scala-collections

I wonder why List(3,2,1).toIndexedSeq.sortBy(x=>x) doesn't work: scala> List(3,2,1).toIndexedSeq.sortBy(x=>x) // Wrong <console>:8: error: missing parameter type List(3,2,1).toIndexedSeq.sortBy(x=>x) ^ <console>:8: error: diverging implicit expansion for type scala.math.Ordering[B] starting with method Tuple9 in object Ordering List(3,2,1).toIndexedSeq.sortBy(x=>x) ^ scala> Vector(3,2,1).sortBy(x=>x) // OK res: scala.collection.immutable.Vector[Int] = Vector(1, 2, 3) scala> Vector(3,2,1).asInstanceOf[IndexedSeq[Int]].sortBy(x=>x) // OK res: IndexedSeq[Int] = Vector(1, 2, 3) scala> List(3,2,1)

I've discovered a strange behavior for mutable sets which I cannot understand: I have a object which I want to add to a set. The equals method for the class is overridden. When I add two different objects to the set, which produces the same output for equals method, I get a different behavior between mutable and immutable sets for the contains method. Here is the code snippet: class Test(text:String){ override def equals(obj:Any) = obj match { case t: Test => if (t.text == this.text) true else false case _ => false } override def toString = text } val mutableSet:scala.collection.mutable.Set

This SO answer describes how scala.collection.breakOut can be used to prevent creating wasteful intermediate collections. For example, here we create an intermediate Seq[(String,String)] : val m = List("A", "B", "C").map(x => x -> x).toMap By using breakOut we can prevent the creation of this intermediate Seq : val m: Map[String,String] = List("A", "B", "C").map(x => x -> x)(breakOut) Views solve the same problem and in addition access elements lazily: val m = (List("A", "B", "C").view map (x => x -> x)).toMap I am assuming the creation of the View wrappers is fairly cheap, so my question is:

I'm trying to convert back from a parallel collection to a regular map. According to the api, if I call toMap on any appropriately defined parallel collection, it's supposed to return a standard Map, but it's returning ParMap over a flattened collection of iterables. I have a val task: Stream[Future[Iterable[Tuple2[String, String]]]] And from which I get: val res: ParSeq[Iterable[Tuple2[String, String]]] = tasks.par.map(f => f.apply()) Finally: val finalresult = res.flatten.toMap Unfortunately, the type of finalresult is ParMap[String, String] . On the other hand, if I call it like: tasks.par

For any given list or array, for instance val list = (1 to 3).toList val array = (1 to 3).toArray and a given function that maps from and onto the collection type, for instance def f(v: Int): Int = v + 10 how to apply f to the ith element of list or array so that list.myApply(f, ith = 2) res: List(1,12,3) and also array.myApply(f, ith = 2) res: Array(1,12,3) tl;dr import scala.collection.SeqLike import scala.collection.generic.CanBuildFrom implicit class Seq_[A, Repr, S : ({type L[X] = X => SeqLike[A, Repr]})#L](seq: S) { def myApply[B >: A, That](f: A => B, ith: Int) (implicit bf:

Let's assume I want to create a trait that I can mix in into any Traversable[T]. In the end, I want to be able to say things like: val m = Map("name" -> "foo") with MoreFilterOperations and have methods on MoreFilterOperations that are expressed in anything Traversable has to offer, such as: def filterFirstTwo(f: (T) => Boolean) = filter(f) take 2 However, the problem is clearly that T is not defined as a type parameter on MoreFilterOperations. Once I do that, it's doable of course, but then my code would read: val m = Map("name" -> "foo") with MoreFilterOperations[(String,String)] or if I

How to do pattern matching on a Map in Scala ? A (non working) attempt includes, Map("a"->1, "b"->2, "c"->3) match { case Map(a,b,_*) => a } which errs with value Map is not a case class, nor does it have an unapply/unapplySeq member case Map(a,b,_*) => a The error is indicative enough, yet how to enrich Map with an unapply method for pattern matching ? Many Thanks Update Following @Paul's comment, a neater use case may be like this, Map("a"->1, "b"->2, "c"->3) match { case Map("b"->2,_*) => "222" } namely, in this case, if map contains key b that maps onto value 2 . Most easy way is tramsform