Scala algorithm: Reverse a String's words efficiently

Published

Algorithm goal

Efficiently, reverse a String's words. For example, 'Hello new world' becomes 'world new Hello'.

Test cases in Scala

assert(reverseWords("").toList == List(""))
assert(reverseWords("A").toList == List("A"))
assert(reverseWords("AA").toList == List("AA"))
assert(reverseWords("AA BB").toList == List("BB", "AA"))
assert(reverseWords("AA  BB").toList == List("BB", "", "AA"))
assert(reverseString("cannot") == "cannot")
assert(reverseString("") == "")
assert(reverseString(" a b ") == " b a ")
assert(reverseString("AA BB") == "BB AA")
assert(reverseString("cannot do") == "do cannot")
assert(
  reverseString("cannot do without Scala") == "Scala without do cannot"
)

Algorithm in Scala

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

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Explanation

While this may look a long-winded solution, it is very explicit and also streaming-based.

An efficiency portion of this algorithm comes from the fact that rather than appending every character that we come across to an existing string, and thus ending up with allocations of a new String per character, we can extract out the positions of the spaces, and come up with the shape of our target String by describing it in terms of the edges and where the spaces are. (this is © from www.scala-algorithms.com)

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Scala concepts & Hints

  1. Collect

    'collect' allows you to use Pattern Matching, to filter and map items.

    assert("Hello World".collect {
      case character if Character.isUpperCase(character) => character.toLower
    } == "hw")
    
  2. 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")
    
  3. Sliding / Sliding Window

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

  4. State machine

    A state machine is the use of `sealed trait` to represent all the possible states (and transitions) of a 'machine' in a hierarchical form.

  5. 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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Fully unit-tested, with explanations and relevant concepts; new algorithms published about once a week.

  1. Quick Sort sorting algorithm in pure immutable Scala
  2. Find minimum missing positive number in a sequence
  3. Longest increasing sub-sequence length
  4. Counting inversions of a sequence (array) using a Merge Sort
  5. Check if an array is a palindrome
  6. Monitor success rate of a process that may fail
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  8. Find combinations adding up to N (unique)
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  10. Compute a Roman numeral for an Integer, and vice-versa
  11. Matching parentheses algorithm with foldLeft and a state machine
  12. Traverse a tree Breadth-First, immutably
  13. Read a matrix as a spiral
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  15. Merge Sort: stack-safe, tail-recursive, in pure immutable Scala, N-way
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  17. Merge Sort: in pure immutable Scala
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  21. Tic Tac Toe MinMax solve
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  24. Balanced parentheses algorithm with tail-call recursion optimisation
  25. Reverse a String's words efficiently
  26. Count number of changes (manipulations) needed to make an anagram with foldLeft and a MultiSet
  27. Compute nth row of Pascal's triangle
  28. Run-length encoding (RLE) Decoder
  29. Check if a number is a palindrome
  30. In a range of numbers, count the numbers divisible by a specific integer
  31. Find the index of a substring ('indexOf')
  32. Reshape a matrix
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  37. Rotate Array right in pure-functional Scala - using an unusual immutable efficient approach
  38. Length of the longest common substring
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  42. Count binary gap size of a number using tail recursion
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Explore the 21 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. View
  21. Zip

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