Appending to Map with value as a list - scala

Question

I have initialized a mutable Map (not sure if I should use a mutable here, but to start with I use mutable):
val aKey = "aa"
val myMap = scala.collection.mutable.Map[String, List[String]]()
if (myMap.exists(_._1 == aKey))
myMap(aKey) = myMap.get(aKey) :: "test"
But on myMap.get(aKey) I get the following error:
Type Mismatch expected List[String] actual option[String]
I thought the way I did is correct to append to list.

Answer 1

You can append to a mutable map with +=.
scala> myMap += ("placeholders" -> List("foo", "bar", "baz"))
res0: scala.collection.mutable.Map[String,List[String]] = Map(placeholders -> List(foo, bar, baz))
To append a new item to the list for aKey as mentioned in the commments.
myMap.get("placeholders") match {
case Some(xs:List[String]) => myMap.update("placeholders", xs :+ "buzz")
case None => myMap
}
res22: Any = ()
scala> myMap
res23: scala.collection.mutable.Map[String,List[String]] = Map(placeholders -> List(foo, bar, baz, buzz))

Answer 2

If you have mutable Map and inside that map immutable List. This is a simple example on how to do it. The example is also defining using withDefaultValue - so that you always get something back from Map.
var posts: collection.mutable.Map[Int, List[String]] = collection.mutable.Map().
withDefaultValue List.empty[String]
def addTag(postID: Int, tag: String): Unit = posts.get(postID) match {
case Some(xs: List[String]) => posts(postID) = xs :+ tag
case None => posts(postID) = List(tag)
}
posts(42)
// List[String] = List()
addTag(42, "tag-a")
addTag(42, "tag-b")
addTag(42, "tag-c")
posts(42)
// List[String] = List(tag-a, tag-b, tag-c)

Answer 3

Everything is fine. Except for list append operator
To add an element to the list. The operator should be something like
myList = element :: myList
myList = myList :: element // wrong
so your program should be
val aKey = "aa"
var myMap = scala.collection.mutable.Map[String, List[String]]().withDefaultValues(List())
myMap(aKey) = "test" :: myMap(aKey)

Answer 4

Starting Scala 2.13, you could alternatively use Map#updateWith on mutable Maps (or Map#updatedWith on immutable Maps):
map.updateWith("a")({
case Some(list) => Some("test" :: list)
case None => Some(List("test"))
})
def updateWith(key: K)(remappingFunction: (Option[V]) => Option[V]): Option[V]
For instance,
val map = collection.mutable.Map[String, List[String]]()
// map: collection.mutable.Map[String, List[String]] = HashMap()
val key = "key"
// key: String = "key"
if the key doesn't exist:
map.updateWith(key)({ case Some(list) => Some("test" :: list) case None => Some(List("test")) })
// Option[List[String]] = Some(List("test"))
map
// collection.mutable.Map[String, List[String]] = HashMap("key" -> List("test"))
and if the key exists:
map.updateWith(key)({ case Some(list) => Some("test2" :: list) case None => Some(List("test2")) })
// Option[List[String]] = Some(List("test2", "test"))
map
// collection.mutable.Map[String, List[String]] = HashMap("key" -> List("test2", "test"))

Answer 5

I figured the how to do it:
val aKey = "aa"
var myMap = scala.collection.mutable.Map[String, List[String]]()
if (myMap.exists(_._1 == aKey))
myMap.get(aKey).get :+ "foo"
else {
myMap(aKey) = List("zoo")
}
This might not be the scala way of doing but this gives the correct results.

Answer 6

first you shouldn't Mutable Map :). for add one item on List within Map, you can use method get of Map.
val m = Map(1 -> List(2))
val m2 = m.get(1).fold{
// In this case don't exist any List for this key
m + (1 -> List(3))
}{ list =>
// In this case exist any List for this key
m + (1 -> (3 :: list))
}