Algorithm goal

Scala has an immutable Queue structure. Write an algorithm to reverse the order of the first n elements of this queue, but not affect the rest.

Test cases in Scala

assert(Queue.apply(1, 2, 3, 4, 5).dequeue._1 == 1)
assert(Queue.apply(1, 2, 3, 4, 5).enqueue(6).dequeue._1 == 1)
assert(
  Queue.empty[Int].reverseFirst(nElements = 3) == Queue.empty[Int],
  "Empty queue, reversed, is still empty"
)
assert(
  Queue.apply(1, 2, 3, 4, 5).reverseFirst(nElements = 3) ==
    Queue.apply(3, 2, 1, 4, 5),
  "First 3 elements are reversed"
)
assert(
  Queue.apply(1, 2).reverseFirst(nElements = 5) == Queue.apply(2, 1),
  "Where nElements is > size of queue, the queue is fully reversed"
)

Algorithm in Scala

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

Explanation

By taking items from the front of the queue, we can reverse them into a List (a Stack, effectively).

The queue that remains after dequeuing is the final queue - and we can simply append the elements of it to the list we created prior. (this is © from www.scala-algorithms.com)

Scala concepts & Hints

1. foldLeft and foldRight

   A 'fold' allows you to perform the equivalent of a for-loop, but with a lot less code.

   def foldMutable[I, O](initialState: O)(items: List[I])(f: (O, I) => O): O =
     items.foldLeft(initialState)(f)
   

   ▶ Run code in the IDE

2. Lazy List

   The 'LazyList' type (previously known as 'Stream' in Scala) is used to describe a potentially infinite list that evaluates only when necessary ('lazily').

3. 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

4. Stack Safety

   Stack safety is present where a function cannot crash due to overflowing the limit of number of recursive calls.

   This function will work for n = 5, but will not work for n = 2000 (crash with java.lang.StackOverflowError) - however there is a way to fix it :-)

   In Scala Algorithms, we try to write the algorithms in a stack-safe way, where possible, so that when you use the algorithms, they will not crash on large inputs. However, stack-safe implementations are often more complex, and in some cases, overly complex, for the task at hand.

   def sum(from: Int, until: Int): Int =
     if (from == until) until else from + sum(from + 1, until)
   
   def thisWillSucceed: Int = sum(1, 5)
   
   def thisWillFail: Int = sum(1, 300)
   

   ▶ Run code in the IDE