Scala: Sum values across arrays based on key

Question

Very novice Scala programmer so hopefully this is pretty simple.

I have an array of tuples that looks like this:

Array((1,Array(1.0,0.0,5.2,0.0), 
      (1,Array(1.0,0.0,6.3,0.0),
      (2,Array(0.0,1.0,0.0,1.2),
      (2,Array(0.0,1.0,0.0,2.5))

I want to sum up the corresponding values in the second part of the tuple based on the key in the first. So the result would look like this:

Array((1,(2.0,0.0,11.5,0.0),
      (2,(0.0,2.0,0.0,3.7))

The function I came up with is:

def sumByKeys[A](tuples: Array[(String, Array[Double])]) = {
    tuples.groupBy(_._1).mapValues(_.map(_._2).sum)
}

There error I get is

error: could not find implicit value for parameter num: Numeric[Array[Double]]
       tuples.groupBy(_._1).mapValues(_.map(_._2).sum)
                                                  ^

I'm hoping this is something simple where I am just messing up the data types.

Thanks.

why didn't you just use a Map instead?

Electric Coffee
– Electric Coffee

2014-09-26 15:56:38 +00:00
Commented Sep 26, 2014 at 15:56 — Electric Coffee
– Electric Coffee, Commented Sep 26, 2014 at 15:56

The Archetypal Paul · Accepted Answer · 2014-09-26 15:56:41Z

4

scala> a.groupBy(_._1).mapValues(_.map(_._2).transpose.map(_.sum)).toArray
res2: Array[(Int, Array[Double])] = Array((2,Array(0.0, 2.0, 0.0, 3.7)),
                                          (1,Array(2.0, 0.0, 11.5, 0.0)))

edited Sep 26, 2014 at 15:56

The Archetypal Paul

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answered Sep 26, 2014 at 15:26

Marth

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1 Comment

J Calbreath Over a year ago

This does it exactly. Thanks!

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