How to access/initialize and update values in a mutable map?

Starting Scala 2.13, Map#updateWith serves this exact purpose:

map.updateWith("a")({
  case Some(count) => Some(count + 1)
  case None        => Some(1)
})

def updateWith(key: K)(remappingFunction: (Option[V]) => Option[V]): Option[V]

---

For instance, if the key doesn't exist:

val map = collection.mutable.Map[String, Int]()
// map: collection.mutable.Map[String, Int] = HashMap()

map.updateWith("a")({ case Some(count) => Some(count + 1) case None => Some(1) })
// Option[Int] = Some(1)
map
// collection.mutable.Map[String, Int] = HashMap("a" -> 1)

and if the key exists:

map.updateWith("a")({ case Some(count) => Some(count + 1) case None => Some(1) })
// Option[Int] = Some(2)
map
// collection.mutable.Map[String, Int] = HashMap("a" -> 2)

You could create the map with a default value, which would allow you to do the following:

scala> val m = collection.mutable.Map[String, Int]().withDefaultValue(0)
m: scala.collection.mutable.Map[String,Int] = Map()

scala> m.update("a", m("a") + 1)

scala> m
res6: scala.collection.mutable.Map[String,Int] = Map(a -> 1)

As Impredicative mentioned, map lookups are fast so I wouldn't worry about 2 lookups.

Update:

As Debilski pointed out you can do this even more simply by doing the following:

scala> val m = collection.mutable.Map[String, Int]().withDefaultValue(0)
scala> m("a") += 1
scala> m
 res6: scala.collection.mutable.Map[String,Int] = Map(a -> 1)