a union b. Note that for ordered collections the order of the operands is important: in the resulting collection, the elements of the first operand go before the elements of the second.

To find an intersection between two collections (elements present in both of them), use intersect(). To find collection elements not present in another collection, use subtract(). Both these functions can be called in the infix form as well, for example, a intersect b.

fun main() {
//sampleStart
    val numbers = setOf("one", "two", "three")

    println(numbers union setOf("four", "five"))
    println(setOf("four", "five") union numbers)

    println(numbers intersect setOf("two", "one"))
    println(numbers subtract setOf("three", "four"))
    println(numbers subtract setOf("four", "three")) // same output
//sampleEnd
}

{kotlin-runnable=”true” kotlin-min-compiler-version=”1.3”}

You can also apply union, intersect, and subtract to List. However, their result is always a Set, even on lists. In this result, all the duplicate elements are merged into one and the index access is not available.

fun main() {
//sampleStart
    val list1 = listOf(1, 1, 2 ,3, 5, 8, -1)
    val list2 = listOf(1, 1, 2, 2 ,3, 5)
    println(list1 intersect list2) // result on two lists is a Set
    println(list1 union list2)     // equal elements are merged into one
//sampleEnd
}

{kotlin-runnable=”true” kotlin-min-compiler-version=”1.3”}