Working with collections

The standard library in the kotlin.collections package offers numerous extension functions that extend the collections types. Most of them extend the base Iterable interface, so they can be used with any collection type. 

The library is too big to cover every function, so we’ll see here the most useful ones. We can group them based on their operation.

Filtering functions

These functions are used to remove or filter items from a collection.

Drop

Drop removes first n elements from the collection.

val numbers = listOf(1, 2, 3, 4, 5)
val dropped = numbers.drop(2)

The dropped list now contains [3, 4, 5].

Filter

Filter applies the supplied predicate function to the collection and returns the following result:

val numbers = listOf(1, 2, 3, 4, 5)
val smallerThan3 = numbers.filter { n -> n < 3 }

The resulting list has these numbers [1, 2].

FilterNot

This is the inverted filter function:

val numbers = listOf(1, 2, 3, 4, 5)
val largerThan3 = numbers.filterNot { n -> n < 3 }

The...