Search for items in a Map - scala

Question

I am new to scala, my objective is to iterate over list and check if items in the list exist the map as its keys and if they exist return the value for that key.
I did the following:
def getMatchingValues(listItmes: List[String]) = {
for (item <- listItems) {
theMap.keys.foreach { i =>
if (i.equals(item)) {
theMap(i)
}
"NoMatch"
}
}
}
I am trying to figure if there is a better way to do this in scala?

Answer 1

Map has a getOrElse method which does what you want:
def getMatchingValues(listItems: List[String]) = listItems map (theMap.getOrElse(_,"NoMatch"))
At least I think that is what you want. Here's an example:
scala> val theMap = Map("a"->"A", "b" -> "B")
theMap: scala.collection.immutable.Map[String,String] = Map(a -> A, b -> B)
scala> val listItems = List("a","b","c")
listItems: List[String] = List(a, b, c)
scala> listItems map (theMap.getOrElse(_,"NoMatch"))
res0: List[String] = List(A, B, NoMatch)

Answer 2

A possible solution with flatMap:
/* Return Some[String] if found, None if not found, then flatten */
def getMatchingValues(listItems: List[String], theMap: Map[String, String]): List[String] =
listItems.flatMap(item => theMap.get(item))
/* Same thing with some syntactic sugar */
def getMatchingValuesSmartass(listItems: List[String], theMap: Map[String, String]): List[String] =
listItems flatMap theMap.get
val l = List("1", "3", "5", "7")
val m = Map("5" -> "five", "2" -> "two", "1" -> "one")
getMatchingValues(l, m)
getMatchingValuesSmartass(l, m)

Answer 3

You could use the map.get method and handle the result with pattern matching
list.map { x => map.get(x) match {
case None => "No match"
case Some(i) => (x, i)
}}
The above code returns a list of pairs where each pair represents the elements of the list and the value associated in the map ("No match" if not found)

Answer 4

If I was you, I would do two steps. Given this Map:
val map = Map("a" -> "b", "b" -> "c", "c" -> "d", "d" -> "e")
and this List:
val list = List("a", "c", "e")
At first I would map an Option value to every item in the List. Giving you if there is a value for the item.
val mapped = list.map(item => item -> map.get(item))
This will give you this:
mapped: List[(String, Option[String])] =
List(("a",Some("b")), ("c",Some("d")), ("e", None))
Calling get on the map returns a wrapped result. If there is a result, you will get the result wrapped in a Some. Otherwise you will get a None. Both are subclasses of Option Option is a closure-construct, that provides you a null-value without having to deal with null. Now you are able to map again, to reach your goal.
val result = mapped.map(tuple => tuple._1 -> tuple._2.getOrElse("No match"))
result: List[(String, String)] = List(("a","b"), ("c","d"), ("e","No match"))
getOrElse extracts the value of a Some or falls back to the parameter if it is a None.
To make it look more professional, we can write this expression in one line ;)
val result = list.map(item => item -> map.get(item).getOrElse("No match"))
This will give you the exact same result.