What is the 'spread' operator in Kotlin and how to use it?

The 'spread' operator in Kotlin corresponds to the prefix * before an array or vararg. It is used to pass the elements of such a collection to a function expecting a variable number of arguments (vararg). Essentially, it "spreads" the collection into individual arguments.

Example of usage:

fun printNumbers(vararg numbers: Int) {
    for (number in numbers) {
        println(number)
    }
}

fun main() {
    val intArray = intArrayOf(1, 2, 3)
    printNumbers(*intArray) // Using the spread operator
}

Without the * operator, attempting to pass intArray to printNumbers would result in a compilation error, as printNumbers expects a variable number of Int, not an Int array.

The 'spread' operator can also be used when creating a new array by combining an existing array with individual elements or other arrays:

fun main() {
    val array1 = arrayOf(1, 2)
    val array2 = arrayOf(3, 4)
    val combinedArray = arrayOf(*array1, 5, *array2) // Using the spread operator
    println(combinedArray.contentToString()) // Outputs [1, 2, 5, 3, 4]
}

Key points:

• Applied only to arrays (Array, primitive arrays) and vararg.
• Spreads the collection's elements into separate function arguments or elements of a new array.
• Denoted with the prefix *.