Looking for an FP ranking implementation which handles ties (i.e. equal values)

Question

Starting from a sorted sequence of values, my goal is to assign a rank to each value, using identical ranks for equal values (aka ties):

Input: Vector(1, 1, 3, 3, 3, 5, 6)

Output: Vector((0,1), (0,1), (1,3), (1,3), (1,3), (2,5), (3,6))

A few type aliases for readability:

type Rank = Int
type Value = Int
type RankValuePair = (Rank, Value)

An imperative implementation using a mutable rank variable could look like this:

var rank = 0
val ranked1: Vector[RankValuePair] = for ((value, index) <- values.zipWithIndex) yield {
  if ((index > 0) && (values(index - 1) != value)) rank += 1
  (rank, value)
}

// ranked1: Vector((0,1), (0,1), (1,3), (1,3), (1,3), (2,5), (3,6))

To hone my FP skills, I was trying to come up with a functional implementation:

val ranked2: Vector[RankValuePair] = values.sliding(2).foldLeft((0 , Vector.empty[RankValuePair])) {
  case ((rank: Rank, rankedValues: Vector[RankValuePair]), Vector(currentValue, nextValue)) =>
    val newRank = if (nextValue > currentValue) rank + 1 else rank
    val newRankedValues =  rankedValues :+ (rank, currentValue)
    (newRank, newRankedValues)
}._2

// ranked2: Vector((0,1), (0,1), (1,3), (1,3), (1,3), (2,5))

It is less readable, and – more importantly – is missing the last value (due to using sliding(2) on an odd number of values).

How could this be fixed and improved?

Answer 1

This works well for me:

// scala
val vs = Vector(1, 1, 3, 3, 3, 5, 6)
val rank = vs.distinct.zipWithIndex.toMap
val result = vs.map(i => (rank(i), i))

The same in Java 8 using Javaslang:

// java(slang)
Vector<Integer>                  vs = Vector(1, 1, 3, 3, 3, 5, 6);
Function<Integer, Integer>       rank = vs.distinct().zipWithIndex().toMap(t -> t);
Vector<Tuple2<Integer, Integer>> result = vs.map(i -> Tuple(rank.apply(i), i));

The output of both variants is

Vector((0, 1), (0, 1), (1, 3), (1, 3), (1, 3), (2, 5), (3, 6))

*) Disclosure: I'm the creator of Javaslang