What is the Scala way of summing fields of list of tuples?

Question

I want to reduce list of tuples that have a common field such as x by summing y fields.

Input:

List(("x" -> "foo", "y" -> 1000),
     ("x" -> "foo", "y" -> 1),
     ("x" -> "bar", "y" -> 101))

Output:

List(("x" -> "foo", "y" -> 1001),
     ("x" -> "bar", "y" -> 101))

What is the good approach for that? foldLeft or reduce?

You should use json as output format, dont't use manipulate data. — blueiur, Aug 23 '15 at 05:02
The input and output are invalid. Could you fix that please. — Alexandr Nikitin, Aug 23 '15 at 09:58
@AlexandrNikitin I think, the inputs and outputs are correct now. — Muhtar Lupen, Aug 23 '15 at 10:22
This seems two entirely different questions. How to reduce/sum by key a list of tuples, and how to convert a List of tuples into JSON. Why do you think they are related? — The Archetypal Paul, Aug 23 '15 at 10:41
@Paul, blueiur's comment seemed reasonable so I wanted to update the input so that the input is different from JSON. Anyway, you're right, too. We can skip JSON conversion. I will edit the question. — Muhtar Lupen, Aug 23 '15 at 10:58

score 1 · Accepted Answer · edited Aug 23 '15 at 10:44

1

Your item should be tuple:

List(("x" -> "foo", "y" -> 1000),("x" -> "foo", "y" -> 1), ("x" -> "bar", "y" -> 101))

use groupBy to group your key, and mapValues to generate the target:

List(("x" -> "foo", "y" -> 1000),("x" -> "foo", "y" -> 1), ("x" -> "bar", "y" -> 101))
 .groupBy(x => x._1)
 .mapValues(t => t.head._2._1 -> t.foldLeft(0.0)(_ + _._2._2))

The Output:

scala.collection.immutable.Map[(String, String),(String, Double)] = 
    Map((x,foo) -> (y,1001.0), (x,bar) -> (y,101.0))

edited Aug 23 '15 at 10:44

The Archetypal Paul

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answered Aug 23 '15 at 03:53

chengpohi

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This seems a reasonable answer to the first part of the question. Not sure why it got a downvote – The Archetypal Paul Aug 23 '15 at 10:43

What is the Scala way of summing fields of list of tuples?

1 Answers1