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.