Question

Can someone please give an example of how to use the HashMap forall() method? I find the Scala docs to be impenetrable.

What I want is something like this:

val myMap = HashMap[Int, Int](1 -> 10, 2 -> 20) 
val areAllValuesTenTimesTheKey = myMap.forall((k, v) => k * 10 == v)

but this gives:

error: wrong number of parameters; expected = 1

Answer 1

You need instead

val myMap = HashMap[Int, Int](1 -> 10, 2 -> 20) 
val areAllValuesTenTimesTheKey = myMap.forall { case (k, v) => k * 10 == v }

The problem is that forall wants a function that takes a single Tuple2, rather than two arguments. (We’re thinking of a Map[A,B] as an Iterable[(A,B)] when we use forall.) Using a case statement is a nice workaround; it’s really using pattern matching here to break apart the Tuple2 and give the parts names.

If you don’t want to use pattern matching, you could have also written

val areAllValuesTenTimesTheKey = myMap.forall(p => p._1 * 10 == p._2 }

but I think that’s less helpful.