Combining a filter within a map - scala

Question

I have a list which I am combining to a map in this way, by calling the respective value calculation function. I am using collection.breakout to avoid creating unnecessary intermediate collections since what I am doing is a bit combinatorial, and every little bit of saved iterations helps.
I need to filter out certain tuples from the map, in my case where the value is less than 0. Is it possible to add this to the map itself rather than doing a filter afterwards (thus iterating once again)?
val myMap: Map[Key, Int] = keyList.map(key => key -> computeValue(key))(collection.breakOut)
val myFilteredMap = myMap.filter(_._2 >= 0)
In other words I wish to obtain the second map ideally at one go, so ideally in the first call to map() I filter out the tuples I don't want. Is this possible in any way?

Answer 1

You can easily do this with a foldLeft:
keyList.foldLeft( Map[Key,Int]() ) {
(map, key) =>
val value = computeValue(key)
if ( value >= 0 ) {
map + (key -> value)
} else {
map
}
}

Answer 2

It would probably be best to do a flatMap:
import collection.breakOut
type Key = Int
val keyList = List(-1,0,1,2,3)
def computeValue(i: Int) = i*2
val myMap: Map[Key, Int] =
keyList.flatMap { key =>
val v = computeValue(key)
if (v >= 0) Some(key -> v)
else None
}(breakOut)
You can use collect
val myMap: Map[Key, Int] =
keyList.collect {
case key if computeValue(key) >= 0 => key -> computeValue(key)
}(breakOut)
But that requires re-computing computeValue(key), which is silly. Collect is better when you filter then map.
Or make your own method!:
import scala.collection.generic.CanBuildFrom
import scala.collection.TraversableLike
implicit class EnrichedWithMapfilter[A, Repr](val self: TraversableLike[A, Repr]) extends AnyVal {
def maptofilter[B, That](f: A => B)(p: B => Boolean)(implicit bf: CanBuildFrom[Repr, (A, B), That]): That = {
val b = bf(self.asInstanceOf[Repr])
b.sizeHint(self)
for (x <- self) {
val v = f(x)
if (p(v))
b += x -> f(x)
}
b.result
}
}
val myMap: Map[Key, Int] = keyList.maptofilter(computeValue)(_ >= 0)(breakOut)