Question

I have a 3-tuple list like the following [I added line breaks for readability]:

(2, 127, 3)
(12156, 127, 3)
(4409, 127, 2) <-- 4409 occurs 2x
(1312, 127, 12) <-- 1312 occurs 3x

(4409, 128, 1) <-- 
(12864, 128, 1)
(1312, 128, 1) <-- 
(2664, 128, 2)

(12865, 129, 1)
(183, 129, 1)
(12866, 129, 2)
(1312, 129, 10) <--

I want to sum up based on the first entry. The first entry should be unique.

The result should look like this:

(2, 127, 3)
(12156, 127, 3)
(4409, 127, 3) <- new sum = 3
(1312, 127, 23) <- new sum = 23

(12864, 128, 1)
(2664, 128, 2)

(12865, 129, 1)
(183, 129, 1)
(12866, 129, 2)

How can I achieve this in Scala?

Answer 1

Try this:

list groupBy {_._1} mapValues {v => (v.head._1, v.head._2, v map {_._3} sum)}

The middle entry is preserved and it always takes the first one that appeared in the input list.