Can I use for-comprehenion / yield to create a map in Scala?

Question

Can I "yield" into a Map?

I've tried

val rndTrans = for (s1 <- 0 to nStates;
                    s2 <- 0 to nStates
                        if rnd.nextDouble() < trans_probability)
                            yield (s1 -> s2);

(and with , instead of ->) but I get the error

TestCaseGenerator.scala:42: error: type mismatch;
 found   : Seq.Projection[(Int, Int)]
 required: Map[State,State]
    new LTS(rndTrans, rndLabeling)

I can see why, but I can't see how to solve this :-/

Pythia

Answer 1

scala> (for(i <- 0 to 10; j <- 0 to 10) yield (i -> j)) toMap
res1: scala.collection.immutable.Map[Int,Int] = Map((0,10), (5,10), (10,10), (1,10), (6,10), (9,10), (2,10), (7,10), (3,10),  (8,10), (4,10))

Hays

Hmm, probably close to what I'm looking for, but I get: error: value toMap is not a member of Seq.Projection[(Int, Int)] – Pythia

Thats strange. Which version of Scala are you using? – Pythia

Seq.Projection is deprecated in 2.8; so it should not be the result of a built-in for loop. Could you check your version again? (println(util.Properties.versionString)) – Gene

Yeah. I'm on 2.7 :-( when will the Ubuntu repos be updated to provide 2.8?? Thanks anyway... – Pythia

@Pythia FWIW, it's fairly easy to get a later version of scala running in Ubuntu without using the Ubuntu repos. You can a) install the package for your favorite IDE, b) install SBT which will automatically download Scala, or c) download the debian package. See scala-lang.org/node/292#distributions and/or Daniel's answer to this question: #4161960. – Newhall

Awsome! Thanks! However, I still can't get sbt working. See my other question. – Pythia

@Pythia There's a ticket on Debian about upgrading the Scala package. I hope they do get to it. – Quadrisect

Answer 2

An alternate solution in Scala 2.8:

Welcome to Scala version 2.8.1.r23457-b20101106033551 (Java HotSpot(TM) Client VM, Java 1.6.0_22).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import scala.collection.breakOut            
import scala.collection.breakOut

scala> val list: List[(Int,Int)] = (for(i<-0 to 3;j<-0 to 2) yield(i->j))(breakOut)
list: List[(Int, Int)] = List((0,0), (0,1), (0,2), (1,0), (1,1), (1,2), (2,0), (2,1), (2,2), (3,0), (3,1), (3,2))

scala> val map: Map[Int,Int] = (for(i<-0 to 3;j<-0 to 2) yield(i->j))(breakOut)    
map: Map[Int,Int] = Map((0,2), (1,2), (2,2), (3,2))

scala> val set: Set[(Int,Int)] = (for(i<-0 to 3;j<-0 to 2) yield(i->j))(breakOut)
set: Set[(Int, Int)] = Set((2,2), (3,2), (0,1), (1,2), (0,0), (2,0), (3,1), (0,2), (1,1), (2,1), (1,0), (3,0))

scala> 

Postpositive

+1 for the ever mysterious (but useful!) breakOut. Correct me if I'm wrong, but I think it affords slightly better performance because it provides a builder that allows direct generation of the map (instead of building another collection which is converted afterward). – Ugrian

@Zwirb You are correct. And +1 because I didn't know you could use it with for/yield syntax! :-) – Quadrisect

Answer 3

Alternative (works on 2.7):

scala> Map((for(i <- 0 to 10; j <- 0 to 10) yield (i -> j)): _*)
res0: scala.collection.immutable.Map[Int,Int] = Map((0,10), (5,10), (10,10), (1,10), (6,10), (9,10), (2,10), (7,10), (3,10), (8,10), (4,10))

Thetos

Great. I'm stuck with 2.7 for now, so I'll use this one. – Pythia

Answer 4

val rndTrans = (
  for {
    s1 <- 0 to nStates
    s2 <- 0 to nStates if rnd.nextDouble() < trans_probability
  } yield s1 -> s2
  ) (collection.breakOut[Any, (Int, Int), Map[Int, Int]])

Staciastacie