Using the 'flatMap' Function for Transforming Nested Collections

The 'flatMap' function is a powerful tool for transforming and flattening nested collections in Kotlin. It allows you to apply a transformation function to each element of a collection and then merge the results into a single collection. This is particularly useful when dealing with collections of collections, such as lists of lists, where you want to flatten the structure into a single list while applying some transformation to the inner elements.

val nestedList = listOf(
    listOf(1, 2, 3),
    listOf(4, 5),
    listOf(6, 7, 8, 9)
)

val flattenedList = nestedList.flatMap { it.map { number -> number * 2 } }
// flattenedList will be [2, 4, 6, 8, 10, 12, 14, 16, 18]