Question

One of the things I commonly get hooked up on in ruby is recursion patterns. For example, suppose I have an array, and that may contain arrays as elements to an unlimited depth. So, for example:

my_array = [1, [2, 3, [4, 5, [6, 7]]]]

I’d like to create a method which can flatten the array into [1, 2, 3, 4, 5, 6, 7].

I’m aware that .flatten would do the job, but this problem is meant as an example of recursion issues I regularly run into – and as such I’m trying to find a more reusable solution.

In short – I’m guessing there’s a standard pattern for this sort of thing, but I can’t come up with anything particularly elegant. Any ideas appreciated

Answer 1

Recursion is a method, it does not depend on the language. You write the algorithm with two kind of cases in mind: the ones that call the function again (recursion cases) and the ones that break it (base cases). For example, to do a recursive flatten in Ruby:

class Array
  def deep_flatten
    flat_map do |item|
      if item.is_a?(Array)
        item.deep_flatten
      else
        [item]
      end
    end
  end
end 

[[[1]], [2, 3], [4, 5, [[6]], 7]].deep_flatten
#=> [1, 2, 3, 4, 5, 6, 7]

Does this help? anyway, a useful pattern shown here is that when you are using recusion on arrays, you usually need flat_map (the functional alternative to each + concat/push).