Scala Cookbook

11.19. Reversing Keys and Values

Problem

You want to reverse the contents of a map, so the values become the keys, and the keys become the values.

Solution

You can reverse the keys and values of a map with a for comprehension, being sure to assign the result to a new variable:

val reverseMap = for ((k,v) <- map) yield (v, k)

But be aware that values don’t have to be unique and keys must be, so you might lose some content. As an example of this, reversing the following map—where two values are $5—results in one of the items being dropped when the keys and values are reversed:

scala> val products = Map(
     |   "Breadsticks" -> "$5",
     |   "Pizza" -> "$10",
     |   "Wings" -> "$5"
     | )
products: scala.collection.mutable.Map[String,String] =
  Map(Wings -> $5, Pizza -> $10, Breadsticks -> $5)

scala> val reverseMap = for ((k,v) <- products) yield (v, k)
reverseMap: scala.collection.mutable.Map[String,String] =
  Map($5 -> Breadsticks, $10 -> Pizza)

As shown, the $5 wings were lost when the values became the keys, because both the breadsticks and the wings had the String value $5.

See Also

• Recipe 3.4

• Recipe 10.13