"-" not works for normal sortBy arguments

Question
1 
scala> x
val res18: List[(String, Int)] = List((a,2), (b,1), (c,3))

scala> x.sortBy(-_._2)
val res20: List[(String, Int)] = List((c,3), (a,2), (b,1))

scala> x.sortBy(x=>-(x._2))
                 ^
       error: value =>- is not a member of List[(String, Int)]
                       ^
       error: value _2 is not a member of List[(String, Int)]

The first sortBy works. While the second does not work.
Can you help explain it?

Thanks in advance.

2

It seems that all you need to do is add some white spaces to help the parser.

// This works:
 x.sortBy(x => -(x._2))

// Or even just:
 x.sortBy(x => -x._2)
1 Like
3

You need a space after => so (x=> -(x._2)) would work, but Scala programmers invariably put space on both sides of => and the parens around x._2 are unnecessary. So the idiomatic way of writing this would be (x => -x._2).

2 Likes
4

Unrelated to syntax, this may not always do what you expect.

scala> val ps = List("a" -> Int.MinValue, "b" -> 0, "c" -> Int.MaxValue)
val ps: List[(String, Int)] = List((a,-2147483648), (b,0), (c,2147483647))

scala> ps.sortBy(- _._2)
val res0: List[(String, Int)] = List((a,-2147483648), (c,2147483647), (b,0))

More verbose, but safer:

scala> ps.sorted(Ordering.by[(String, Int), Int](_._2)).reverse
val res1: List[(String, Int)] = List((c,2147483647), (b,0), (a,-2147483648))

…or (pairs are ordered by right element by default):
[EDIT: Nonsense, sorry, default ordering for pairs is by first/left element, then by right.]

scala> ps.sorted(implicitly[Ordering[(String, Int)]].reverse)
val res2: List[(String, Int)] = List((c,2147483647), (b,0), (a,-2147483648))
1 Like