Scala sum Map values

Question

I have a List

    val l : List[Map[String,Any]] = List(Map("a" -> 1, "b" -> 2.8), Map("a" -> 3, "c" -> 4), Map("c" -> 5, "d" -> "abc"))

and I used the following code to find the sum for the keys "a" (Int), "b" (Double) and "c" (Int). "d" is included as noise.

    l.map(n => n.mapValues( v => if (v.isInstanceOf[Number]) {v match {
    case x:Int => x.asInstanceOf[Int]
    case x:Double => x.asInstanceOf[Double]
    }} else 0)).foldLeft((0,0.0,0))((t, m) => (
t._1 +  m.get("a").getOrElse(0), 
t._2 + m.get("b").getOrElse(0.0), 
t._3 + m.get("c").getOrElse(0)))

I expect the output would be (4, 2.8, 9) but instead I was trashed with

<console>:10: error: overloaded method value + with alternatives:
  (x: Int)Int <and>
  (x: Char)Int <and>
  (x: Short)Int <and>
  (x: Byte)Int
 cannot be applied to (AnyVal)

I think the exception was trying to tell me that '+' doesn't work with AnyVal. How do I get this to work to get my the result that I want? Thanks

agold · Accepted Answer · 2016-06-15 09:59:49Z

8

m.foldLeft(0)(_+_._2)

it's a very clear and simple solution. reference: http://ktuman.blogspot.com/2009/10/how-to-simply-sum-values-in-map-in.html

edited Jun 15, 2016 at 9:59

agold

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answered Jun 15, 2016 at 9:42

Jiayu Wang

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Comments

senia · Accepted Answer · 2012-04-04 10:35:39Z

6

You can use foldLeft function:

scala> val l : List[Map[String,Any]] = List(Map("a" -> 1, "b" -> 2.8), Map("a" -> 3, "c" -> 4), Map("c" -> 5, "d" -> "abc"))
l: List[Map[String,Any]] = List(Map(a -> 1, b -> 2.8), Map(a -> 3, c -> 4), Map(c -> 5, d -> abc))

scala> val (sa, sb, sc) = l.foldLeft((0: Int, 0: Double, 0: Int)){
     |   case ((a, b, c), m) => (
     |     a + m.get("a").collect{case i: Int => i}.getOrElse(0),
     |     b + m.get("b").collect{case i: Double => i}.getOrElse(0.),
     |     c + m.get("c").collect{case i: Int => i}.getOrElse(0)
     |     )
     |   }
sa: Int = 4
sb: Double = 2.8
sc: Int = 9

Updated using incrop's idea of collect instead of match.

edited Apr 4, 2012 at 10:35

answered Apr 4, 2012 at 9:27

senia

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3 Comments

senia Over a year ago

@paradigmatic: Not "exactly": with Option[Any].getOrElse you'll get Any, with match/case you'll get the type you want.

senia Over a year ago

@paradigmatic: Instead of m.get("a").getOrElse(0) you can use m.getOrElse("a", 0), but you'll still get Any.

incrop Over a year ago

You can use collect to get desired type: m.get("a").collect{case x: Int => x}.getOrElse(0)

Didier Dupont · Accepted Answer · 2012-04-04 09:55:13Z

First, you totally miss the point of pattern matching

{case i: Int => i
 case d: Double => d
 case _ => 0}

is the proper replacement of all your function inside mapValues. Yet this is not the problem, your writing, while complex, does the same thing.

Your function in mapValues returns Double (because some branches return Int and others return Double, and in this case, Int is promoted to Double. If it were not, it would return AnyVal). So you get a List[Map[String, Double]]. At this point, you have lost the Ints.

When you do m.get("a"), this returns Option[Double]. Option[A] has method getOrElse(default: A) : A (actually, default: => X) but it makes no difference here).

If you call getOrElse(0.0) instead of getOrElse(0), you get a Double. Your code still fails, because your fold start with (Int, Double, Double), and you would return (Double, Double, Double). If you start your fold with (0.0, 0.0, 0.0), it works, but you have lost your Ints, you get (4.0, 2.8, 9.0)

Now, about the error message. You pass an Int to a method expecting a Double (getOrElse), the Int should normally be converted to Double, and it would be as if you called with getOrElse(0.0). Except that Option is covariant (declared trait Option[+A]). if X is an ancestor of A, an Option[A] is also an Option[X]. So an Option[Double] is also Option[AnyVal] and Option[Any]. The call getOrElse(0) works if the option is considered an Option[AnyVal], and the result is AnyVal (would work with Any too, but AnyVal is more precise and this is the one the compiler chooses). Because the expression compiles as is, there is no need to promote the 0 to 0.0. Thus m.get("a").getOrElse(0) is of type AnyVal, which cannot be added to t._1. This is what your error message says.

You have knowledge that "a" is associated with Int, "b" with double, but you don't pass this knowledge to the compiler.

Destin · Accepted Answer · 2012-04-04 16:11:06Z

0

A nifty one-liner:

l.map(_.filterKeys(_ != "d")).flatten groupBy(_._1) map { case (k,v) => v map { case (k2,v2: Number) => v2.doubleValue} sum }

res0: scala.collection.immutable.Iterable[Double] = List(9.0, 4.0, 2.8)

answered Apr 4, 2012 at 16:11

Destin

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Comments

Community · Accepted Answer · 2017-05-23 11:46:13Z

0

In general, if you don't know the keys, but just want to sum values you can do

val filtered = for {
    map <- l
    (k, v) <- map
    if v.isInstanceOf[Number]
  } yield k -> v.asInstanceOf[Number].doubleValue

val summed = filtered.groupBy(_._1) map { case (k, v) => k -> v.map(_._2).sum }

scala> l
res1: List[Map[String,Any]] = List(Map(a -> 1, b -> 2.8), Map(a -> 3, c -> 4), Map(c -> 5, d -> abc))

scala> filtered
res2: List[(String, Double)] = List((a,1.0), (b,2.8), (a,3.0), (c,4.0), (c,5.0))

scala> summed
res3: Map[String,Double] = Map(c -> 9.0, a -> 4.0, b -> 2.8)

Update

You can filter map by type you want, for example

scala> val intMap = for (x <- l) yield x collect { case (k, v: Int) => k -> v }
intMap: List[scala.collection.immutable.Map[String,Int]] = List(Map(a -> 1), Map(a -> 3, c -> 4), Map(c -> 5))

and then sum values (see linked question)

scala> intMap reduce { _ |+| _ }
res4: scala.collection.immutable.Map[String,Int] = Map(a -> 4, c -> 9)

edited May 23, 2017 at 11:46

CommunityBot

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answered Apr 4, 2012 at 9:50

4e6

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2 Comments

thlim Over a year ago

While it is concise, using this solution I am left with all doubles. Is there a way to retain the type?

4e6 Over a year ago

In this case I can only think about filtering map by some type.

sprague44 · Accepted Answer · 2015-08-25 09:19:04Z

-2

Am I missing something or can you not just do:

map.values.sum

?

answered Aug 25, 2015 at 9:19

sprague44

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1 Comment

The Archetypal Paul Over a year ago

Yes, you're missing something. The sum of each key separately is wanted, and the values have different types

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