Negation of function reference in Kotlin

Issue

We can filter all items in a list that are also contained in another list by

list.filter(otherList::contains)

However, I want to filter all items in a list that are NOT contained in the other list. How can I negate the above?

Ofc, I can pass a lambda, for example

list.filter { !otherList.contains(it) }

but I looking for a better Kotlin-way approach.

Solution

Use the filterNot to filter out items that are not contained in another list. This returns a list of elements for which the predicate yields false.

val filteredList = list.filterNot { otherList.contains(it) }

The operation has a time complexity of O(n*m), where n is the size of the original list and m is the size of the other list. If performance becomes an issue due to large lists, consider converting otherList to a HashSet, which would allow for constant-time lookups:

val setFromOtherList = otherList.toHashSet()
val filteredList = list.filterNot { setFromOtherList.contains(it) }

The above code will perform faster for large lists because checking membership in a HashSet is generally much quicker than checking membership in a List.

Answered By - Keyboard Corporation