What's wrong with this piece of scala code? - scala

Question

I have the following code snippet which can not be compiled:
val cids = List(1, 2, 3, 4)
val b = Map.newBuilder[Int, Int]
for (c <- cids) {
b += (c, c*2)
}
b.result()
Compiler reports that
console>:11: error: type mismatch;
found : Int
required: (Int, Int)
b += (c, c*2)
I have no idea what's the mistake.

Answer 1

This would work:
for (c <- cids) {
b += ((c, c*2))
}
The parenthesis are parsed by compiler as the argument-list parenthesis of the += function, and not as a tuple. Adding nested parenthesis means a tuple is passed as the argument. It is confusing...

Answer 2

You can fix it the following way:
b += (c->c*2)

Answer 3

This is a duplicate question.
Normally, supplying an untupled arg list works as shown, but it doesn't work when the method is overloaded, because it will choose the method you didn't intend, and not bother to try auto-tupling your args.
scala> class C[A] { def f(a: A) = 42 }
defined class C
scala> val c = new C[(String, Int)]
c: C[(String, Int)] = C#3a022576
scala> c.f("1", 1)
res0: Int = 42
scala> class C[A] { def f(a: A) = 42 ; def f(a: A, b: A, rest: A*) = 17 }
defined class C
scala> val c = new C[(String, Int)]
c: C[(String, Int)] = C#f9cab00
scala> c.f("1", 1)
<console>:14: error: type mismatch;
found : String("1")
required: (String, Int)
c.f("1", 1)
^

Answer 4

An approach using (immutable) values,
(cids zip cids.map(_ * 2)).toMap
Using zip we pair each value with its double, and the resulting list is converted to a Map.

Answer 5

If you go to the documentation you will find : this
The supported API is ms += (k -> v) . That is you need to use
for (c <- cids) {
b += (c -> c*2)
}
Alternately you could use the ((c, c*2)) syntax as suggested above. This is happening because the compiler has no way of knowing that the parentheses are for the tuple. It simply understands that argument as two parameters being passed to the += method.