Algorithm goal

Reimplement the 'String#indexOf' Java function.

Test cases in Scala

assert(indexOf("Test", "ested") == None)
assert(indexOf("Test", "ested1234") == None)
assert(indexOf("Test", "est") == Some(1))
assert(indexOf("Tesla Test", "est") == Some(7))
assert(indexOf("est", "est") == Some(0))
assert(indexOf("es", "est") == None)

Algorithm in Scala

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

Explanation

Note that we do not need to check all of the positions of the string - this can be shown by considering where the needle/target string could possibly end.

Then, by using a View, we can compute the tails of the input string, coupled with their start index; If the tail equals to the search string, we found our result - so we return the first such a result. (this is © from www.scala-algorithms.com)

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))
   

   ▶ Run code in the IDE

2. For-comprehension

   The for-comprehension is highly important syntatic enhancement in functional programming languages.

   val Multiplier = 10
   
   val result: List[Int] = for {
     num <- List(1, 2, 3)
     anotherNum <-
       List(num * Multiplier - 1, num * Multiplier, num * Multiplier + 1)
   } yield anotherNum + 1
   
   assert(result == List(10, 11, 12, 20, 21, 22, 30, 31, 32))
   

   ▶ Run code in the IDE

3. Option Type

   The 'Option' type is used to describe a computation that either has a result or does not. In Scala, you can 'chain' Option processing, combine with lists and other data structures. For example, you can also turn a pattern-match into a function that return an Option, and vice-versa!

   assert(Option(1).flatMap(x => Option(x + 2)) == Option(3))
   
   assert(Option(1).flatMap(x => None) == None)
   

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4. 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")
   

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5. View

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

6. 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'))
   

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