I have two lists: listA = listOf<ObjA>(…..) ...

# getting-started

[Daniele B]

I have two lists:
listA = listOf<ObjA>(…..)
listB = listOf<ObjB>(…..)
ObjA and ObjB have a common field called “fieldC”
how can filter listA, so that it removes any ObjA object whose “fieldC” is not present in any element of listB?

[deactivateduser]

is this what you need?
https://pl.kotl.in/EK2DZoBHC

[Daniele B]

thank you!
is this:

listA.removeIf { objA -> objA.field1 !in listB.map { it.field1 } }

the same of this?

listA.filter { objA -> objA.field1 in listB.map { it.field1 } }

[deactivateduser]

yup.. pretty much similar... the above is removing the items in-place whereas below is returning new filtered list 🙂

[ephemient]

can use retainAll instead of removeIf to avoid double-negation, and map the list to a set once for performance: https://pl.kotl.in/ur4cbOd0T

[deactivateduser]

true, separating list of field1 from listB is also preferable 😉

[nanodeath]

@ephemient that's interesting -- I would have done asSequence().map { it.field1 }.toSet(), but I might like your way better...undecided 🙂

[Daniele B]

is this:
val keepC = listB.mapTo(mutableSetOf()) { it.field1 }
equivalent to this?
val keepC = listB.map { it.field1 }.toSet()

[Daniele B]

and what is the difference between:
listA.retainAll { it.field1 in keepC }
and this ?
listA.filter { it.field1 in keepC }

[nanodeath]

in your first example, you get the same end result, but the second line (listB.map) creates an intermediate list that you don't need

[nanodeath]

in your second example, the first one mutates listA, while the second one returns a new list

[Daniele B]

very interesting, thanks!