Scala Map - Use map function to replace key->value

Question

I want to change the keys and values for the keys key1 and key2 only when their values are val1 and val2 (both these mappings should be present for the transformation to take place). I am able to do it using the following code, but I do not think this is very elegant or efficient.

Is there a better way to do the same thing, perhaps using just one .map function applied over map?

Code:

val map = Map(
  "key1" -> "val1",
  "key2" -> "val2",
  "otherkey1" -> "otherval1"
)

val requiredKeys = List("key1", "key2")

val interestingMap = map.filterKeys(requiredKeys.contains) // will give ("key1" -> "val1", "key2" -> "val2").

val changedIfMatched =
  if (interestingMap.get("key1").get.equalsIgnoreCase("val1") && interestingMap.get("key2").get.equalsIgnoreCase("val2"))
    Map("key1" -> "newval1", "key2" -> "newval2")
  else
    interestingMap

print(map ++ changedIfMatched) // to replace the old key->values with the new ones, if any.

Also can ++ operation to update the old key->value mappings be made more efficient?

Answer 1

Just do the check ahead of time:

 map
   .get("k1").filter(_.equalsIgnoreCase("v1"))
   .zip(map.get("k2").filter(_.equalsIgnoreCase("v2")))
   .headOption
   .fold(map) { _ =>
      map ++ Map("key1" -> "newVal1", "key2" -> "newVal2")
   }

Answer 2

You can use the following code:

val interestingMap = 
    if(map.getOrElse("key1", "") == "val1" && map.getOrElse("key2", "") == "val2")
      map - "key1" - "key2" + ("key1New" -> "val1New") + ("key2New" -> "val2New")
    else map

The check part(if statement) can be tweaked to suit your specific need.

if any of these key-value pairs are not present in the map, the original map will be returned, otherwise, you will get a new map with two updates at the requested keys.

Regarding efficiency, as long as there are only two keys to be updated, I do not think there is a real performance difference between using + to add elements directly and using ++ operator to overwrite the keys wholesale. If your map is huge though, maybe using a mutable map proves to be a better option in the long run.

Answer 3

Here's an approach that checks that both key value pairs match.

EDIT: Added a mapValues method to the Map class. This technique can be used to do further checks on the values of the map.

val m = Map("key1" -> "val1", "key2" -> "VAL2", "otherkey1" -> "otherval1")
val oldKVs = Map("key1" -> "val1", "key2" -> "val2")
val newKVs = Map("newkey1" -> "newval1", "newkey2" -> "newval2")

implicit class MapImp[T,S](m: Map[T,S]) {
  def mapValues[R](f: S => R) = m.map { case (k,v) => (k, f(v)) }
  def subsetOf(m2: Map[T,S]) = m.toSet subsetOf m2.toSet
}

def containsKVs[T](m: Map[T,String], sub: Map[T,String]) =
  sub.mapValues(_.toLowerCase) subsetOf m.mapValues(_.toLowerCase)

val m2 = if (containsKVs(m, oldKVs)) m -- oldKVs.keys ++ newKVs else m

println(m2)
// Map(otherkey1 -> otherval1, newkey1 -> newval1, newkey2 -> newval2)

It takes advantage of the fact that you can convert Maps into Sets of Tuple2.

Answer 4

Answer to modified question

val map = Map(
  "key1" -> "val1",
  "key2" -> "val2",
  "otherkey1" -> "otherval1"
)

val requiredVals = List("key1"->"val1", "key2"->"val2")
val newVals = List("newval1", "newval2")

val result =
  if (requiredVals.forall{ case (k, v) => map.get(k).exists(_.equalsIgnoreCase(v)) }) {
    map ++ requiredVals.map(_._1).zip(newVals)
  } else {
    map
  }

This solution use forall to check that all the key/value pairs in requiredKeys are found in the map by testing each pair in turn.

For each key/value pair (k, v) it does a get on the map using the key to retrieve the current value as Option[String]. This will be None if the key is not found or Some(s) if the key is found.

The code then calls exists on the Option[String]. This method will return false if value is None (the key is not found), otherwise it will return the result of the test that is passed to it. The test is _.equalsIgnoreCase(v) which does a case-insensitive comparison of the contents of the Option (_) and the value from the requireKeys list (v).

If this test fails then the original value of map is returned.

If this test succeeds then a modified version of the map is return. The expression requiredVals.map(_._1) returns the keys from the requireVals list, and the zip(newVals) associates the new values with the original keys. The resulting list of values is added to the map using ++ which will replace the existing values with the new ones.

Original answer

val map = Map(
  "key1" -> "val1",
  "key2" -> "val2",
  "otherkey1" -> "otherval1"
)

val requiredVals = Map("key1"->"val1", "key2"->"val2")
val newVals = Map("newkey1" -> "newval1", "newkey2" -> "newval2")

val result =
  if (requiredVals.forall{ case (k, v) => map.get(k).exists(_.equalsIgnoreCase(v)) }) {
    map -- requiredVals.keys ++ newVals
  } else {
    map
  }

Note that this replaces the old keys with the new keys, which appears to be what is described. If you want to keep the original keys and values, just delete "-- requiredVals.keys" and it will add the new keys without removing the old ones.

Answer 5

I think this will be the most generic and resuable solution for the problem.

object Solution1 extends App {

  val map = Map(
    "key1" -> "val1",
    "key2" -> "val2",
    "otherkey1" -> "otherval1"
  )

  implicit class MapUpdate[T](map: Map[T, T]) {
    def updateMapForGivenKeyValues: (Iterable[(T, T)], Iterable[(T, T)]) => Map[T, T] =
      (fromKV: Iterable[(T, T)], toKV: Iterable[(T, T)]) => {

        val isKeyValueExist: Boolean = fromKV.toIterator.forall {
          (oldKV: (T, T)) =>
            map.toIterator.contains(oldKV)
        }

        if (isKeyValueExist) map -- fromKV.map(_._1) ++ toKV else map
      }
  }


  val updatedMap = map.updateMapForGivenKeyValues(List("key1" -> "val1", "key2" -> "val2"),
    List("newKey1" -> "newVal1", "newVal2" -> "newKey2"))

  println(updatedMap)

}

So the method updateMapForGivenKeyValues takes the List of old key value and new key value tuple. If all the key value pairs mentioned in the first parameter of the method exist in the map then only we will update the map with new key value pairs mentioned in the second parameter of the method. As the method is generic will can be used on any data type like String, Int, some case class etc.

we can easily re-use the method for different type of maps without even changing a single line of code.