Question

I have the following map in Scala:

var m = Map[Int,Set[Int]]()
m += 1 -> Set(1)
m(1) += 2

I’ve discovered that the last line doesn’t work. I get “error: reassignment to val”.

So I tried

var s = m(1)
s += 2

Then when I compared m(1) with s after I added 2 to it, their contents were different. So how can I add an element to a set which is the value of a map?

I come from a Java/C++ background so what I tried seems natural to me, but apparently it’s not in Scala.

Answer 1

You’re probably using immutable.Map. You need to use mutable.Map, or replace the set instead of modifying it with another immutable map.

Here’s a reference of a description of the mutable vs immutable data structures.

So…

import scala.collection.mutable.Map
var m = Map[Int,Set[Int]]()
m += 1 -> Set(1)
m(1) += 2