Don't mutate stuff while iterating

gistfile1.scala

scala> val s = MSet() ++ (1 to 20)
s: scala.collection.mutable.Set[Int] = Set(5, 9, 15, 16, 18, 14, 19, 2, 20, 6, 1, 13, 4, 12, 7, 3, 8, 17, 11, 10)

scala> s.size
res16: Int = 20

scala> s foreach { n => if (n % 2 == 0) s -= n }

scala> s.size
res18: Int = 11

scala> s
res19: scala.collection.mutable.Set[Int] = Set(15, 9, 18, 19, 1, 13, 17, 7, 3, 11, 5)

scala> s foreach { n => if (n % 2 == 0) s -= n }

scala> s.size
res21: Int = 10

Ignore the fact that this can be done with calling filter on the set. The point here is that ConcurrentModificationException does not occur and instead, the foreach silently skips elements.