Idiomatic way to update value in a Map based on previous value

scala map immutability
ffriend picture ffriend · Jan 25, 2012 · Viewed 21.1k times · Source

Let's say I store bank accounts information in an immutable Map: 

val m = Map("Mark" -> 100, "Jonathan" -> 350, "Bob" -> 65)

and I want to withdraw, say, $50 from Mark's account. I can do it as follows: 

val m2 = m + ("Mark" -> (m("Mark") - 50))

But this code seems ugly to me. Is there better way to write this?

Answer

Travis Brown picture Travis Brown · Jan 25, 2012

There's no adjust in the Map API, unfortunately. I've sometimes used a function like the following (modeled on Haskell's Data.Map.adjust, with a different order of arguments):

def adjust[A, B](m: Map[A, B], k: A)(f: B => B) = m.updated(k, f(m(k)))

Now adjust(m, "Mark")(_ - 50) does what you want. You could also use the pimp-my-library pattern to get the more natural m.adjust("Mark")(_ - 50) syntax, if you really wanted something cleaner.

(Note that the short version above throws an exception if k isn't in the map, which is different from the Haskell behavior and probably something you'd want to fix in real code.)

Related questions

Scala immutable map, when to go mutable?

My present use case is pretty trivial, either mutable or immutable Map will do the trick. Have a method that takes an immutable Map, which then calls a 3rd party API method that takes an immutable Map as well def …

Read More
Scala best way of turning a Collection into a Map-by-key?

If I have a collection c of type T and there is a property p on T (of type P, say), what is the best way to do a map-by-extracting-key? val c: Collection[T] val m: Map[P, T] One …

Read More
What is the syntax for adding an element to a scala.collection.mutable.Map?

What is the syntax for adding an element to a scala.collection.mutable.Map ? Here are some failed attempts: val map = scala.collection.mutable.Map map("mykey") = "myval" map += "mykey" -> "myval" map.put("mykey","myval")

Read More