Ensure list contains only specific values - scala

Question

How can I make sure a list only contains a specific set of items?
List[Int]
A function to make sure the list only contains the values 10, 20 or 30.
I'm sure this is built in by I can't find it!

Answer 1

Your question doesn't specify what you want to happen when the list doesn't contain the requisite items.
The following will return true if all the items in the List match your criteria, false otherwise:
val ints1: List[Int] = List(1, 2, 3, 4, 5, 6, 7)
val ints2: List[Int] = List(10, 10, 10, 10)
ints1.forall(i => List(10, 20, 30).contains(i)) // false
ints2.forall(i => List(10, 20, 30).contains(i)) // true
The following will return a List with only those items which matched the criteria:
val ints1: List[Int] = List(10, 20, 30, 40, 50, 60, 70)
val ints2: List[Int] = List(10, 10, 10)
ints1.filter(i => List(10, 20, 30).contains(i)) // List(10, 20, 30)
ints2.filter(i => List(10, 20, 30).contains(i)) // List(10, 10, 10)

Answer 2

forall
You may use forall with a Set containing elements which are valid or legal and you want to see in the list.
list.forall(Set(10, 20, 30).contains) //true means list only contains 10, 20, 30
Set is Function
You need not use contains method as Set extends Int => Boolean. You can use Set like a function
list forall Set(10, 20, 30)
Filter
You can use filter to filter out the elements which are not in the given list. Again you can use Set as function as Set extends Function.
list.filter(Set(10, 20, 30)).nonEmpty //true means list only contains 10, 20 and 30
Collect if you like pattern matching
Collect takes a Partial function. If you like pattern matching just use collect
list.collect {
case 10 => 10
case 20 => 20
case 30 => 30
}.nonEmpty //true means list only contains 10, 20 and 30
Scala REPL
scala> val list = List(10, 20, 30, 40, 50)
list: List[Int] = List(10, 20, 30, 40, 50)
scala> list forall Set(10, 20, 30)
res6: Boolean = false

Answer 3

If you simply want to determine whether all of the values in your list are "legal", use forall:
def isLegal(i: Int): Boolean = ??? // e.g. is it 10, 20, or 30
val allLegal = list forall isLegal
If you want to trim down your list so that only legal values remain, use filter:
val onlyLegalValues = list filter isLegal
Note that a Set[Int] counts as a Int => Boolean function, so you could use that in place of your isLegal method:
val isLegal = Set(10, 20, 30)
val allLegal = list forall isLegal
val onlyLegalValues = list filter isLegal