Scala - type mismatch; found Int, required String

Question

I have a problem with type mismatch in Scala. Compiler returns an error:

error: type mismatch; found : Int required: String val v3:Int = (v1+v2)

This is code of my function:

    def sumMaps[T, Int](m1: Map[T, Int], m2: Map[T, Int]): Map[T, Int] = {
        val sameKeys = m1.keySet.intersect(m2.keySet)
        var m3: Map[T, Int] = m1 ++ m2
        for (key <- sameKeys) {
            val v1:Int = m1(key)
            val v2:Int = m2(key)
            val v3:Int = (v1+v2)
            //val v:Int = (m1(key) + m2(key))
            m3 = m3 updated (key, v3)
        }
    m3
}

Why? Any idea what could be the problem?

David Frank · Accepted Answer · 2015-02-04 11:00:26Z

9

You are redeclaring Int as a type parameter when it is a concrete type.

You should change the method head to:

def sumMaps[T](m1: Map[T, Int], m2: Map[T, Int]): Map[T, Int] = {

When Int is a type parameter, the compiler replaces (v1+v2) with a string addition, so it becomes (v1.toString + v2.toString), this gave you the problem.

UPDATE

I came up with a simpler solution to your problem:

def sumMaps[T](m1: Map[T, Int], m2: Map[T, Int]): Map[T, Int] = {
  (for (key <- m1.keySet ++ m2.keySet) yield {
    val v1: Int = m1.getOrElse(key, 0)
    val v2: Int = m2.getOrElse(key, 0)
    key -> (v1 + v2)
  }).toMap
}

Or an even simpler one:

def sumMaps[T](m1: Map[T, Int], m2: Map[T, Int]): Map[T, Int] = {
  (m1 ++ m2).transform {
    case (key, _) => m1.getOrElse(key, 0) + m2.getOrElse(key, 0)
  }
}

edited Feb 4, 2015 at 11:00

answered Feb 4, 2015 at 10:02

David Frank

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3 Comments

Zoltán Over a year ago

@user3858210 Yes, the part in the square brackets before the parentheses in the method definition lists names which you will use to refer to generic types in your method. The only generic type you are using is T, whereas Int is a concrete type, so there's no need to list it in the square brackets.

Wojciech Kałuski Over a year ago

@David Frank Thank you. Earlier I wrote subMap[T, U], but I decided to change U to Int. This caused my error. But Scala Compiler named this error so indistinctly then I coudn't find it. Sorry for my poor english.

David Frank Over a year ago

@user3858210 Please have a look at my update, it might be useful.

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