Algorithm goal

A String is a palindrome when reversing it yields it itself. For example, 'ab' is not a palindrome, but 'aba', and 'abba' are.

This problem is similar to CheckArrayIsAPalindrome and CheckNumberIsAPalindrome.

Test cases in Scala

assert(!isPalindrome("abcd"))
assert(isPalindrome("abcdcba"))
assert(isPalindrome("abcddcba"))
assert(isPalindrome(Array(1).toIndexedSeq))
assert(!isPalindrome(Array(1, 2).toIndexedSeq))

Algorithm in Scala

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

Explanation

Like in CheckArrayIsAPalindrome, note that we can remap an Array to its reverse without allocating a new array (efficient!).

The same can, in fact, be done for a String, as String meets a specification of 'IndexedSeq' (which is also met by an Array). (this is © from www.scala-algorithms.com)

To our advantage, we utilise the fact that a palindrome is its reverse, and that 'IndexedSeq' can be converted into a view, and then re-mapped into that reverse.

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")
   

   ▶ Run code in the IDE

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'))
   

   ▶ Run code in the IDE