Scala algorithm: Find k closest elements to a value in a sorted Array


Algorithm goal

Find k closest elements to a value in a sorted Array. Assume no duplicates.

Test cases in Scala

assert(findClosest(arr = Array.empty, target = 5, k = 3) == Set.empty)
  findClosest(arr = (1 to 10).toArray, target = 5, k = 3) == Set(4, 5, 6)
  findClosest(arr = Array(1, 2, 3, 4, 6, 8, 9, 10), target = 5, k = 3) ==
    Set(3, 4, 6)

Algorithm in Scala

14 lines of Scala (version 2.13), showing how concise Scala can be!

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First, we need to find the closest element to the given value: Scala already provides us with a BinarySearch implementation which gives us a way to find an insertion point in an Array!

Then, based on this insertion point, we have a left range and a right range. (this is © from

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Scala concepts & Hints

  1. Def Inside Def

    A great aspect of Scala is being able to declare functions inside functions, making it possible to reduce repetition.

    def exampleDef(input: String): String = {
      def surroundInputWith(char: Char): String = s"$char$input$char"
    assert(exampleDef("test") == "-test-")

    It is also frequently used in combination with Tail Recursion.

  2. Drop, Take, dropRight, takeRight

    Scala's `drop` and `take` methods typically remove or select `n` items from a collection.

    assert(List(1, 2, 3).drop(2) == List(3))
    assert(List(1, 2, 3).take(2) == List(1, 2))
    assert(List(1, 2, 3).dropRight(2) == List(1))
    assert(List(1, 2, 3).takeRight(2) == List(2, 3))
    assert((1 to 5).take(2) == (1 to 2))
  3. Lazy List

    The 'LazyList' type (previously known as 'Stream' in Scala) is used to describe a potentially infinite list that evaluates only when necessary ('lazily').

  4. Ordering

    In Scala, the 'Ordering' type is a 'type class' that contains methods to determine an ordering of specific types.

    assert(List(3, 2, 1).sorted == List(1, 2, 3))
    assert(List(3, 2, 1).sorted(Ordering[Int].reverse) == List(3, 2, 1))
    assert(Ordering[Int].lt(1, 2))
    assert(!Ordering[Int].lt(2, 1))
  5. Pattern Matching

    Pattern matching in Scala lets you quickly identify what you are looking for in a data, and also extract it.

    assert("Hello World".collect {
      case character if Character.isUpperCase(character) => character.toLower
    } == "hw")
  6. Range

    The (1 to n) syntax produces a "Range" which is a representation of a sequence of numbers.

    assert((1 to 5).toString == "Range 1 to 5")
    assert((1 to 5).reverse.toString() == "Range 5 to 1 by -1")
    assert((1 to 5).toList == List(1, 2, 3, 4, 5))

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