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I'm converting some code from Ruby to Scala. Problem is that I never programmed Ruby in my life. It's going well, but now I reached a line that I don't know because I'm new in Scala and I don't understand the sorting mechanism. So I want to translate this ruby line to scala:

fronts[last_front].sort! {|x,y| crowded_comparison_operator(x,y)}

fronts is Vector[Vector[Map[String, Any]]]

last_front is an Int

crowded_comparison_operator(x,y) returns -1, 0 or 1, x and y are Map[String, Any]

flyist
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  • Have you looked at [the docs for sortBy](https://www.scala-lang.org/api/current/scala/collection/Seq.html#sortBy[B](f:A=>B)(implicitord:scala.math.Ordering[B]):Repr)? – Brian McCutchon May 13 '18 at 17:10
  • Sure, but `crowded_comparison_operator(x,y)` takes two arguments and that is confusing to me. Sorry i'm a bit noob in Scala/Java – flyist May 13 '18 at 17:14

1 Answers1

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You have two possibilities with standard Scala collections:

  • Convert the -1, 0, 1 output of crowded_comparison_operator into a boolean that tells you whether the first element is less than the second element, then use sortWith.
  • define a new Ordering, pass it explicitly to the sorted method.

The sortWith method

The first element is less than the second element if and only if crowded_comparison_operator returns -1, so you could do this:

fronts(last_front).sortWith{ (x, y) => crowded_comparison_operator(x, y) < 0 }

Defining an Ordering for sorted

The sorted method takes an implicit Ordering parameter. You can define your own custom ordering, and pass it explicitly:

import scala.math.Ordering

fronts(last_front).sorted(new Ordering[Vector[Map[String, Any]]] {
  def compare(
    x: Vector[Map[String, Any]], 
    y: Vector[Map[String, Any]]
  ): Int = crowded_comparison_operator(x, y)
})

or shorter, with scala versions supporting SAM (since 2.11.5, if I remember correctly): 

fronts(last_front).sorted(
  (x: Vector[Map[String, Any], y: Vector[Map[String, Any]]) => 
    crowded_comparison_operator(x, y)
)

Note that, as @mikej has pointed out, Ruby's sort! sorts the array in-place. This cannot work for an immutable vector, so you have to adjust your code accordingly.

Andrey Tyukin
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    This answer is great! The only thing to add is that `sort!` in Ruby performs an in place sort. There is no equivalent in Scala so you might need to adjust other parts of your Scala to allow for a new `Vector[Map[String, Any]]` being returned rather than the original being modified. – mikej May 13 '18 at 17:30
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    @mikej Ah, thanks, that's an important difference. Unfortunately, this cannot be changed easily, since `Vector` is immutable. Thank you for pointing it out. – Andrey Tyukin May 13 '18 at 17:32
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    As of [Scala 2.12](https://www.scala-lang.org/news/2.12.0/), you can just write `fronts(last_front).sorted(crowded_comparison_operator)` for the last one (assuming `crowded_comparison_operator` is a method; if it's a function, you might need `fronts(last_front).sorted(crowded_comparison_operator(_, _))`). – Brian McCutchon May 13 '18 at 17:39
  • @BrianMcCutchon Thanks, updated. I assume that `crowded_comparison_operator` is not an actual method name, but rather a placeholder for a lengthy body of a ruby in-line function literal. I therefore included the explicit type annotations `Vector[Map[String, Any]]`, because otherwise the compiler whines at me because of "error: missing parameter type". – Andrey Tyukin May 13 '18 at 19:01