Reverse an array without using a loop in ruby

Question

I have a coding challenge to reverse a an array with 5 elements in it. How would I do this without using the reverse method?

Code:

def reverse(array)
 a         


        

Answer 1

The obvious solution is to use recursion:

def reverse(array)
  return array if array.size < 2
  reverse(array.drop(1)) + array.first(1)
end

We can make this tail-recursive using the standard accumulator trick:

def reverse(array, accum=[])
  return accum if array.empty?
  reverse(array.drop(1), array.first(1) + accum)
end

But of course, tail recursion is isomorphic to looping.

We could use a fold:

def reverse(array)
  array.reduce([]) {|accum, el| [el] + accum }
end

But fold is equivalent to a loop.

def reverse(array)
  array.each_with_object([]) {|el, accum| accum.unshift(el) }
end

Really, each_with_object is an iterator and it is the side-effectful cousin of fold, so there's actually two reasons why this is equivalent to a loop.

Answer 2

Konsolebox is right. If they are asking for the method without loops, that simply means that you cannot use any kind of loop whether it is map, each, while, until or any even built in methods that use loops, like length, size and count etc.

Everything needs to be recursive:

def recursive_reversal(array)

  return array if array == [] # or array.empty?
  last_element = array.pop
  return [last_element, recursive_reversal(array)].flatten

end

Ruby uses recursion to flatten, so flatten will not entail any kind of loop.