Get unique values from a List by a property, keeping certain values on duplicates

I have a list like this:

val l= List(("Agent", "PASS"), ("Agent", "FAIL"), ("Agent 1", "FAIL"), ("Agent", "PASS"), ("Agent 2", "PASS") )

I need to end up with a list like this:

val filteredList= List(("Agent", "FAIL"), ("Agent 1", "FAIL"), ("Agent 2", "PASS") )

This:

("Agent", "PASS"), ("Agent", "FAIL")

becomes:

("Agent", "FAIL")

because if there is at least one "FAIL", I need to keep that entry.

The entries for "Agent 1" and "Agent 2" stay the same because there are just one entry for each.

The closest answer I found is How in Scala to find unique items in List, but I cannot tell how to keep the entries with "FAIL".

Solution

Is this what you want?

jem@Respect:~$ scala
Welcome to Scala version 2.8.0.final (Java HotSpot(TM) Client VM, Java 1.6.0_21).
Type in expressions to have them evaluated.
Type :help for more information.

scala> val l= List(("Agent", "PASS"), ("Agent", "FAIL"), ("Agent 1", "FAIL"), ("Agent", "PASS"), ("Agent 2", "PASS") )
l: List[(java.lang.String, java.lang.String)] = List((Agent,PASS), (Agent,FAIL), (Agent 1,FAIL), (Agent,PASS), (Agent 2,PASS))

scala> l.foldLeft(Map.empty[String, String]){(map,next) =>
     |   val (agent, result) = next
     |   if ("FAIL" == result) map.updated(agent, result)
     |   else {           
     |     val maybeExistingResult = map.get(agent)
     |     if (maybeExistingResult.map(_ == "FAIL").getOrElse(false)) map
     |     else map.updated(agent, result)
     |   }
     | }
res0: scala.collection.immutable.Map[String,String] = Map((Agent,FAIL), (Agent 1,FAIL), (Agent 2,PASS))

scala> res0.toList
res1: List[(String, String)] = List((Agent 2,PASS), (Agent 1,FAIL), (Agent,FAIL))

Or here is a shorter and more obscure solution:

scala> l.groupBy(_._1).map(pair => (pair._1, pair._2.reduceLeft((a,b) => if ("FAIL" == a._2 || "FAIL" == b._2) (a._1, "FAIL") else a))).map(_._2).toList
res2: List[(java.lang.String, java.lang.String)] = List((Agent 2,PASS), (Agent 1,FAIL), (Agent,FAIL))