How to sum a list of tuples

Question

Given the following list of tuples...

val list = List((1, 2), (1, 2), (1, 2))

... how do I sum all the values and obtain a single tuple like this?

(3, 6)

Florida

Note that if you're using Scalaz you automatically get monoid instances for tuples whose elements have monoid instances, so you can just write list.suml. – Fustic

Answer 1

Using the foldLeft method. Please look at the scaladoc for more information.

scala> val list = List((1, 2), (1, 2), (1, 2))
list: List[(Int, Int)] = List((1,2), (1,2), (1,2))

scala> list.foldLeft((0, 0)) { case ((accA, accB), (a, b)) => (accA + a, accB + b) }
res0: (Int, Int) = (3,6)

Using unzip. Not as efficient as the above solution. Perhaps more readable.

scala> list.unzip match { case (l1, l2) => (l1.sum, l2.sum) }
res1: (Int, Int) = (3,6) 

Overture

Answer 2

Very easy: (list.map(_._1).sum, list.map(_._2).sum).

Ens

Doesn't this traverse the list twice? Not that efficient then. – Auxochrome

Answer 3

You can solve this using Monoid.combineAll from the cats library:

import cats.instances.int._ // For monoid instances for `Int`
import cats.instances.tuple._  // for Monoid instance for `Tuple2`
import cats.Monoid.combineAll

  def main(args: Array[String]): Unit = {

    val list = List((1, 2), (1, 2), (1, 2))

    val res = combineAll(list)

    println(res)
    // Displays
    // (3, 6)
  }

You can see more about this in the cats documentation or Scala with Cats.

Noonday

Answer 4

answering to this question while trying to understand aggregate function in spark

scala> val list = List((1, 2), (1, 2), (1, 2))
list: List[(Int, Int)] = List((1,2), (1,2), (1,2))

   scala>  list.aggregate((0,0))((x,y)=>((y._1+x._1),(x._2+y._2)),(x,y)=>(x._1+y._2,y._2+x._2))
res89: (Int, Int) = (3,6)

Here is the link to the SO QA that helped to understand and answer this [Explain the aggregate functionality in Spark

Shoring

Answer 5

Scalaz solution (suggestied by Travis and for some reason a deleted answer):

import scalaz._
import Scalaz._

val list = List((1, 2), (1, 2), (1, 2))
list.suml

which outputs

res0: (Int, Int) = (3,6)

Sheepfold

Answer 6

You can also use a reduce function :

val list = List((1, 2), (1, 2), (1, 2))

val res = list.reduce((x, y) => (x._1 + y._1, x._2 + y._2))

Idealize