Scala algorithm: Rotate Array right in pure-functional Scala - using an unusual immutable efficient approach

Published

Algorithm goal

Rotating the array \([1,2,3]\) by 1 element to the right returns an array \([3,1,2]\). Example:

Array before:
123
Array after rotating by \(n = 1\):
312
Array after rotating by \(n = 2\):
231

Rotating the array \([1,2,3]\) 3 times returns us to the original array \([1,2,3]\).

Test cases in Scala

assert(countToDrop(length = 0, rightShift = 0) == 0)
assert(countToDrop(length = 1, rightShift = 0) == 1)
assert(countToDrop(length = 5, rightShift = 0) == 5)
assert(countToDrop(length = 5, rightShift = 1) == 4)
assert(countToDrop(length = 5, rightShift = 5) == 5)
assert(countToDrop(length = 4, rightShift = 2) == 2)
assert(rotateArrayRight(Array(), 3).isEmpty)
assert(rotateArrayRight(Array(1), 3).toVector == Vector(1))
assert(rotateArrayRight(Array(1, 2), 0).toVector == Vector(1, 2))
assert(
  rotateArrayRight(Array(1, 2, 3, 4), 1).toVector == Vector(4, 1, 2, 3)
)
assert(
  rotateArrayRight(Array(1, 2, 3, 4), 5).toVector == Vector(4, 1, 2, 3)
)

Algorithm in Scala

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

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Explanation

The most tricky part is finding the trick - which is to join two Arrays side-by-side, and then slice that array to get a continuous slice that simulates a rotation - no mutations needed!

In Scala, we use the View concept to turn our array \([1,2,3]\) into an array \([1,2,3,1,2,3]\) with concatenation (without doing any allocations yet!), and then select the expected slice. (this is © from www.scala-algorithms.com)

For example, when \(n = 1\):

Array before:
123123

Full explanation is available to subscribers Scala algorithms logo, maze part, which looks quirky

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. View

    The .view syntax creates a structure that mirrors another structure, until "forced" by an eager operation like .toList, .foreach, .forall, .count.

Scala Algorithms: The most comprehensive library of algorithms in standard pure-functional Scala

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  6. Unit tests, with a button to run them immediately in our in-browser IDE.
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Fully unit-tested, with explanations and relevant concepts; new algorithms published about once a week.

  1. Compute the length of longest valid parentheses
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  4. Remove duplicates from an unsorted List
  5. Make a queue using stacks (Lists in Scala)
  6. Find height of binary tree
  7. Single-elimination tournament tree
  8. Reverse Polish Notation calculator
  9. Quick Sort sorting algorithm in pure immutable Scala
  10. Check word in grid (depth-first search)
  11. Maximum wait at a fuel station
  12. Find minimum missing positive number in a sequence
  13. Least-recently used cache (LRU)
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  18. Matching parentheses algorithm with foldLeft and a state machine
  19. Traverse a tree Breadth-First, immutably
  20. Read a matrix as a spiral
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  24. Leaky Bucket Rate Limiter
  25. Merge Sort: stack-safe, tail-recursive, in pure immutable Scala, N-way
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  28. Reverse first n elements of a queue
  29. Binary search a generic Array
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  31. Merge Sort: in pure immutable Scala
  32. Make a queue using Maps
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  36. Tic Tac Toe MinMax solve
  37. Run-length encoding (RLE) Encoder
  38. Print Alphabet Diamond
  39. Find kth largest element in a List
  40. Balanced parentheses algorithm with tail-call recursion optimisation
  41. Reverse a String's words efficiently
  42. Count number of changes (manipulations) needed to make an anagram with an efficient foldLeft
  43. Count passing cars
  44. Count dist intersections
  45. Establish execution order from dependencies
  46. Counting inversions of a sequence (array) using a Merge Sort
  47. Longest common prefix of strings
  48. Check if an array is a palindrome
  49. Compute missing ranges
  50. Check a directed graph has a routing between two nodes (depth-first search)
  51. Compute nth row of Pascal's triangle
  52. Run-length encoding (RLE) Decoder
  53. Check if a number is a palindrome
  54. In a range of numbers, count the numbers divisible by a specific integer
  55. Merge intervals
  56. Compute minimum number of Fibonacci numbers to reach sum
  57. Find the longest palindrome within a string
  58. Find the index of a substring ('indexOf')
  59. Reshape a matrix
  60. Compute the steps to transform an anagram only using swaps
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  68. Rotate Array right in pure-functional Scala - using an unusual immutable efficient approach
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  77. Monitor success rate of a process that may fail
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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
  3. Collect
  4. Def Inside Def
  5. Drop, Take, dropRight, takeRight
  6. foldLeft and foldRight
  7. For-comprehension
  8. Lazy List
  9. Option Type
  10. Ordering
  11. Partial Function
  12. Pattern Matching
  13. Range
  14. scanLeft and scanRight
  15. Sliding / Sliding Window
  16. Stack Safety
  17. State machine
  18. Tail Recursion
  19. Type Class
  20. Variance
  21. View
  22. Zip

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