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

Think in Scala & master the highest paid programming language in the US

Scala is used at many places, such as AirBnB, Apple, Bank of America, BBC, Barclays, Capital One, Citibank, Coursera, eBay, JP Morgan, LinkedIn, Morgan Stanley, Netflix, Singapore Exchange, Twitter.

Study our 94 Scala Algorithms: 6 fully free, 54 published & 40 upcoming

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
  7. Find combinations adding up to N (non-unique)
  8. Find combinations adding up to N (unique)
  9. Find k closest elements to a value in a sorted Array
  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
  14. Remove duplicates from a sorted list (state machine)
  15. Merge Sort: stack-safe, tail-recursive, in pure immutable Scala, N-way
  16. Binary search a generic Array
  17. Merge Sort: in pure immutable Scala
  18. Is an Array a permutation?
  19. Count number of contiguous countries by colors
  20. Add numbers without using addition (plus sign)
  21. Tic Tac Toe MinMax solve
  22. Run-length encoding (RLE) Encoder
  23. Print Alphabet Diamond
  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
  33. Closest pair of coordinates in a 2D plane
  34. Find the contiguous slice with the minimum average
  35. Compute maximum sum of subarray (Kadane's algorithm)
  36. Binary search in a rotated sorted array
  37. Rotate Array right in pure-functional Scala - using an unusual immutable efficient approach
  38. Length of the longest common substring
  39. Tic Tac Toe board check
  40. Find an unpaired number in an array
  41. Check if a String is a palindrome
  42. Count binary gap size of a number using tail recursion
  43. Remove duplicates from a sorted list (Sliding)
  44. Find sub-array with the maximum sum
  45. Find the minimum absolute difference of two partitions
  46. Find maximum potential profit from an array of stock price
  47. Fibonacci in purely functional immutable Scala
  48. Fizz Buzz in purely functional immutable Scala
  49. Count factors/divisors of an integer
  50. Compute single-digit sum of digits
  51. Traverse a tree Depth-First
  52. Reverse bits of an integer
  53. QuickSelect Selection Algorithm (kth smallest item/order statistic)
  54. Rotate a matrix by 90 degrees clockwise

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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