Processing Set of Sets and return a flat Iterable

Question

val input=Set(Set("a","b"),Set("b","c"))

I want this:

Map("a"->1,"b"->2,"c"->1)

What is the best functional approach for implementing such functionality? Using yield keyword results in nested Iterables:

output = for(firstlevel<-input) yield for(item<-firstlevel) yield item

Answer 1

update: incorporated the suggestion to use input.toSeq.flatten
instead of input.toSeq flatMap { _.toSeq }

convert to a single sequence of values...

input.toSeq.flatten

...group values that match...

input.toSeq.flatten groupBy { identity }

...and count

input.toSeq.flatten groupBy { identity } mapValues { _.size }

Answer 2

If you want to use for-comprehension and yield:

output = for{
    (set,idx) <- input.zipWithIndex
    item <- set
} yield (item -> idx)

The code in your last line can be simplified (but does not what you want):

output = for{
    set <- input
    item <- set
} yield item

Answer 3

Oh boy, that's so ugly...

input.foldLeft(Map[String,Int]())((m,s) => 
   s.foldLeft(m)((n,t) => n + (t -> (1 + n.getOrElse(t,0)))))

[Edit]

The Collection-API needs really a method for "merging" two Maps (or did I just overlook it???), e.g.

def merge[A,B](m1: Map[A,B], m2:Map[A,B])(f: (B,B)=>B):Map[A,B] =
  m1.foldLeft(m2)((m,t) =>
      m + (t._1 -> m.get(t._1).map(k => f(k,t._2)).getOrElse(t._2)))

With this you could write something like:

input.map(_.map(x => x -> 1).toMap).reduceLeft(merge(_,_)(_+_))

[Edit2]

With Kevin's idea merge could be written as

def merge[A,B](m1: Map[A,B], m2:Map[A,B])(f: (B,B)=>B):Map[A,B] =
   m1.keys ++ m2.keys map {k => k ->
        List(m1.get(k), m2.get(k)).flatten.reduceLeft(f)} toMap

Seems like my Scala-Fu is still too weak. What's the best way to express

(o1,o2) match {
    case (Some(x),Some(y)) => Some(f(x,y))    
    case (Some(x), _) => Some(x)    
    case (_, Some(y)) => Some(y)    
    case => error("crack in the time-space-continuum")  
}

?