Behavior of shuffle on Set vs List using scala.util.Random
list, random, scala, set, shuffle
Solution
Scala's sets aren't ordered (just like the mathematical ones). They are iterable, however—you just can't rely on the order that you'll get the items in. Many implementations of sets will iterate the same elements in the same order—i.e.,
scala> Set(1, 2, 3, 4, 5).toList == Set(5, 4, 3, 2, 1).toList
res0: Boolean = true
Which explains the effect you're seeing here. You should never rely on this, though—there could be a perfect valid `Set` implementation for which the above wouldn't hold.
Problem
``` scala> Random.shuffle((1 to 10).toSet) res10: scala.collection.immutable.Set[Int] = Set(5, 10, 1, 6, 9, 2, 7, 3, 8, 4) scala> Random.shuffle((1 to 10).toSet) res11: scala.collection.immutable.Set[Int] = Set(5, 10, 1, 6, 9, 2, 7, 3, 8, 4) scala> Random.shuffle((1 to 10).toSet) res12: scala.collection.immutable.Set[Int] = Set(5, 10, 1, 6, 9, 2, 7, 3, 8, 4) scala> Random.shuffle((1 to 10).toList) res13: List[Int] = List(3, 9, 8, 5, 7, 6, 10, 2, 1, 4) scala> Random.shuffle((1 to 10).toList) res14: List[Int] = List(5, 10, 2, 9, 4, 7, 8, 6, 1, 3) scala> Random.shuffle((1 to 10).toList) res15: List[Int] = List(5, 9, 10, 6, 8, 3, 4, 1, 7, 2) ``` So shuffle can handle Lists just fine, but not sets ? Can't sets be shuffled ? Why is res10 == res11 == res12 ?