map list of objects to be with specific field based on given list of the field elements

I have list of Person, with ids 4,5,6 :

case class Person(id: Int)

val people = List(Person(4),Person(5),Person(6))

now i have int list of ids 1,2,3 :

val ids = List(1,2,3)

and I want to map people to be with the ids in this list, given that list of people and list of ids are the same size, how would you do that?

expected result:

people = List(Person(1),Person(2),Person(3))

people.zip(ids).map{ case (p, id) => p.copy(id = id) }

case classes are generally immutable, which is why you would normally have an approach like the following (you can play around with this code here on Scastie):

case class Person(id: Int)

val people = List(Person(4), Person(5), Person(6))

val ids = List(1, 2, 3)

val newPeople =
  people.zip(ids).map { case (person, id) => person.copy(id = id) }

assert(newPeople == List(Person(1), Person(2), Person(3)))

If you want to mutate something in place, you will need to qualify it as a var (whereas case class attributes are implicitly vals):

class Person(var id: Int)

val people = List(Person(4), Person(5), Person(6))

val ids = List(1, 2, 3)

people.zip(ids).foreach { case (person, id) => person.id = id }

assert(people == List(Person(1), Person(2), Person(3)))

The code above is also available here on Scastie for your to play around with.

In principle you can make a case class attribute a var, but case classes are generally assumed to be immutable, so you might want to decide whether you want a case class or a simple class based on the project you are working on.

In Scala, immutability is generally regarded as preferable unless you have a very good reason not to. While it might cause more pressure on the garbage collector, it generally makes it easier to write code that is self-contained and it's a nice property to have if you have to share your data across threads.