Scala algorithm: Compute missing ranges

Published

Algorithm goal

Given a set of numbers within a range, and the specified range, find the gaps that need to be filled.

Test cases in Scala

assert(missingRanges(items = Nil, range = 0 until 0).isEmpty)
assert(missingRanges(items = List(0), range = 0 until 0).isEmpty)
assert(missingRanges(items = List(0), range = 0 to 0).isEmpty)
assert(missingRanges(items = Nil, range = 0 to 0).isEmpty)
assert(missingRanges(items = List(0), range = 0 to 1) == List(1 to 1))
assert(missingRanges(items = List(0, 1), range = 0 to 2) == List(2 to 2))
assert(missingRanges(items = List(0, 1, 5), range = 0 to 5) == List(2 to 4))
assert(
  missingRanges(items = List(0, 7, 9, 19, 30), range = 0 to 50) ==
    List(1 to 6, 8 to 8, 10 to 18, 20 to 29, 31 to 50)
)

Algorithm in Scala

11 lines of Scala (compatible versions 2.13 & 3.0), showing how concise Scala can be!

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Explanation

We use LazyLists to be able to handle possibly a larger number of items for a potentially large range.

Then, to discover the ranges that are missing, we use a Sliding / Sliding Window to find candidates and then filter them out. (this is © from www.scala-algorithms.com)

Scala concepts & Hints

  1. 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))

  2. For-comprehension

    The for-comprehension is highly important syntatic enhancement in functional programming languages.

    val Multiplier = 10

val result: List[Int] = for {
  num <- List(1, 2, 3)
  anotherNum <-
    List(num * Multiplier - 1, num * Multiplier, num * Multiplier + 1)
} yield anotherNum + 1

assert(result == List(10, 11, 12, 20, 21, 22, 30, 31, 32))

  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. 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))

  5. Sliding / Sliding Window

    Get fixed-length sliding sub-sequences (sliding windows) from another sequence

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Explore the 22 most useful Scala concepts

To save you going through various tutorials, we cherry-picked the most useful Scala concepts in a consistent form.

  1. Class Inside Class
  2. Class Inside Def
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  4. Def Inside Def
  5. Drop, Take, dropRight, takeRight
  6. foldLeft and foldRight
  7. For-comprehension
  8. Lazy List
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  15. Sliding / Sliding Window
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