Scala algorithm: Check if an array is a palindrome

Algorithm goal

A palindrome is a sequence of things that is the same when reversed.

For example, 'aabb' is not a palindrome, 'ababb' is not a palindrome, but 'aba', 'abba' are.

This problem is similar to CheckStringIsAPalindrome and CheckNumberIsAPalindrome

Algorithm in Scala

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

def isArrayPalindrome[T](array: Array[T]): Boolean =
  array.view
    .zip(array.reverse.view)
    .forall({
      case (a, b) =>
        a == b
    })

Test cases in Scala

assert(
  isArrayPalindrome(Array(1, 2, 1)),
  "Array of integers is a palindrome"
)
assert(
  isArrayPalindrome(Array('a', 'b', 'a')),
  "Array of Char is a palindrome"
)
assert(isArrayPalindrome(Array(1, 2, 2, 1)), "Even Array is a palindrome")
assert(!isArrayPalindrome(Array(1, 2, 1, 2)), "Array is not a palindrome")

Explanation

The more standard solution is to check Array indices directly, but there is a Scala-idiomatic solution which lets you do it without checking array sizes and indices and so forth.

You could directly compare the array against its reverse, but this means passing through the array three times: once to reverse, and then twice to zip up the array with its reverse, and then compare. (this is © from www.scala-algorithms.com)

In Scala, 'indexed views' provide you with the ability to map an Array to its reverse, and then 'zip' the two arrays together so that you can then compare the last item of the array against the first, one before last with one after after first and so on, thereby doing the check of whether the array is indeed a palindrom

Scala concepts & Hints

  1. 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")
    
  2. View

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

  3. Zip

    'zip' allows you to combine two lists pair-wise (meaning turn a pair of lists, into a list of pairs)

    It can be used over Arrays, Lists, Views, Iterators and other collections.

    assert(List(1, 2, 3).zip(List(5, 6, 7)) == List(1 -> 5, 2 -> 6, 3 -> 7))
    
    assert(List(1, 2).zip(List(5, 6, 7)) == List(1 -> 5, 2 -> 6))
    
    assert(List(5, 6).zipWithIndex == List(5 -> 0, 6 -> 1))
    
    assert(List(5, 6).zipAll(List('A'), 9, 'Z') == List(5 -> 'A', 6 -> 'Z'))
    
    assert(List(5).zipAll(List('A', 'B'), 1, 'Z') == List(5 -> 'A', 1 -> 'B'))
    

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

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Study our 116 Scala Algorithms: 6 fully free, 65 published & 51 upcoming

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

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  5. Remove duplicates from an unsorted List
  6. Find combinations adding up to N (unique)
  7. Find k closest elements to a value in a sorted Array
  8. Make a queue using stacks (Lists in Scala)
  9. Single-elimination tournament tree
  10. Quick Sort sorting algorithm in pure immutable Scala
  11. Compute a Roman numeral for an Integer, and vice-versa
  12. Matching parentheses algorithm with foldLeft and a state machine
  13. Traverse a tree Breadth-First, immutably
  14. Read a matrix as a spiral
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  17. Binary search a generic Array
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  32. Check if an array is a palindrome
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  34. Compute nth row of Pascal's triangle
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  38. Find the index of a substring ('indexOf')
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  46. Rotate Array right in pure-functional Scala - using an unusual immutable efficient approach
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  61. Compute single-digit sum of digits
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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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