Set-specific operations

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:

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

    // output according to the order
    println(numbers union setOf("four", "five"))
    // [one, two, three, four, five]
    println(setOf("four", "five") union numbers)
    // [four, five, one, two, three]
//sampleEnd
}

To find an intersection between two collections (elements present in both of them), use the intersect() function. To find collection elements not present in another collection, use the subtract() function. 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")

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

To find the elements present in either one of the two collections but not in their intersection, you can also use the union() function. For this operation (known as symmetric difference), calculate the differences between the two collections and merge the results:

fun main() {
//sampleStart
    val numbers = setOf("one", "two", "three")
    val numbers2 = setOf("three", "four")

    // merge differences 
    println((numbers - numbers2) union (numbers2 - numbers))
    // [one, two, four]
//sampleEnd
}

You can also apply union(), intersect(), and subtract() functions to lists. However, their result is always a Set. 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)

    // result of intersecting two lists is a Set
    println(list1 intersect list2)
    // [1, 2, 3, 5]

    // equal elements are merged into one
    println(list1 union list2)
    // [1, 2, 3, 5, 8, -1]
//sampleEnd
}

Last modified: 15 April 2024