scala add _2 s in a list of Tuple 2

Question

I have the following mutable Hashmap in Scala:

HashMap((b,3), (c,4), (a,8), (a,2))

and need to be converted to the following:

HashMap((b,3), (c,4), (a,10))

I need something like reduceByKey function logic.

I added the code here

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

    val m = new mutable.HashMap[String,Tuple2[String,Int]]()
    println("Hello, world")
    m.+=(("xx",("a",2)))
    m.+=(("uu",("b",3)))
    m.+=(("zz",("a",8)))
    m.+=(("yy",("c",4)))

    println(m.values)
  }

Keys in a hashmap are unique, so you do not have that mutable hashmap. By the time you've put those keys into a hashmap, you've already lost the older of them. Where do you get this data from? Can you get it as, perhaps, a list of tuples or something that allows duplicates? — Silvio Mayolo
– Silvio Mayolo, Commented May 6, 2022 at 15:51
@SilvioMayolo I added the code and that was my output of println. — Majid Hajibaba
– Majid Hajibaba, Commented May 6, 2022 at 15:54
What purpose do the keys in the hashmap serve in your code example? It looks like you're grouping by a part of the value, not by the key? Is there any significance to the xx, yy, etc. part, or can we just get rid of that? — Silvio Mayolo
– Silvio Mayolo, Commented May 6, 2022 at 15:55
m.values.groupMapReduce(tuple => tuple._1)(tuple => tuple._2)((a, b) => a + b) — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented May 6, 2022 at 16:13

Guru Stron · Accepted Answer · 2022-05-06 17:12:33Z

3

For pre 2.13 Scala versions you can try using groupBy with map:

m.values
  .groupBy(_._1)
  .mapValues(_.map(_._2).sum)

answered May 6, 2022 at 17:12

Guru Stron

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Comments

Silvio Mayolo · Accepted Answer · 2022-05-06 16:16:49Z

2

It sounds like what you have is not a hashmap but m.values of type Iterable[Tuple2[String, Int]], which is more manageable. In that case, as hinted at in the comments, groupMapReduce does it all in one function. This function groups "matching" elements together, applies a transformation to each element, and then reduces the groups using a binary operation.

m.values.groupMapReduce(_._1)(_._2)(_ + _)

This says "Group the values by the first element of their tuple, then keep the second element (i.e. the number), and then add all of the numbers in each group". This produces a map from the first element of the tuple to the sum.

Map(a -> 10, b -> 3, c -> 4)

Note that this is a Map, not necessarily a HashMap. If you want a HashMap (i.e. for mutability), you'll need to convert it yourself.

answered May 6, 2022 at 16:16

Silvio Mayolo

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2 Comments

Majid Hajibaba Over a year ago

I used Scala 2 and groupMapReduce cannot be resolved by compiler.

Guru Stron Over a year ago

@MajidHajibaba groupMapReduce was introduced in Scala 2 =)

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