# 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!

## 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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