How to call .map() on a list of pairs in Scala

Question

I want to call map() on a list of pairs, but I get a type mismatch error.

For example, suppose I want to map a List of pairs of Int to a list of their sums:

scala> val ll=List((1,2),(3,4),(5,6)) ll: List[(Int, Int)] = List((1,2), (3,4), (5,6))  scala> ll.map((x:Int,y:Int)=>x+y) <console>:9: error: type mismatch;  found   : (Int, Int) => Int  required: ((Int, Int)) => ?               ll.map((x:Int,y:Int)=>x+y)                                   ^ 

By the way, when trying to run foreach() I get a very similar error:

scala> ll.foreach((x:Int,y:Int)=>println(x,y)) <console>:9: error: type mismatch;  found   : (Int, Int) => Unit  required: ((Int, Int)) => ?               ll.foreach((x:Int,y:Int)=>println(x,y))                                   ^ 

What does the ? sign stand for? What am I missing here?

Answer 1

You can use pattern matching to get the elements of the pair.

ll.map{ case (x:Int,y:Int) => x + y } 

You don't even need to specify the types:

ll.map{ case (x, y) => x + y } 

The same works with foreach of course.

The error message tells you that the compiler expected to find a function of one parameter (a pair of ints) to any type (the question mark) and instead found a function of two parameters, both ints.