Scala collections, single key multiple values
Solution 1
Using MultiMap
You possibly want to use MultiMap
, which is a mutable collection isomorphic to Map[K, Set[V]]
. Use as follows:
import collection.mutable
val mm = new mutable.HashMap[Int, mutable.Set[String]] with mutable.MultiMap[Int, String]
Then you add your nodes:
mm addBinding (key, value)
Without MultiMap
The alternative is to stick with immutable values. Assuming you want to avoid using lenses (see scalaz), you can add nodes as follows:
nodes += (key -> (value :: (nodes get key getOrElse Nil)))
Here it is working (in response to your comment):
scala> var nodes = Map.empty[Int, List[String]]
nodes: scala.collection.immutable.Map[Int,List[String]] = Map()
scala> def addNode(key: Int, value: String) =
| nodes += (key -> (value :: (nodes get key getOrElse Nil)))
addNode: (key: Int, value: String)Unit
scala> addNode(1, "Hi")
scala> addNode(1, "Bye")
scala> nodes
res2: scala.collection.immutable.Map[Int,List[String]] = Map(1 -> List(Bye, Hi))
Using Scalaz
Using the scalaz library, you can realize that this is simply using the Empty
pattern:
nodes += (key -> (value :: ~(nodes get key)))
Or you could take advantage of the fact that Map
is a monoid:
nodes = nodes |+| Map(key -> List(value))
Solution 2
In addition to @oxbow_lakes' answer, here's a idea for how you could use an addMap
method that correctly adds two maps together (ie, combining lists for matching keys, adding new lists for new keys):
class EnhancedListMap(self: Map[Int,List[String]]) {
def addMap(other: Map[Int,List[String]]) =
(this.ungroup ++ enhanceListMap(other).ungroup)
.groupBy(_._1)
.mapValues(_.map(_._2))
def ungroup() =
self.toList.flatMap{ case (k,vs) => vs.map(k -> _) }
}
implicit def enhanceListMap(self: Map[Int,List[String]]) = new EnhancedListMap(self)
And you'd use it like this:
val a = Map(1 -> List("a","b"), 2 -> List("c","d"))
val b = Map(2 -> List("e","f"), 3 -> List("g","h"))
a addMap b
//Map(3 -> List(g, h), 1 -> List(a, b), 2 -> List(c, d, e, f))
You can include addNode
, addValue
, and addValues
the same way (to EnhancedListMap
above):
def addNode(key: Int) =
if(self contains key) self else self + (key -> Nil)
def addValue(key: Int, value: String) =
self + (key -> (value :: (self get key getOrElse Nil)))
def addValues(key: Int, values: List[String]) =
self + (key -> (values ::: (self get key getOrElse Nil)))
And then use them together:
var nodes = Map.empty[Int, List[String]]
// Map()
nodes = nodes.addNode(1)
// Map(1 -> List())
nodes = nodes.addValue(1,"a")
// Map(1 -> List(a))
nodes = nodes.addValue(2,"b")
// Map(1 -> List(a), 2 -> List(b))
nodes = nodes.addValues(2,List("c","d"))
// Map(1 -> List(a), 2 -> List(c, d, b))
nodes = nodes.addValues(3,List("e","f"))
// Map(1 -> List(a), 2 -> List(c, d, b), 3 -> List(e, f))
nodes = nodes.addMap(Map(3 -> List("g","h"), 4-> List("i","j")))
// Map(1 -> List(a), 2 -> List(c, d, b), 3 -> List(e, f, g, h), 4 -> List(i, j))
Nabegh
Updated on July 05, 2022Comments
-
Nabegh almost 2 years
I have a list of parent keys, each of which could possibly have zero or more associated values. I am not sure which collection to use.
I am using
Map[Int,List[String]]
I am declaring the Map as
var nodes = new HashMap[Int, List[String]]
Then I have two methods to handle adding new elements. The first is to add new keys
addNode
and the second is to add new valuesaddValue
. Initially, the key will not have any values associated with it. Later on, during execution, new values will be associated.def addNode(key: Int) = nodes += (key -> "") def addValue(key: Int, value: String) = ???
I am not sure how to implement
addValues
Update:
In response to @oxbow-lakes answer, This is the error I am receiving. Please note that keys need not have values associated with them.
scala> var nodes = Map.empty[Int, List[String]] nodes: scala.collection.immutable.Map[Int,List[String]] = Map() scala> nodes += (1->null) scala> nodes += (1 -> ("one" :: (nodes get 1 getOrElse Nil))) java.lang.NullPointerException at .<init>(<console>:9) at .<clinit>(<console>) at .<init>(<console>:11) at .<clinit>(<console>) at $print(<console>) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:704) at scala.tools.nsc.interpreter.IMain$Request$$anonfun$14.apply(IMain.scala:920) at scala.tools.nsc.interpreter.Line$$anonfun$1.apply$mcV$sp(Line.scala:43) at scala.tools.nsc.io.package$$anon$2.run(package.scala:25) at java.lang.Thread.run(Thread.java:680)
Update 2:
The problem with the code above is the line
nodes += (1->null)
the key should be associated withNil
instead. Below is the working code.scala> var nodes = Map.empty[Int, List[String]] nodes: scala.collection.immutable.Map[Int,List[String]] = Map() scala> nodes += (1->Nil) scala> nodes += (1 -> ("one" :: (nodes get 1 getOrElse Nil))) scala> nodes res27: scala.collection.immutable.Map[Int,List[String]] = Map(1 -> List(one))
-
Nabegh almost 12 yearsThis line is failing
value :: (nodes get key getOrElse Nil)
I am getting NullPointerException -
oxbow_lakes almost 12 yearsI have added an extract from a REPL to show it definitively working. The cause of the NPE must be something else, I'm afraid. Perhaps it warrants a separate question?
-
oxbow_lakes almost 12 yearsPerhaps in your example, the
nodes
variable isnull
?